OCR A - A level Chemistry Module 3

Enthalpy

Enthalpy Changes and Enthalpy DiagramsAll chemical reactions have an associated enthalpy change. This is the difference in energy between reactants and products. Enthalpy change isn't the same as temperature change. You can't directly measure the enthalpy of a system but you can measure an enthalpy change.Enthalpy is given the symbol H and enthalpy change is delta H (represented by a small triangle). The units of enthalpy change are KJmol^-1.Each reaction requires an endothermic or exothermic process, the one which requires more energy to either break or form bonds is what gives it the title of being exo or endo. The bonds in the reactants are broken, requiring energy (endo) New bonds are formed between atoms as molecules, releasing energy (exo) If more energy is released in step 2 than step 1, its exothermic. IF more energy is required than is released then its endothermic.Exothermic ReactionsThere is an increase in temperature as the enthalpy change goes down so the products have less energy than the reactants. If energy was released then it was surplus to the reaction which means its given out as heat. More energy is being released than is required so the enthalpy change decreases. The enthalpy change is negative for this reason.

Endothermic ReactionThe temperature decreases as the enthalpy change increases.The reactants have more energy than is being released and so it takes more energy to form the bonds in the product than it does to break the bonds in the reactants. Therefore more energy is required overall and the enthalpy change is positive.

The activation energy is defined as the minimum energy which particles need to collide to start a reaction and Ea stands for the activation energy

`The average bond enthalpy is the enthalpy required to break 1 mole of bonds, forming gaseous atomsThe average bond takes into account all the bonds in their different environments. The table below shows the most commonly used average bond enthalpies.When working out the enthalpy change, you find out how many bonds were formed and multiply this by the average bond enthalpy. Then you look to see whether it is an exo or endo reaction. If there is more release energy when forming the products than is required 1e. a higher value on the right hand side of the equation than on the left side, it is exothermic. Therefore the enthalpy change (deltaH) will be a negative.

Standard Enthalpy Changes Standard enthalpy change of reaction is enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard states and conditions and with all reactants and products in their standard states. Standard Enthalpy Change of Formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states and conditions. Standard Enthalpy Change of Combustion is the enthalpy change when 1 mole of a compound is completely burned in oxygen, under standard conditions and states Standard Enthalpy Change of Neutralisation is the enthalpy change when solutions of an acid and an alkali react to form 1 mole of water under standard conditions and states

To measure enthalpy change in a solution we use:Q= mc(delta)TEnthalpy change = mass of solution x heat capacity x temperature changeTo calculate the enthalpy change of reaction you: Substitute the vales into the Q=mc(delta)T equation o work out the energy change Work out the number of moles the reactants use, include equation Divide q by the number of moles and divide by 1000 to get KJmol if that's needed

Core Practical:One where substances are mixed in an insulated container and the temperature rise is recorded - this could be a solid dissolving or reacting in a solution or it could be two solutions reacting together.Method: Washes equipment with solutions to be used Dry cup after washing Put polystyrene cup in beaker for insulation and support Clamp thermometer into place making sure thermometer is immersed in liquid Measure initial temperature of solution Transfer reagents into cup and stir Measure final temperature - optimum EvaluationIf reaction is slow then the exact temperature rise can be difficult to obtain as cooling occurs simultaneously with the reaction. To counteract this, we take readings at regular time intervals and extrapolate the temperature curve back to the time the reactants were added togetherWe also take the temperature of the reactants for a few minutes before they're added to get a better average temperatureErrors: Heat loss from container to surroundings Approximation in specific heat capacity of solution. method assumes all solutions have heat capacity of water Reaction or dissolving may be incomplete or slow

The Periodic Table

We're going to skip the history of the periodic table, essentially a guy called Mendeleev (1869) produced a fairly accurate one which is similar to the one we have today. He arranged the known elements by atomic mass but he left gaps in the table so other elements could be discovered. Nowadays, the modern periodic table has the elements arranged in order of increasing atomic number.The periodic table is arranged into periods (which are the rows) and groups (which are the columns). All the elements within a period have the same number of shells and this results in repeating patterns of physical and chemical properties across a period (this is known as periodicity). The period number tells you the number of shells the element has.All the chemicals within a group have the same number of electrons in their outer shell and this means they have similar physical and chemical properties. The group number tells you how many electrons there are in their outer shell.

