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Description
Flashcards by Chima Power, updated more than 1 year ago
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Created by Chima Power
about 10 years ago
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| Question | Answer |
| John Dalton | Arranged elements in order of their masses which had been measured in various chemical reaction. In 1808 published table of elements in his book A New System of Chemical Physilosophy |
| John Newland | In 1864 grew on John Dalton ideas with his 'law of octaves' (eight notes). Arranged elements in order of mass noticed each eight element seemed similar. Produced a table illustrating this but assumed all elements had been found so when new found his some elements weren't similar. Not accepted in generation. |
| Dmitri Mendeleev | In 1869 when 50 elements had been identified, Mendeleev was able to arrange these in a table, placing them in order of atomic mass. Then arranged so periodic, reoccurring, pattern in their properties could be seen. Fascinating thing was able to leave space for elements that had not been discovered, then able to use table to predict what these would be. new elements appeared that closely matched these predictions |
| Limitations of Mendeleev's periodic table | Not all elements fit into this pattern when put in order of atomic masses. For example argon, a noble gas, had a greater mass than potassium resulting in argon being in the same group as extremely reactive metals like sodium and lithium. At start of 20th century scientists began to find out more about the structure of elements were they able to see problem. Able to fix it by arranging metals in order of atomic, proton, number. elements with similar properties together and important summary of the electronic structure of all elements. |
| Element groups | Elements in same group on periodic table have similar properties. As atoms have same number of electrons on highest occupied energy levels. Plus the group number in the periodic table tells us the number of electrons in the outermost shell of an atom. |
| Reactivity within groups | Further down a group the greater number of shells occupied by electrons increases and atoms become bigger. means that: larger atoms lose electrons more easily larger atoms gain electrons less easily. Happens because outer electrons further from attractive protons in nucleus and inner shells shield outer electrons from positive charge in nucleus. Occurs in alkali metals and halogens. |
| Group 1 reactivity | Reactivity increases going down the group as atoms get bigger single electrons in outermost shell attracted less strongly to the positive nucleus. As further away from nucleus and inner shells shield to attractive forces. Such that outermost electron easier to lose, so elements lower down group 1 are more reactive |
| Group 7 Reactivity | reactivity decreases going down the group as atoms get bigger electrons added to the outermost shell attracted less strongly to positive nucleus. further away and inner shells screen positive attraction forces, you know the story. Such that extra electron less likely to be attracted to the outer shell, so elements lower down group seven less likely to be reactive |
| Metal reaction | Metals react by losing an electron to become positive, non-metals react by with metals by gaining electrons to become negative ions |
| Alkali Metals | This is the first group of the periodic table, consists of metals lithium, sodium, potassium, rubidium, caesium and francium. They are highly reactive |
| Properties of alkali metals | Very reactive must be stored in oil which stops them from reacting with oxygen in air. Have low density compared with other metals, lithium, sodium and potassium all float on water. Alkali metals are soft can be cut with a knife. have silvery, shiny surfaces when first cut but goes dull as reacts with oxygen in air. That forms a layer of oxide on the shiny surface. Reactive as only have one electron on outer shell so only have to lose one electron to become stable. Become positively charge always form ionic compounds |
| Melting and boiling points of alkali metals | melt and boil at relatively low temperature for metals lower down the group lower the boiling point |
| Alkaline metals reaction with water | Lithium, sodium or potassium in water float moving around a fizzing, fizzing happens because the metal reacts with water to form hydrogen gas. Potassium reaction so vigorous that hydrogen gas catches fire burns with a lilac flame. Reaction between water and alkali metal produces metal hydroxide hence name 'alkali metals', soluble in water with a high pH |
| Alkali metals reactions | React vigorously with non-metals like chlorine produce metal chlorides which are white solids. Chlorides readily dissolve in water to form colourless solutions. react in similar way with fluorine, bromine, and iodine. All these ionic compounds of the alkali metals are white and dissolve in water and are a colourless solution |
| Transition elements | In centre of periodic table |
| Physical properties of transition metals | Have typical metal properties, metallic bonding and giant structures explain properties. -Good conductors of electricity and energy -Hard and strong -Have high densities -Have high melting points( exception mercury) |
| Chemical properties of transition metals | Much less reactive than metals in Group 1 don't as easily react with water or oxygen. So if corrode do so very slowly. Plus physical properties makes them very useful as structural material |
| Compounds of transition metals | Many of transition metals form coloured compounds for example copper sulphate is blue. Name of compound containing a transition metal usually includes Roman number for example iron(III) oxide. Because transition metals can form more than one ion. Iron may exist as Fe^2+ or Fe^3+. Compounds of these ion are different colours Fe2+ gives compounds of green colour, but iron Fe3+ ions give reddish-brown colour. Transition metals very important to chemical industry as catalysts |
| Group 7- Halogens | Group 7 elements are called halogens. group of poisonous non-metals that have coloured vapours, fairly typical properties of non- metals -Low melting and boiling points -Poor conductors of energy and electricity As elements exist as molecules made up of pair of atoms joined together by covalent bonds |
| Reactions of halogens | Electronic structure determines way react have 7 electrons on out most shell (highest energy level). Need to gain just one electron to achieve the stable electronic structure of noble gas so have ionic and covalent bonds. All react with metals gain an electron to form -1 charge forming an ionic bond. When reacts with non- metals forming a covalent bond gives atoms of both elements a stable electronic structure |
