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| What is an acid and a base? | An acid is a substance that forms an aqueous solution with a pH of less than 7. Acids from H+ ions in water. A base is any substance that will react with an acid to form a salt. An alkali is a base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH- ions in water. |
| What is the general equation for a neutralisation reaction? | acid + base --> salt + water |
| What is the general equation for a neutralisation reaction between acids and alkalis in terms of H+ and OH- ions? | H+ + OH- --> H2O When an acid neutralises a base (or the other way) the products are neutral (seen by the indicator). |
| Name the 5 main apparatus used in a titration? | Pipette (measures one volume of solution) Burette (measure different volumes and let drop by drop) Acid (in burette) Scale (on burette to measure volume of acid) Conical flask (contains alkali) |
| What is the purpose of titrations? | Titrations allow you to find out exactly what volume of acid is needed to neutralise a measured volume of alkali (or the other way). You can then use this data to work out the conc of the acid/alkali. |
| How would you do a titration? | 1. using a pipette and a pipette filler, add a set volume of the alkali to a conical flask with a couple drops of indicator. 2. use a funnel to fill a burette with some acid of known conc (do below eye level so none spills). Record initial volume of acid. 3. use burette to add the acid to the alkali a bit at a time, giving the conical flask a regular swirl (slow down when you think you are near the endpoint). 4. the indicator changes colour when all the alkali has been neutralised (phenolphthalein is pink in alkaline conditions, but colourless in acidic). 5. record the final volume of acid in burette, and use it, along with the initial to find the volume of acid needed to neutralise the alkali. |
| How would you increase the accuracy of your titration? | Take multiple consistent readings to spot any anomalous results. The first titration should be rough then repeat to get similar answers then calculate a mean result (excluding anomalies). |
| What does each single indicator colour change to? | Phenolphthalein - colourless in acids to pink in alkalis. Litmus - red in acids to blue in alkalis. Methyl orange - red in acids to yellow in alkalis. |
| What do acids produce in water? | Acids produce protons in water. They ionise in aqueous solution, producing hydrogen ions, H+ e.g. |
| What determines whether an acid is strong? | Strong acids (e.g.sulfuric, hydrochloric and nitric acids) ionise completely in water. All the acids particles dissociate to release H+ ions. |
| What determines whether an acid is weak? | Weak acids (e.g. ethanoic, citric and carbonic acids) do not fully ionise in solution. Only a small proportion of acid particles dissociate to release H+ ions. The ionisation of a weak acid is reversible which sets up an equilibrium between the undissociated and dissociated acid. Since only a few of the acid particles release H+ ions, the position of the equilibrium lies well to the left. |
| What does pH tell us about the concentration of H+ ions in the solution? | The pH of an acid/alkali is a measure of the conc of H+ ions in the solution. For every decrease of 1 on the pH scale, the conc of H+ ions increases by a factor of 10. General rule is; Factor H+ ion concentration changes by = 10^-x. The pH of a strong acid is always lower than the pH of a weaker acid if they have the same concentration. |
| What is the difference between strong/concentrated acids? | Acid strength tells you what proportion of the acid molecules ionise in water. The concentration of an acid measures how much acid there is in a certain volume of water. The larger the amount of acid there is in a certain volume of liquid, the more concentrated the acid is. So you can have a strong dilute acid or a concentrated weak acid. |
| What are metal oxides and hydroxides? | Some metal oxides and hydroxides dissolve in water (alkalis). Even bases that won't dissolve in water will still take part in neutralisation reactions with acids, so all metal oxides/hydroxides react with acids to form a salt and water. Acid + metal oxide/hydroxide --> salt + water. |
| What do acids and metal carbonates produce? | Metal carbonates are also bases. They will react with acids to produce a salt, water and carbon dioxide. Acid + Metal carbonate --> Salt + Water + Carbon dioxide. |
| How can you make soluble salts using an insoluble base? | 1. you need to pick the right acid, plus an insoluble base such as an insoluble hydroxide, metal oxide or carbonate. 2. gently warm the dilute acid using a bunsen, then turn off. 3. add the insoluble base to the acid a bit at a time until no more reacts (i.e. the base is in excess). You'll know when the acid has been neutralised as the excess solid will just sink to the bottom. 4. filter out excess solid to get the salt solution. 5. to get pure salt crystals, gently heat the solution using a water bath to evaporate some water (make more concentrated) and then stop and leave to cool. Crystals should form which can be filtered out of the solution and dried (crystallisation). |
| What is the reactivity series? | It lists metals in order of their reactivity towards other substances. For metals, their reactivity is determined by how easily they lose electrons (further up, more easily). |
| Why are carbon and hydrogen in the reactivity series? | Gives you information about how metals will react with them; Metals that are less reactive than carbon can be extracted from ores by reduction (more reactive cannot) Metals that are more reactive than hydrogen will react with acids (less reactive will not). |