Electron ConfigurationsThe periodic table is split into blocks, the s-block, p-block, d-block and f-block. This tells you which subshell the electrons go into. ] The s-block elements have an outer shell electron configuration of s1 or s2 The p-block elements have an outer shell configuration of s2 p1 to s2 p6. The d-block elements are slightly harder as you don't write the subshells in the order they're being filled but you write them in order of increasing period number. This means that even though the d subshells are filled last, they aren't written at the end of the line You don't encounter f block elements at a level. When you've got the periodic table labelled with the shells and sub shells, its pretty easy to read off the electron structure of any elements by starting at the top and working your way across and down until you get to your element.

Ionisation Energy

When electrons have been removed from an atom or molecule, its been ionised. The energy needed to remove the first electron is the first ionisation energy. The technical definition for this is :The first ionisation energy is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms Ionisation is an endothermic process Its measured in Kjmol-1 You must use the gas state symbol because ionisation energies are for gaseous atoms Always refer to 1 mole of atoms The lower the ionisation energy, the easier it is to form an ion

Factors affecting ionisation energy Nuclear Charge - The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger attraction for the electrons Atomic Radius - Attraction falls off very rapidly with distance. An electron close to the nucleus will be much more strongly attracted than one further away Shielding - As the number of electrons between the outer electron and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge. The lessening of the pull of the nucleus by inner shells of electrons is called shielding. Ionisation Energy TrendsAs you go down a group in the periodic table, ionisation energies generally fall (gets easier to remove electrons from their outer shell). This is because, as you go down the group, the number of electron shells increase. The increase in electron shells means there is a greater shielding effect as more shells means more electrons between the nucleus and the outer electrons. There is also a large atomic radius as the distance between the outer electron and the nucleus increases as well. Both of these factors make it easier to remove outer electrons resulting in a lower ionisation energy. The nuclear charge does increase the further down the group you go, but this effect is overridden by the extra shells.Periodicity of Ionisation EnergyAs you move across a period, the general trend is for the ionisation energy to increase (harder to remove electrons). This is because there is a greater nuclear charge due to the increasing number of protons in the nucleus. There is little shielding effect due to the fact that there are the same number of shells even though there are different orbital types. Although, the periodicity generally increases, there are some small drops between different groups. These drops are due to the outer electrons being in different orbitals. For example, there is a drop between a group 2 element and a group 3 element. This is because the outer electrons have gone from being in an s orbital to a p orbital, making the ionisation energy decrease.

Successive Ionisation EnergiesEach time you remove an electron, there's a successive ionisation energy. These are called second ionisation energy, third ionisation energy etc. To define these ionisation energies (say second ionisation energy) you write:The energy needed to remove 1 electron from each ion in 1 mole of gaseous atoms to from 1 mole of gaseous 2+ ions.If you have the successive energies of an element you can work out the number of electrons in each shell of the atom and which element the group is in. A graph of successive ionisation energies provides evidence for the shell structure of atoms.

You can see here, with the successive ionisation energies of sodium, that the 1 electron in the third shell is only weakly attracted to the nucleus and therefore its ionisation energy is low. The other eight electrons in its second shell have a greater ionisation energy and then its first shell has two electrons where the ionisation energy is highest due to its proximity to the nucleus.Within each shell, successive ionisation energies increase. This is because electrons are being removed from an ever increasing positive ion. There is less repulsion amongst the remaining so more energy is needed to remove them. The big jumps in ionisation energy happen when a new shell is reached.

Group 2 - Alkaline Earth Metals

When group 2 elements react they lose electrons, forming positive ions. The easier it is to lose electrons, the more reactive the element is. So because ionisation energy decreases down the group, reactivity increases down the groupGroup 2 elements all have 2 electrons in their outer shell so they form 2+ ions, so when they are oxidised, their state goes from 0 to +2.