| Displacement reaction between halogens | Can use a more reactive halogen to displace a less reactive halogen from solutions of its salts. Bromine displaces iodine from solution because it is more reactive than iodine. Fluorine the most reactive will displace all others. But reacts so strongly with water that we cannot carry out any reactions in aqueous solutions |
| Hard Water | Water is some areas of the country easily form a rich thick lather with soap - water called soft water. Other places difficult to form lather with soap and water - hard water. Hard water makes it difficult to wash ourselves makes it difficult to wash a bath or sink as hard water contains dissolved compounds that react with soap to form scum. Scum formed by hard water isn't a problem in modern washing machines as contain soapless detergants. |
| How hard water forms | Contains dissolved calcium and magnesium ions dissolve when streams or rivers run over rocks containing calcium or magnesium compounds - ex. gypsum calcium sulfate and limestone calcium carbonate. Water in streams and rivers also become slightly acidic. Calcium carbonate weakly reacts with acid forming a calcium hydrocarbonate which is soluble in water hence calcium ions Ca2+ get into the water making it hard. Equation: CaCO3 (s) + H20 (I) + CO2 (aq) --> Ca2+ ((aq) hardness)) + 2HCO3- (aq) Dissolved ions carried into reservoirs and into our domestic water supply - its the dissolved calcium ions and magnesium ions that react with soap to form scum. |
| How hard water wastes soap | Hard water expensive as use much more soap, before soap can remove dirt some wasted reacting with calcium and magnesium ions in the water - forms insoluble salts that form appear as scum. Only once all the magnesium and calcium ions have reacted then can the soap form a lather. Reaction: sodium stearate (soap) + Ca2+ ions --> calcium stearate precipitate (scum) + Na+ ions soluble in water |
| How scale (limescale) is formed | Hard water also often forms scale insoluble solid that can form when we heat a type of hard water. Example forms in washing machines, pipes, immersion heaters and other parts of hot water systems pipes will eventually be blocked. Scale form in kettles 'furring up' of the heating element makes it less efficient as scale is a poor conductor of heat. Making boiling water longer and using more energy - costing more money. |
| Advantages of hard water | Calcium ions in drinking water help in the development strong bones and teeth Evidence that suggests that helps heart disease |
| Removing hardness | Soft water does not contain dissolved salts that cause scale and scum to form. Soften hard water by removing calcium and magnesium ions that make it hard. The benefits are don't waste soap and have scum form and when heating no scale is formed. People advised to keep drinking hard water believed there benefits to health. Soft water also important in industrial processes as hardness can produce scale in boilers making it expensive to run and may also interfere in chemical processes like dyeing. |
| Temporary hardness | Temporary hard water is water that can have its hardness removed by boiling as scale is formed removing the calcium and magnesium ions. |
| Permanent hardness | Solutions where calcium ions and magnesium ions from some salts sulfates aren't removed by heating - causing permanent hard water. Though even this can be softened. |
| Explaining the effect of heating hard water | What happens when we heat temporary hard water: Ca2+ ((aq) hardness)) + 2HCO3- (aq) heat-> CaCO3 ((s)scale)) + H2O (I) + CO2 (g) 1. When we boil water containing hydrogencarbonate ions it decomposes: 2HCO3- (aq) heat-> CO3^2- (aq) + H2O (I) + CO2 (g) 2. Carbonate ions, CO3^2- (aq) react with calcium and magnesium ions in hard water: Ca2+(aq) + CO3^2- (aq) --> CaCO3 ((s) scale)) Removing hardness by heating water would waste energy and be costly. |
| Softening water - Using washing soda | One way to soften water is to add sodium carbonate to it also called 'washing soda' used when washing clothes. When add sodium carbonate to hard water, reaction takes place. Soluble carbonate ions precipitate out calcium and magnesium ions, dissolved metal ions form insoluble carbonates: Ca2+ ((aq) hardness) + CO3^2- ((aq) from sodium carbonate) --> CaCO3 (s) Reaction similar to formation of scale when temporary hard water is heated. Though using sodium carbonate it happens very quickly where and when wanted without wasting energy. |
| Softening water - Using an ion-exchange column | Water can be softened by removing Ca2+ or Mg2+ ions using an ion exchange column. Column usually contains a resin packed with sodium ions (Na+), the hard water passes through the column. The sodium ions are exchanged for the magnesium and calcium ions in hard water. Some columns work by swapping hydrogen ions for the aqueous calcium or magnesium ions - how domestic water softening units work. Resin can be recharged with sodium ions after they have been exchanged for calcium and magnesium ions. Resin is washed with salt (sodium chloride) solution - putting the sodium ions back in. Why water softeners must continually be topped up with salt. |
| Water treatment - boreholes | Water vital and useful for cooking and washing and cleaning and drinking. In industry used as a raw material, solvent and coolant. Drinking water is an issue in the world. Water from boreholes usually fairly clean, has been filtered as it passes through the rocks around the borehole. Normally only need to sterilise this water with chlorine to make it drinkable - chlorine kills the microbes in water. |
| Water treatment - Rivers and reservoirs | Treatment involves several physical and chemical processes. Water source is chosen so it contains as few dissolved chemicals as possible - water then passes five stages. |
| Water treatment - Filter jugs | Some people use filter jugs in their homes usually have a top part from which water is inserted. As water goes from top to bottom passes through a filter cartridge. Usually contains activated carbon, an ion exchange resin and silver. - Carbon in filter reduces the levels of chlorine, as well as pesticides and other organic impurities in the water - Ion exchange resin removes calcium, magnesium, lead, copper and aluminium ions - Some filters contain silver - silver particles discourage the growth of bacteria in the filter. In most filter jugs the filter cartridge needs to be changed every few weeks. |
| Pure - or just fit to drink? | Even water that has been treated and then passed through a filter jugs isn't pure will still contain some substances dissolved in it. Yet it's fit to drink. Can attain pure water by distilling water which involves boiling the impure water. The liquid water evaporates to a gaseous steam we then cool the steam to condense and collect the pure water. Use distilled water in chemistry particle as it's pure. Though this distilled method isn't used to make drinking water as distilling large volumes of impure water would need vast amounts of energy - making the process very expensive. |