| What is the general formula for the reaction between an acid and a metal? | Acid + Metal --> Salt + Hydrogen |
| How can we tell the reactivity of a metal by reacting it with an acid? | The speed of the reacting is indicated by the rate of hydrogen bubbles given off. The more reactive the metal, the faster the reaction. |
| What is the general formula for the reaction between metal and water? | Metal + Water --> Metal hydroxide + Hydrogen. Only potassium, sodium, lithium and calcium will react with water (zinc, iron and copper will not). |
| What do we have to do with most metals? | Most metals aren't found in their pure form. Formation of metal ore; oxidation = gain of oxygen, extraction of metal; reduction = loss of oxygen. |
| How is metal extracted using carbon? | Some metals are extracted from their ores by reduction using carbon, the ore is reduced (no O2), and carbon is oxidised (gains the oxygen). However, metals more reactive than carbon have to be extracted by electrolysis. Gold is mined in its pure form. |
| What is redox reaction? | If electrons are transferred, it's a redox reaction (reduction and oxidation at the same time). A loss of electrons is oxidation. A gain of electrons is reduction. |
| Give examples of redox reactions? | Metals reacting with acids - All reactions of metals with are redox. E.g. the reaction between iron and dilute sulfuric acid, iron atoms lose electrons to become iron ions (oxidised by halogen ions), hydrogen ions gain electrons to become hydrogen atoms (reduced by iron atoms). Halogen displacement reactions - a more reactive halogen will displace a less reactive halogen from a salt solution. E.g. chlorine can displace bromine from potassium bromide solution, chlorine atoms gain electrons to become chloride ions (reduced by bromide ions), bromide ions lose electrons to become bromine atoms (oxidised by chlorine atoms). |
| How are metal displacement reactions redox? | A more reactive metal will displace a less reactive metal from its compound. In metal displacement reactions it's always the less reactive metal ion that gains electrons and is reduced, the more reactive always loses an electron and is oxidised. |
| What is electrolysis? | During electrolysis, an electric current is passed through an electrolyte. The electrolyte is a molten or dissolved ionic compound (must be molten/dissolved so electrons a free to move). The ions move towards the electrodes, where they react, and the compound decomposes (positive ions to the negative electrode, negative ions to the positive electrode). This creates a flow of charge through the electrolyte as ions travel to the electrodes. As ions gain/lose electrons, they form the uncharged element an are discharged from the electrolyte. |
| How do you form elements by electrolysis of molten ionic solids? | An ionic solid cant be electrolysed because the ions are in fixed positions and cant move but when molten they can because the ions can move freely and conduct electricity so can be electrolysed. E.g. molten lead bromide; positive metal ions are reduced to the element at the cathode (-). Negative non-metal ions are oxidised to the element at the anode (+). |
| Explain the electrolysis of metals from their ores? | Very expensive as lots of energy is required to melt the ore and produce the required current. E.g. for aluminium oxide, aluminium oxide has a very high melting point so is mixed with cryolite to lower its MP (mixture has free ions so conducts electricity). The Al3+ ions are attracted to negative electrode where they pick up three electrons and turn into neutral aluminium atoms, sinking to the bottom of the tank. The negative O2- ions are attracted to the positive electrode where they each lose two electrons, the neutral oxygen atoms combine to form O2 molecules. |
| What is a cryolite? | Cryolite is an aluminium based compound with a lower melting point than aluminium oxide. |
| Why might it be easier to discharge ions from water in aqueous solution? | In aqueous solutions, as well as the ions from the ionic compound, there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water. Which ions are discharged at the electrodes when the solution is electrolysed depends on the reactivity of the ions in the solution. |
| What are the rules for each electrode? | Cathode (-) - if H+ and metal ions are present, hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen. If the metal ions form an elemental metal that is less reactive than hydrogen, a solid layer of the pure metal will be produced instead, coating the cathode. Anode (+) - if OH- and halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed. If no halide ions are present, the OH- ions from the water will be discharged and oxygen gas (and water) will be formed. |
| Explain the electrolysis of a copper sulfate solution? | Copper(II) sulfate contains; Cu2+, SO42-, H+ and OH-. Copper metal is less reactive than hydrogen, so at the cathode, copper metal is produced and coats the electrode. There aren't any halide ions present, so at the anode oxygen and water are produced: the oxygen can be seen as bubbles. |
| Explain electrolysis of sodium chloride solution? | A solution of sodium contains four different ions; Na+, Cl-, OH- and H+. Sodium metal is more reactive than hydrogen so at the cathode, hydrogen gas is produced. Chloride ions are present in the solution, so at anode chlorine gas is produced. |
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