Reactions with WaterThe group 2 metals react with water to give a metal hydroxide and hydrogen. The metal hydroxide that forms dissolves in water to produce hydroxide ions (OH-). These make the solutions strongly alkaline with a pH of about 12-13The elements react more readily down the group as the ionisation energy decreases. This is due to the size of the atomic radii and the increasing shielding effect. Reactions with OxygenWhen group 2 metals burn in oxygen, you get solid white metal oxides. The oxides react readily with water to form hydroxides which then dissolve, this makes the solution alkaline with pH 12-13. There is an exception with this, magnesium oxide only reacts slowly and the hydroxide isn't very soluble. The oxides that form more strongly alkaline solutions are further down the group. Reactions with Dilute AcidsWhen group 2 metals react with dilute hydrochloric acid, you get a metal chloride and hydrogen. Different acids will produce different salts for example sulphuric acid can be used which will produce a metal sulphate

Uses of Group 2 CompoundsGroup 2 elements are known as the alkaline earth metals and many of their compounds are used for neutralising acids. Calcium hydroxide is used in agriculture to neutralise acidic solid. Magnesium hydroxide and calcium carbonate are used in some indigestion tablets as antacids, neutralising excess stomach acid.

Group 7 - The Halogens

The table below gives information of some of the main properties of the first four halogens.

Boiling Point TrendThe boiling and melting points of the halogens increase down the group. This is due to the increasing strength of the London dispersion forces as the number of electrons increases when the size and relative mass of the atoms increases. As substance is said to be volatile if it has a low boiling point so volatility decreases down the group.Reactivity TrendHalogen atoms react by gaining an electron in their outer shell. This means they're reduced and therefore oxidising agents. When halogens are reduced, halide ions are formed. As you go down the group, the atomic radius increases so the outer electrons are further from the nucleus. The outer electrons are also shielded more from the attraction of the positive nucleus, because there are more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion, despite the increased charge on the nucleus. So, larger atoms are less reactive and so less oxidising.Displacement ReactionsThe halogens relative oxidising strengths can be seen in their displacement reactions with halide ions. More reactive halogens will oxidise and displace the halide ions of less reactive halogens. Reactivity decreases down the group so a halogen will displace a halide from solution if the halide is below it in the periodic table. Chlorine will displace bromide and iodide Bromine will displace iodide Iodine has no reaction With these displacement reactions, there are colour changes Chlorine water is colourless and will displace potassium bromide solution and potassium iodide solution Bromine water is orange and will displace potassium iodide solution Iodine solution is brown and does not displace any halides The brown and orange of bromine water and iodine solution can often look similar so reacting the mixture with an organic solvent like hexane will dissolve with the halogen and the different halogens form different colours. A violet/pink colour shows the presence of iodine An orange red colour shows bromine A very pale yellow/green shows chlorine.

Testing for Halides (PAG 4) First of all, if you're testing solid substances for halide ions, you'll first need to dissolve each of them separately in distilled water to make solutions. Once you've got solutions of the substances that you're testing, use a pipette to add about 3cm^3 of each solution to separate test tubes. Add some dilute nitric acid to each test tube. This gets rid of any unwanted ions Add some silver nitrate to each test tube. If there are chloride, iodide and bromide ions present a precipitate of the silver halide will be formed The colour of the precipitate identified the halides. Chloride ions give a white precipitate, bromide ions give a cream precipitate and iodide ions give a yellow precipitate Sometimes, like the displacement reactions, its difficult to identify the different halides. To confirm the different halides, ammonia can be used. Add dilute ammonia to each of the silver halide precipitates and record any observations If none of the precipitates are dissolved, then add concentrated ammonia to the solutions and observe What you should find with this, is that if there was chloride ions present then the precipitate would dissolve in the dilute ammonia, if bromide ions were involved then the precipitate would dissolve in concentrated ammonia but if iodide ions were present then the precipitate would be insoluble in both dilute and concentrated ammonia.