| Water treatment - Rivers and reservoirs - Steps | 1. Water source rivers and reservoirs 2. As water enters treatment works, passes through a screen made from bars of metal placed close together that catch large objects like leaves and twigs. 3. In the settlement tank the sand and soil settle out 4. Aluminium sulfate and lime are added to the water small particles of dirt clump together so they sink to the bottom of the water, the sludge that forms like this is dumped in a landfill site where it forms mud 5. Water passed through a special filter of fine sands to remove any remaining particles of mud or grit, so water is clear. 6. Although water now looks clean may still contain some bacteria small amount of chlorine is added to kill the bacteria. 7. pH of water checked and corrected so it's neutral 8. Then stored in large tanks and service reservoirs ready to be pumped to homes, schools, offices and factories -etc. |
| To soften or not to soften? | Most people chosen to soften hard water buying a water softener and continually topping it up with salt (sodium chloride) is expensive. Also other products that can be add to your washing machine or central heating system to protect it from scale. Money spent on these items could save you the expense of repair or replacing parts. Hard water helps strengthens bones and reduces risk of heart disease but filter jugs and cartridges often soften water. Believe hard water better for health, ion exchange columns that exchange the magnesium and calcium ions with sodium ions aren't good for health as sodium ions has been linked to high blood pressure. |
| Chlorine in water | Estimated 50,000 die daily from diseases spread from water would be greater without chlorine. Chlorine used throughout the world to kill microbes in water it plays a large role in many sewage plants worldwide. Monitor effects on humans and environment poisonous chlorine compounds first detected in paper mills lots of chlorine used to bleach paper. Chlorine reacts with harmless organic compounds and can form toxic compounds. Some think adding chlorine to water is a health risk - buy filters to absorb chlorine before it their taps or showers. Though household water uses a lot less than paper mills. Majority of people believe that benefits of chlorine use outweigh the risk. Using ozone is an alternative to using chlorine to kill microbes in water. |
| Fluoride in water | Some places have fluoride in water supply to prevent tooth decay so if you drink tap water taking in small doses of fluoride whether like it or not. Debate whether fluoride should be added to public water. Evidence for arguments often criticized for being unable to control control variables difficult to as in two areas with and without fluoride in water many other variables differ between two areas. Can make a more 'fair test' by increasing sample size and matching groups to get higher quality data. |
| Advantages of fluoridation | - Some areas have fluoridated for 50 yrs no- one has proved harmful effects except from fluorosis - Effect is 30% reduction in cavities in teeth, in 1960's shows five times more likely to have tooth decay if water wasn't fluoridated reduction in effect as bacteria causing tooth decay are dying out due to success of fluoridation - Need fluoridation to protect those with poor dental hygiene habits and don't regularly visit dentist - Bacteria associated with tooth decay also causes types of heart disease so fluoridation protects from this - Fluoride in only added in small parts (1 part per million) anyway |
| Disadvantages of fluoridation of water | - What happens to teeth reflects what happens to bones. Fluorosis condition when children take too much fluoride - white streaks or tips appear on their teeth - deposits of calcium fluoride. They are porous and can be stained. Fluorosis could be a sign of other changes in bones. Studies linked excess fluoride to weakening bones (increased number of fractures) and bone cancer. - Benefit of fluoridation for teeth is significant: reduction of one filing per person - not worth risk? Toothpastes and dental care has improved since 1960s claim of huge benefits not proved don't need it ? - Ethically wrong to give people treatment they haven't consented to: people have right to choose. - Studies show excess fluoride (some studies) affects the brain owing to learning difficulties, associated with Alzheimer's disease in old peoples. - Can't set safe limits of fluoride as can't control people's intake. |
| Comparing the energy released by fuels | Not all exothermic reactions release the same amount of energy when they burn, some combustion reaction are more exothermic than others. Often very important to know how much energy a fuel releases when it burns. In school lab difficult to identify, actual amount of energy released by a burning fuel in an experiment is related to the rise of temperature of the water in a calorimeter. The larger the rise in temperature the more energy will be released. |
| Calculating the energy released | Equation used measured in Joules (J) and given as Joules or Kilojoules: energy released= mass of water heated * specific heat capacity of water * rise in temperature Written as: Q=mc/\T Where: Q = energy released by fuel m = mass of water heated by calorimeter (1cm^3 of water has mass 1g) c= specific heat capacity (amount of energy needed to raise 1g by 1 degree celsuis) /\T = rise in temperature (the final temperature of water minus the initial temperature) |
| Calculate the energy released per gram or per mol | Useful to be able to compare the energy content of different fuels combustion given in joules or kilojoules of energy released per gram or per mole of fuel burned. If the fuel was weighed before and after burning it can be found the mass change ex. 0.2g so energy given out in the experiment will be multiplied by the number of ex. 0.2g in 1g energy released per gram = 5.25 * (1/0.2) kJ/g = 26.25 kJ/g If we know the relative formula mass of the fuel is 46 can now also work out the energy released per mole. One mole of energy has a mass of 46g, so the energy given out in the experiment will be multiplied by the number of 0.2g there are in 46g: energy released per mole = 5.25*(46/0.2)kJ/mol = 1207.5 kJ/mol |
| Simple calorimeter | Used to measure energy changes in a solution polystyrene or styrofoam is a good thermal insulator so helps minimise energy transfer through sides of the container during reactions, a lid on the calorimeter reduces energy transfer to the surroundings even further. The thermometer is used to measure the change in temperature taking place in the reaction, chemicals are mixed in the cup. |
| Energy transfers in solutions | Can calculate energy changes using: Q=mc/\T In these calculation assume solutions behave like water: 1cm^3 of solution has a mass of 1g, solutions have a specific heat capacity of 4.2J/gC - 4.2J of energy raise the temperature of 1g of solution by 1 degree. |
| Worked example of energy transfer in solutions | Simple calorimeter used to measure change in reaction: A+B->C 60cm^3 of a solution containing 0.1moles of A is mixed with 40 cm^3 of a solution containing 0.1moles of B. Temperature of both solutions before being mixed was 19.6 degrees celsius after mixing them the maximum temperature was 26.1 degrees celsius. Step 1- calculate the temperature change: temperature change = 26.1- 19.6 degrees celsius = 6.5 degrees celsius Step 2- calculate the energy change: Q=mc/\T mass of solutions heated = 60g + 40g = 100g energy change = 100g * 4.2J/gC *6.5 =2370J =2.37kJ Energy change when 0.1 moles of reactants A and B are mixed so when 1 mole of reactants are mixed there will be ten times as much energy released (1 mole is 10*0.1moles) = 2.73 kJ *10 =27.3 kJ So the experiment gives the energy change for the reaction: A+B->C as 27.3kJ/mol (temperature rises so is exothermic) - temperature rise is proportional to the amount of energy released |
| Energy level diagrams | Can find out more about what happened in a reaction using a energy diagram. These diagrams shows us the relative amount of energy contained in the reactants and the products. Energy is measured in kilojoules per mole (kJ/mol) |
| Energy level diagram for an exothermic reaction | Products are a lower energy level than the reactants therefore when the reactants form the products energy is released. Difference between the energy levels of the reactants and the products is the energy change during the reaction measured in kJ/mol. Difference in energy between the products and reactants is released to the surroundings. Therefore in exothermic reactions the temperature of the surroundings increases - surroundings get hotter. |
| Energy level diagram for endothermic reactions | Here the products are at a higher energy than the reactants - as reactants react to form products energy is absorbed from the surroundings. Temperature of the surroundings decrease as energy is taken in during the reaction, surroundings get colder, products are at a higher energy level than the reactants. |
| Activation energy and catalysts | The minimum amount of energy required to begin a reaction is the activation energy which can be shown in energy level diagrams. Catalysts can increase the rate of a reaction as they provide an alternative pathway to the products, which has a lower activation energy. Which means that a higher proportion of reactant particles now have enough energy to react which can shown in an energy level diagram. |
| Bond breaking and bond making | In a chemical reaction the chemical bonds between the atoms or ions are broken then new chemicals can be formed to make products - bond making. -Energy must be supplied to break chemical bonds, means that breaking bonds is an endothermic process - energy is taken in from the surroundings. - When new bonds are formed energy is released so making new bonds is an exothermic process. Breaking bonds aBsorbs energy, foRming bonds Releases energy |
| Making and breaking bonds | Always a balance between the energy needed to break bonds and the energy released when new bonds are made in a reaction - it's what decides if a reaction is exothermic or endothermic. - In reactions where the energy released to form new bonds (the products) is more than the energy needed to break the bonds in the reactants. The reactions overall transfer energy to the surroundings - they are exothermic. - In reactions where the energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products the reactions transfer energy from the surroundings to the reacting chemicals - they are endothermic. |
| Bond energy | Energy needed to break the bonds between two atoms is called the bond energy for that bond. Bond energies are measured in kJ/mol. We can bond energies to work out the energy change (/\H) for many chemical reactions. To calculate the energy change for a chemical reaction we must work out: - How much energy is needed to break the chemicals bonds in reactants - Then how much energy is released when the new bonds are formed in the products Data in the table is the energy required to break bonds when we want to know the energy released as these bonds are formed the amount of energy is the same. Ex. Bond energy of C-C bond is 347kJ/mol which means that the energy released forming a C-C bond is also 347kJ/mol. |
| Consequences of burning fuels | Supplies of fossil fuels are running out its a finite resource though has been an increase in their use since the industrial revolution in the last 200 yrs. Resulted in increasing levels of carbon dioxide in the air, most scientist agree that this human activity has contributed to global warming. Research for new alternative fuels is urgent, much of world's pollution causation of increasing number of vehicles on our roads. |
| Hydrogen powered vehicles | Scientists developing hydrogen as a fuel - burns well and produces no pollutants: hydrogen + oxygen -> water 2H2 + O2 -> 2H2O. Could aid fight global warming as reaction doesn't produce carbon dioxide. Though problems with safety and storage which must be solved and issues with supplying the hydrogen to burn in cars - we can use electrolysis but this will generate electricity from non-renewable fossil fuels which doesn't help the environment. Power stations will be producing carbon dioxide and using our limited resources. More efficient use of energy from oxidising hydrogen is in a fuel cell. Cells are fed with hydrogen and oxygen which produce water, most of the energy released in the reaction is transferred to electrical energy can be used to run a vehicle. Though we need a constant supply of hydrogen to run the fuel cell. Scientists aware that replacing engines powered by fossil fuels with cleaner energy fuels there could be great benefits - we have developed many types of fuel cell and hydrogen powered engines. Challenge to match performance, convenience and price fossil fuels |
| Hydrogen fuel cells | A hydrogen fuel cell with an alkaline electrolyte and the only waste product is water. Hydrogen refuelling stations tend to have no roof then if hydrogen leaks it will escape upwards into the atmosphere reducing the risk of explosion. |
| Test for positive ions | Scientists working in environmental monitoring, industry, medicine and forensic science need to analyse and identify substances. To identify unknown substances there are variety of different chemical tests. |
| Test for positive ions: Flame test- Though Lithium is crimson, potassium lilac, barium green, calcium red only one that's right is sodium. | Some metal ions produce flames with a characteristic colour. To carry out a flame test: - Put a small amount of the compound to be tested in a nichrome test loop (wire loop should be dipped in concentrated hydrochloric acid and heated first to clean it then dipped in acid before dipping in the metal compound). - Hold the loop in the roaring blue flame of a Bunsen burner - Use the colour of the Bunsen flame to identify the metal ion in the compound |