Disproportionation

Disproportionation is when a single element is simultaneously oxidised and reduced. The halogens undergo disproportionation when they react with a cold dilute alkali solution such as sodium hydroxide

If you mix chlorine gas with cold dilute aqueous sodium hydroxide, you get sodium chlorate solution, which is the common bleach. In this reaction some of the chlorine is oxidised and some of it is reduced so its a disproportionation reaction. The sodium chlorate solution has lots of uses; water treatment, bleach paper, cleaning toilets.When you mix chlorine with water, disproportionation happens again and you end up with hydrochloric acid and chloric acid. (HClO)Aqueous chloric acid ionises to make chlorate ions when its combined with water. The chlorate ions kill bacteria. So, adding chlorine to water can make it safe to drink. Chlorine is an important part of water treatment as it kills bacteria and prevents reinfection, growth of algae and eliminates discolouration from organic compounds. However there are risks with using chlorine: Chlorine gas is toxic - irritates the respiratory tract Liquid chlorine on skin/eyes causes severe chemical burns Chlorine can react with other compounds in water and form chlorinated hydrocarbons which are carcinogenic Alternatives to chlorine: Ozone - strong oxidising agent but its expensive to produce and it has a short half life (treatment is temporary) UV light - kills microorganisms but its ineffective in cloudy water and has is only temporary

Ion Testing

To identify unknown ions in a solution, you have to avoid having false positives (thinking that a certain ion is there when it actually isn't) and to prevent these false positives, you can conduct the tests in a certain order or sometimes add a sample to remove the ions that are creating a false positive test.The order that you should do your test is:Test for carbonates --------> Test for sulphates --------> Test for halides

Carbonate Testing: Add a dilute strong acid to unknown sample If carbonates are present then carbon dioxide will be released You can test to see if carbon dioxide is produced by using a delivery tube feeding into limewater. If the limewater goes cloudy then carbon dioxide is being released.

Sulphate Testing If there were no carbonates present then a sulphate test should be conducted next. Barium sulphate is insoluble so we can use this fact to determine if sulphate ions are in the unknown solution Add a few drops of barium nitrate If you get a white precipitate of barium sulphate then your mystery compound is a sulphate. In the carbonate test, you add a dilute strong acid and it will react with any carbonates or sulphites present so they wont interfere with the test results. This make sure that they wont interfere with the test for sulphate as well (they could cause a false positive otherwise)

Halide Testing Add nitric acid, then silver nitrate solution If any of the halogens are present then a precipitate will form. The colour of the precipitate depends on the halogen. To further identify the halogen, you can test their solubility in ammonia

Ammonia Testing Ammonia gas is alkaline and so you can use damp red litmus paper If the litmus paper turns blue then ammonia is present This test can be used to determine whether a substance contains ammonium ions. Add a few drops of sodium hydroxide to your mystery substance in a test tube and warm the mixture Hold litmus paper over the test tube, if it changes colour then ammonium ions are present

Hess's Law and Reaction Rates

Hess's Law states that the total enthalpy change for a reaction is independent of the route taken by which the chemical change takes placeHess cycles are used to measure the enthalpy change for a reaction that cant be measured directly by experiments. Instead alternative reactions are carried out that can be measured

Reaction Rates:Collision theory - reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules. This minimum energy is the activation energy.At higher concentrations and pressures, there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of collisions. The rate of reaction is defined as the change in concentration of a substance in unit times. When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the reaction rate. The initial rate is the rate at the start of the reaction where its fastest.

Maxwell Boltzmann DistributionThese show the spread of energies that molecules of a gas or liquid have at a particular temperature. Things to consider about a Maxwell Boltzmann Distribution curve: A few particles have low energies because collisions cause some particles to slow down It has to start form the origin to show 0 molecules with 0 energy Area under curve represents total number of particles present Only a few particles have greater energy than the Ea It should never meet the x axis

Effect of Temperature As the temperature increases, the distribution shifts towards having more molecules with higher energies. At higher temperatures the mean energy and peak shift to higher values although the number of molecules with that value decreased Total area under curve remains constant At higher temperatures the molecules have a wider range of energies than at lower temperatures At higher tm