| Reactions with sodium hydroxide | Reactions with sodium hydroxide solutions can also help us identify some positive ions. Aluminum ions, calcium ions and magnesium ions all form white precipitates with sodium hydroxide solution. So if a white precipitate form we know an unknown compound contains either Al3+, Ca2+ or Mg2+ ions. If we add excess sodium hydroxide aluminium ions dissolve though the white precipitate formed with calcium and magnesium ions will not. Calcium and magnesium ions can be distinguished with a flame test - calcium ions give a brick red flame but magnesium ions produce no colour at all. Some metal ions form coloured precipitates with sodium hydroxide. If we add sodium hydroxide solution to a substance containing: - copper (II) ions, a blue precipitate forms - iron (II) ions, a green precipitate will form - iron (III) ions, a brown precipitate is formed |
| Tests for negative ions: Carbonates | If we add a dilute acid to a carbonate it fizzes and produces carbon dioxide gas - good test to see if unknown substance is a carbonate. Can represent the reaction by just showing the ions that change in the reaction - called an ionic equation: 2H+((aq)acid)) + CO3^2- ((aq)carbonate ions)) --> CO2(g) + H2O(I) In lime water the carbon dioxide reacts with calcium hydroxide forming a white precipitate of calcium carbonate which turns limewater cloudy. |
| Test for negative ions: Halides (chloride, bromide and iodide) | Simple test to identify if chloride, bromide or iodide ions are present in compounds. First add dilute nitric acid, then add silver nitrate solution. If precipitate forms there are halide ions present, nitric acid is first added to remove any carbonate acids which would also form a precipitate with the silver ions so interfere with the test. Colour of the precipitate will tell us which halide it is: - Chloride ions give a white precipitate - Bromide ions give a cream precipitate - Iodide ions give a pale yellow precipitate |
| Test for negative ions: Sulfates | Can test for sulfate ions by adding dilute hydrochloric acid followed by barium chloride solution. Add dilute hydrochloric acid first to remove carbonate ions that would form a precipitate with the barium chloride solution. A white precipitate tells us sulfate ions are present, the white precipitate is the insoluble salt, barium sulfate. Ionic equation: Ba2+ (aq) + SO4^2- (aq) -> BaSO4 (s) |
| Titration | Acid and alkali (soluble base) react together and neutralise each other forming a salt and water in the process. Solution made will only be neutral if we add exactly the right quantities of acid and alkali. If we begin with more acid than alkali the alkali will be neutralised but the solution will be acidic as there will be excess acid and same if there's more alkali than acid. We can measure the exact volumes of acid and alkali needed to react with each other through titration. Point at which the acid and alkali completely reacted is the end point of the reaction. This can be shown by using an indicator. |
| Carrying out titration | 1. Measure a known volume of alkali into a conical flask using a pipette. Before doing so should wash pipette with distilled water and then with some alkali. 2. Now add an indicator solution to the solution in the flask 3. Pour the acid you are going to use into a burette, a long tube with a tap at one end, the tube has marking on it to enable you accurately measure volumes (often to nearest 0.05cm^3). Before doing this the burette should first be washed with distilled water, and then with some acid. 4. Record the reading on the burette, then open the tap to release a small amount of acid into the flask, swirl the flask to ensure the two solutions are mixed. 5. Keep on repeating the previous step until the indicator in the flask changes colour, which shows the alkali in the flask has completely reacted with the acid added to the burette. Record the reading on the burette and calculate the volume of acid run into the flask. (Your first go will likely be a rough estimate as you will probably run too much acid into the flask ). |
| Carrying out titration 2 | 6. Repeat the whole process a minimum of three times discarding any anomalous data then calculate an average value to give the most accurate results possible. Alternatively you could repeat the titration until you attain two identical results. 7. Now results can be used to calculate the concentration of the alkali. |
| Titration calculations: Calculating concentrations | The concentration of a solute in a solution is the number of moles of solute dissolved in one cubic decimetre of solution - units: moles per decimetre cubed or mol/dm^3. If we know the mass of the solute dissolved in a certain volume of solution the concentration can be worked out. Can work out the mass of one mole of a substance using the relative formula mass - adding the atomic masses of the elements. |
| Titration calculation: Calculating concentrations - Example | Use 40g of sodium hydroxide to make 500cm^3 of solution instead of 1dm^3 (1000cm^3) To find the concentration of solution must work how much sodium hydroxide there will be 1000cm^3 of solution: 40g of NaOH dissolved in 500cm^3 so 40/500g will be amount in 1cm^3. 40/500 *1000g = 80g of NaOH dissolved in 1000cm^3 of solution. Mass of one mole of NaOH is 40g (using relative formula masses) so 80g of NaOH is 80/40moles = 2 moles. 2 moles of NaOH dissolved in 1dm^3 of solution so concentration 2mol/dm^3 |
| Titration calculation + Example 3 | In a titration always have concentration of one solution know accurately put this in the burette. Then place other solution with unknown concentration in a conical flask, using a pipettte. Ensures we know the volume of the solution accurately. Result from the titration is used to calculate the number of moles of the substance in the conical flask Example 3: Sudent put 25cm^3 of sodium hydroxide in a conical flask using pipette, sodium hydroxide reacted exactly with 20cm^3 of 0.5 mol/dm^3 of sulfric acid added from the burette. What was the concentration of sodium hydroxide solution? Equation: 2NaOH(aq) + H2SO4(aq) -> Na2SO4(aq) + 2H2O (I) Tells us 2 moles of NaOH reacts with 1 mole of H2SO4. Concentration of the H2SO4 is 0.5mol/dm^3 so 0.5moles of H2SO4 are dissolved in 1000cm^3 of acid, and 0.5/1000 moles of H2SO4 are dissolved in 1cm^3 of acid hence 0.5/1000 * 20moles of H2SO4 are dissolved in 20cm^3 of acid. So there are 0.01 moles of H2SO4 dissolved in 20cm^3 of acid. Tells us that 0.01 moles of H2SO4 will react with exactly 2*0.01 moles of naOH. Means there must be 0.02 |
| Titration calculations: Example 3 continued | Means there must be 0.02 of naOH in the 25cm^3 of solution in the conical flask. To claculate the concentration of NaOH in the solution in the flask we must calculate the number of moles of NaOH in 1 dm^3 of solution. 0.02 moles of NaOH are dissolved in 25cm^3 of solution so 0.02/25 moles of NaOH are dissolved in 1cm^3 of solution and there will be 0.02/25 * 1000 = 0.8 moles of NaOH in 1000cm^3 of solution. The concentration of the sodium hydroxide solution is 0.8 mol/dm^3 |
| Titrations Calculations: Example 2 | Q: What mass of potassium sulfate is there in 250cm^3 of a 1mol/dm^3 S: In 1dm^3 of solution would be 1 mole of K2SO4, the mass of 1 mole of K2SO4 is (2*39) + 32 + (4*16)g = 174g so in 1000cm^3 of solution there would be 174g of K2SO4 and in 1cm^3 of solution ther are 174/1000g of K2SO4. So in 250cm^3 of solution there are 174/1000 *250g of K2SO4 = 43.5 of K2SO4. There is 43.5g of K2SO4 in 250cm^3 of 1mol/dm^3 potassium sulfate solution. |
| Chemical analysis | Chemists have instruments to halp analyse unknown substances, can use traditional chemistry test known as 'wet' chemistry (Test for negative and positive ions) which are qualitative testing. The titration calculations are quantitative testing. Whether instrumental or 'wet' chemistry is used the accuracy of the data collected depends on the expertise of the tester. |
| Analysis in foresnic science | Forensic chemistry use both qualitative and quantitative analysis, chemists in forensic labs help solve crimes by analysing: drugs, paints, remnants of explosives, fire debris, gunshot residues, fibres, soil samples, toxic chemicals (used in chemical weapons), biological toxins (used in biological weapons) Technique called gel electrophoresis used to analyse DNA, which produces a plate that carries a series of bands according to the composition of the DNA, the bands are unique to each individual (except identical twins) |
| Analysis in pollution control | Environmental scientist need to monitor cases of environmental pollution for instance if a river becomes polluted will test water and trace origin of any pollutants. |
| Analysis in medicine | Use of genetic fingerprinting is in the treatment of leukaemia (blood disease) bone marrow is transplanted from a healthy donor to the patient. After the operation samples of blood from the patient and donor are analysed for their DNA. Doctors are looking for a match between the bands of the two electrophoresis plates, if they're the same then the patients blood cell and DNA in them have come from the transplanted bone marrow - meaning the transplant has been sucessful. Doctors can also study metal ion concentrations of a few parts per billion in patients, ex. they can look for cobalt and chromium ions in the patients blood with hip replacements a concentration of metal ions above 7 parts per billion can indicate that the joint will fail. |
| Chemical equilibrum | Some reactions are reversible, the products can react together to make the original reactants again. What happens when we start with just reactants in a reversible reaction: 1) A+B ----------> (Reactants only at start of reaction) 2) A+B ----------> Rate of products form C +D <----- greater than the reverse at first. 3) A+B -----> Rate products form slow down as reactants get used up C+D <------ Rate at which reverse occur increases as C+D build up 4) A+B -----> Eventually rates products C+D <----- form and reverse occurs are the same In a closed system no reactants, products or energy can get in or out. So in a reversible reaction as the concentration of products build up, the rate at which they react to re-form reactants increases.. As this starts to happen, the rate of the forward reaction decreases. As the concentration of reactans is decreasing from its original maximum value. Eventually both forward and reverse reaction are going at the same rate. When this happens the reactants are making products at the same rate at which products are making reactants. |
| Chemical equilibrium continued | So overall there isn't any change in the amount of products and reactants so say that the reaction has reached equilibrium. At equilibrium the rate of the forward reactions equals the rate of the reverse reaction due to the continuous reaction taking place we sometimes say there is a state of 'dynamic' equilibrium. |
| Chemical equilibrium example | Example of reversible reaction is reaction between iodine monochloride (ICL) and chlorine gas. Iodine monochloride is a brown liquid while chlorine is a green gas. We can react these substances together to make yellow crystals of iodine trichloride (ICL3) The equilibrium mixture can be changed by adding or removing chlorine from the mixture. When there's plenty of chlorine gas the forward reaction makes iodine trichloride crystals which are quite stable but if we lower the concentration of chlorine gas the rate of the forward reaction decreases. The reverse reaction becomes the faster of the two reactions. This starts turning more iodine trichloride back to iodine monochloride and chlorine until equilibrium is again established. We can change the relative amounts of the reactants and products in a reacting mixture by changing the conditions, important in the chemical industry. In a process with a reversible reaction we need conditions that give as much product as possible. though there are other economic and safety factors to consider. |
| Altering conditions: Pressure and equilibrium | find that the position of equilibrium shifts as if to try cancel out any change in conditions when we change concentration. So when we increase the concentration of a reactant, the position of equilibrium shifts to the right to reduce the concentration of that reactant - it opposes the change we introduce. If a reversible reaction involves changing number of gas molecules, pressure can also affect the equilibrium mixture. In many reversible reactions there are more molecules of gas on one side of the equation ahtn on the other. By changing the pressure at which we carry out these reactions we can change the amount of products we make. If the forward reaction produces more molecules of gas an increase in pressure decreases the amount of products formed and a decrease in pressure increases the amount of products formed. If the forward reaction produces fewer molecules of gas an increase in pressure increases the amount of products formed and a decrease in pressure decreases the amount of products formed. |
| Altering conditions: Pressure and equilibrium - Example | In reversible reaction: 2NO2(g) <--> N2O4(g) brown gas pale yellow gas In this reaction can see from balanced symbol equation that we have: 2 molecules of gas on the left hand side of the equilibrium equation equation and 1 molecule of gas on the right hand side. If we increase the pressure in the reaction vessel, the position of equilibrium will shift to reduce the presure it will move in favour of the reaction that produces fewer gas molecules. Here this is the right in favour of the forward reaction so more N2O4 gas will be made, the colour of the gaseous mixture will get lighter. |
| Altering conditions: Energy and equilibrium | When we have a closed system nothing is added or taken away from the reaction mixture, in a closed system the relative amounts of the reactants and products in a reaction at equilibrium depend on the temperature. By changing the temperature we can plan to get more of the products and less of the reactants. If the forward reaction is exothermic an increase in temperature decreases the amount of products formed and a decrease in temperature increases the amount of products formed. If the forward reaction is endothermic an increase in temperature increases the amount of products formed and a decrease in temperature decreases the amount of products formed. |
| Altering conditions: Energy and Equilibrium - Example | Show in reversible reaction: (exothermic)> 2NO2 (g) <--> N2O4 (g) <(endothermic) brown gas pale yellow gas If we increase the temperature the equilibrium shifts as if to try to reduce the temperature, the reaction that is endothermic (taking in energy) will cool it down, so in this case the reverse reaction is favored and more NO2 is formed. Heat up Cool down <- 2NO2 (g) <--> N2O4 (g) -> + No2 formed + N2O4 formed Left favors endothermic reaction Right favors exothermic reaction |
| Making ammonia | Need plants for food and as a way of maintaining oxygen in the air, plants need nitrogen to grow even though nitrogen gas makes up 80% of the air, most plants cannot directly use it. Instead plants absorb soluble nitrates from the soil through their roots, when we harvest crops the nitrogen in plants is lost as the plants don't die and decompose to replace the nitrogen in the soil. So farmers need to put nitrogen back into the soil for the next year's crops. Now we usually do this by adding nitrate fertilisers to the soil we make these fertilisers using a process invented by German chemist called Fritz Haber. |
| The Haber process | Provides us with a way of turning the nitrogen in the air into ammonia, can use ammonia in many different ways one of the most important of these is to make fertilisers. Raw material for making ammonia are: - nitrogen from the air - hydrogen mainly get from natural gas (containing methane, CH4) Nitrogen and hydrogen are purified their passed over an iron catalyst at high temperatures (about 450 degrees celsius) and pressures (about 200 atmospheres). Product of this reversible reaction is ammonia. 1. Gas streams containing hydrogen and nitrogen input 2. Then nitrogen and hydrogen mixture is compressed to pressure of 200atm and heated to 450 degrees celsius 3. Reaction vessels contains iron catalyst 4. Mixture of gases emerging from the reactor is cooled, ammonia liquefies and is separated 5. Unreacted nitrogen and hydrogen are returned to the reaction vessel through the compressor The reaction used in the process is reversible, which means that the ammonia breaks down again into nitrogen and hydrogen, we remove the ammonia by cooling the gases so that the ammonia liquefies. |
| The Haber process continued | The liquefied ammonia can be separated from the unreacted nitrogen and hydrogen remaining as a gases, the unreacted nitrogen and hydrogen are recycled back into the reaction mixture. They then have a chance to react again. N2 +3H2 <---> 2NH3 nitrogen + hydrogen ammonia By removing the ammonia that forms we can reduce the rate of the reverse reaction this helps to stoop the ammonia that is formed from breaking down into nitrogen and hydrogen. We carry out the Haber process in carefully chosen conditions, these are decided to give a reasonable yield of ammonia as quickly as possible. |
| Economics of the Haber process - Effect of pressure | The volume of the reactants is greater than the volume of the products so an increase in pressure will shift the position of equilibrium to the right producing more ammonia. To get the maximum possible yield of ammonia pressure should be as high as possible, but very high pressures require a lot of energy to compress the gases, high pressures also need expensive reaction vessels and pipes. Must be strong enough to withstand very high pressures otherwise there's danger of explosion. To avoid the higher costs of building a stronger chemical plant, the haber process uses a pressure of 200 atmospheres which is a good compromise - it gives a lower yield than it would with even higher pressures but it reduces expenses. |
| Economics of Haber process - Effect of temperature | Forward reaction is exothermic, so if temperature is low this would increase the amount of ammonia in the reaction mixture as equilibrium but at a low temperature the rate of the reaction would be very slow, as the particles would collide less often and would have less energy. To make ammonia commercially we must get the reaction to go quickly, we don't want to waste time waiting for the ammonia to be produced. to do this we need a compromise, a reasonably high temperature is used to get the reaction at a reasonable rate even though this reduces the yield of ammonia. |
| Economics of Haber process - Effect of catalyst | Can use an iron catalyst to speed up the reaction a catalyst speeds up the rate of both the forward and reverse reactions by the same amount hence it does not affect the actual yield of ammonia but we get it formed more quickly. A lower temperature can also reduce the effectiveness of the iron catalyst. |
| Structures of alcohols, carboxylic acids and esters | Substances that form the basis of living things are organic compounds. Organic molecules all contain carbon atoms, tend to form the 'backbone' of organic molecules. Organic compounds include alkanes and alkenes, both of which are only made from carbon and hydrogen atoms. Though there are many more 'families of organic compounds that also contain a few other types of atom. Some organic compounds are made up of carbon, hydrogen and oxygen - the three 'families' are alcohols, carboxylic acids and esters. |
| Alcohols | Ethanol, a biofuel, is an alcohol. An alcohol is similar to an alkane molecule but has a H atom removed and replaced with and O-H group giving us an alcohol molecule. The -OH group of atoms is an example of a functional group. A functional group gives each type of organic compound their characteristic reactions. All the compounds with the same functional group is called a homologous series. Name alcohols from the alkane with the same number of carbon atoms, take the 'e' from the end of the alkane's name and replace it with 'ol'. Formula of ethanol may be written as C2H6O but more information can given in a structural formula which for ethanol would be CH3CH2OH often shortened to C2h5OH |
| Carboxylic acids | Ethanoic acid is the main acid in vinegar all carboxylic acids contain the -COOH functional group. We show th structural formula of each of the carboxylic acis as HCOOH, CH3COOH and CH3CH2COOH (C2H5COOH). |
| Esters | These are closely related to carboxylic acids if we replace the H atom in the -COOH group by a hydrocarbon group we get an ester. On the other side is an ester called ethyl ethanoate. An ester's structural formula always has the -COO- functional group in it, teh strucutral formula of ethyl ethanoate is CH3COOCH2CH3 (or CH3COOC2H5) |
| Properties and uses of alcohols | Alcohols, especially ethanol commonly used in everyday products, have already seen how ethanol is the main alcohol we refer to in alcoholic drinks. Made by fermenting sugars from plant material it's becoming an important alternative fuel to petrol and diesel. Ethanol can be made from ethene and steam in industry. Alcohols dissolve many of the same substances as water, those alcohols with smaller molecules mix very well with water, giving neutral solutions. The alcohols can also dissolve many other organic molecules, property makes them useful as solvents for example we can remove ink stains from permanent marker pens using methylated spirits. Methylated spirits ('meths') mainly ethanol but has the more toxic methanol mixed with it. Also has a purple dye added and other substances to make it unpleasant to drink, alcohols are also used as solvents in products like perfumes, aftershaves and mouthwashes. |
| Uses of alcohols: Combustion | Use of ethanol (and methanol) as fuels shows that the alcohols are flammable, ethanol is used in spirit burners it burns with a 'clean' blue flame: ethanol+oxygen-> carbon dioxide+water C2H5OH + 3O2 -> 2CO2 + 3H2O |
| Uses of alcohols: Reaction with sodium | Alcohols react in similar way to water when sodium is added, the sodium fizzes giving of hydrogen gas and dissolves away to form a solution. Reactions aren't as vigorous as the reaction we observe with water |
| Uses of alcohols: Oxidation | Combustion is a way to oxidise an alcohol though when we use chemical oxidising agents, like potassium dichromate (vi) we get different products. An alcohol is oxidised to a carboxylic acid when boiled with acidified potassium dichromate (vi) solution. So ethanol can be oxidised to ethanoic acid the same reaction takes place if ethanol is left exposed to air. Microbes in the air produce ethanoic acid from the ethanol which is why bottles of beer or wine taste and smell like vinegar when left open. |
| Carboxylic acids uses | Carboxylic acids form acidic solutions when they dissolve in water, they have the typical reactions of all acids: ethanoic acid + sodium carbonate -> sodium ethanoate + water + carbon dioxide |
| Why carboxylic acids are called 'weak acids'? | CO2 gas given off more slowly when a carbonate reacts with a carboxylic acid than with hydrochloric acid of the same concentration. Carboxylic acids are called weak acids as opposed to strong acids like hydrochloric acid. pH of a 0.1mol/dm^3 solution of hydrochloric acid is 1. Yet a 0.1mol/dm^3 solution of ethanoic acid has a higher pH of 2.9. The solution of ethanoic acid isn't as acidic even though the two solutions have the same concentration. As acids must dissolve in water before they show their acidic properties as in water all acids ionise. Their molecules split up to form a negative ion and H+ (aq) ions. It's the H+ (aq) ions that all acidic solutions have in common, for instance in hydrochloric acid the HCL molecules all ionise in water: HCl(aq) --> H+(aq) + Cl-(aq) Say the strong acids completely ionise in the solution, though, in weak acids most of the molecules stay as they are. Only some will ionise (split up) in their solutions a position of equilibrium is reached in which the molecules and ions are present so in ethanoic acid we get: CH3COOH(aq) <--> CH3COO-(aq)+H+ |
| Why carboxylic acids called weak acids - continued | Therefore given to aqueous solutions of equal concentration, the strong acid will have a higher concentration of H+ (aq) ions than the solution of the weak acid. So a weak acid has a higher pH (and reacts more slowly with a carbonate). |
| Making esters | Carboxylic acids also react with alcohols to make esters, water is also formed in this reversible reaction. An acid, usually sulfuric acid is used as a catalyst: sulfuric acid catalyst ethanoic acid + ethanol <--> ethyl ethanoate + water In general: carboxylic acid + alcohol<-->ester + water Esters formed has a distinctive smells, they are volatile (evaporate easily), many smell sweet and fruity - makes them ideal to use in perfumes and food flavourings. |
| Organic issues: Ethanol in drinks | Alcoholic drinks help people relax, it can relieve stress for some after hard day at work. too many people are drinking more than maximum recommended amount of alcohol, puts their health at risk. Alcohol has been associated with high blood pressure and heart disease. Excess alcohol can also damage the liver in extreme cases a liver transplant is the only way to avoid death. Alcohol is a socially acceptable drug, some become dependant on it, alcoholics are addicted to ethanol, many ruin their lives as their behaviour changes as a result of drinking alcohol. Ethanol is used in methylated spirits as a solvent, some desperate will drink this as it contains a lot of ethanol and is cheap even though it has had toxic methanol added to it. Also contains emetics (substances that make you vomit) and foul tasting chemicals. Drinking 'meths' causes liver failure, blindness and an early death. Other chemicals also added to methylated spirits to make it more difficult to distill off the ethanol. By including chemicals with smaller boiling points, people can't separate off the ethanol for drinking. |
| Ethanol in drinks | Alcoholic drinks usually more expensive than methylated spirits as they have tax added on (demerit goods), government can use the income generated for good causes. Though e should weigh the costs of dealing with: health problems, days lost at work, policing anti social behaviour. |
| Ethanol and esters as biofuel | Ethanol used as biofuel made by fermenting sugars from crops, we also looked at biodiesel made from plant oils which are esters. In processing these esters, the oils are broken down into long-chain carboxylic acids. Then reacted with methanol or ethanol (in presence of a catalyst) to make the esters use as biodiesel. Though the land used for biofuel crops could be used for food crops, with an ever-increasing world population, feeding ourselves will become more of an issue. Will need more land for farming both for fuels and dood. New farming land is often made by cutting down and burning tropical rainforests. Which destroys habitats of wildlife and contributes toward increasing the percentage of carbon dioxide in the atmosphere. Yet alternatives to crude oil are needed and that's all she wrote. |
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