AQA GCSE Chemistry C2.5 Salts and Electrolysis

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C2.5.1 Acids and Alkalis

  • Dissolving a substance in water makes it an aqueous solution, which could be acidic, alkaline or neutral, depending on the substance dissolved.
  • Soluble hydroxides are alkalis makes solutions which are alkaline. E.g. NaOH solution.
  • Bases, including alkalis, are substances that can neutralise acids-metal oxides and metal hydroxides are bases. E.g. Iron oxide.
  • Acids, including citric acids taste very sour, though many are too dangerous to taste. Enthanoic acid is used for vinegar, citric acid in citrus fruit and fizzy drinks-acids we can eat.
  • E.g.: hydrochloric acid forming when gas hydrogen chloride (HCl) dissolves in water: HCl(g) (+water)-->H+(aq) + Cl-(aq)
  • All acids form  H+ ions when added to water, which makes a solution acidic  (aq)-a state symbol, showing that ions are in an aqueous solution-dissolved in water.
  • Alkalis: bases dissolve in water. Sodium hydroxide becomes sodium hydroxide solution: NaOH(s) (+water)-->Na+(aq) + OH-(aq).
  • All alkalis form OH- hydroxide ions when added to water, making a solution alkaline.
  • Measuring pH: Indicators are substances that change colour when added to acids/alkalis. E.g.: Litmus paper.
  • A pH scale is used to measure acidity, from 0 to 14, most acidic to most alkaline.
  • Universal indicator, UI, can be used to find the pH. It's madfe from different dyes and changes colour as the pH changes; red to yellow to green to blue to purple.
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C2.5.2 Making salts from metals or bases

  • MASH reactions: Metal+Acid-->Salt+Hydrogen.
  • Acid+insoluble-->salt+water
    • This is a neutralisation reaction.
  • The salt made depends on the metal/base used, as well as the acid. Bases with sodium ions will always make sodium salts.
  • Salts formed when hydrochloric acid is neutralised are always chlorides.
  • Surlfuric acid-sulphates.
  • Nitric acids-nitrates
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C2.5.3 Making salts from solutions

  • Acid+alkali-->salt+water
    • This is a neutralisation reaction
    • Neutralisation can be thought of in terms of H+(aq) ions and OH-(aq) ions-H+(aq)+OH-(aq) --> H2O(l)
  • Ammonium reacts with water to form a weakly alkaline solution:
    • Ammonium solution reacts with an acid (e.g. nitric acid):
    • acid + ammonium solution-->ammonium salt+water.
    • Ammonium nitrate-high proportion of nitrogen, very soluble in water, so an ideal source of nitrogen for plants to take up through roots instead of nitrogen found in soil.
    • Ammonium salts-made by adding ammonia solution to an acid until there's an excess of ammonia, which is detected by using universal indicator. We crystallise the ammonium salt from its solution and excess ammonia evaporates.
  • Making insoluble salts
    • Salts can be made by reacting 2 soluble salts, reacting to make an insoluble one-this is a precipitation reaction; the insoluble solid formed is a precipitate.
    • Lead nitrate solution+potassium iodide solution-->lead iodide precipitate+potassium nitrate solution.
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C2.5.4 Electrolysis

  • Definition: 'splitting up using electricity'-using an electric current to break down an ionic substance (electrolyte).
  • Two electrodes dip into the electrolyte. They are conducting rods; one connected to the positive terminal of the power supply and the other to the negative, They are often made of an unreactive/unert substance-often graphite or sometimes platinum so electrodes don't react with the electrolyte or the products.
  • During electrolysis, positively charged ions move to the negative electrode and negatively charged ions move to the positive electrode.
  • Ions reaching electrodes-lose charge and become atoms-gases given off or metals deposited.
  • Ionic substances conduct electricity when melted as ions are free to move and carry charge towards electrodes.
  • Many ionic substance-high melting points, but some dissolve in water.
  • However, electrolysing solutions means it's harder to predict what will form as water also forms ions.
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C2.5.5 Changes at Electrodes

  • When ions go to the electrodes, they either lose or gain electrons. Negative ions lose electrons (oxidised) at the positive electrode and positive ions gain electrons at the negative electrode (reduced).
  • OILRIG-oxidation is loss, reduction is gain (of electrons).
  • Half equations: (If it was lead and bromine ions) must balance out.
    • Negative electrode: Pb2++23-àPb
    • Positive electrode: 2Br - àBr2+ 2e- or 2Br 2e-à Br
  • Effect of water:
    • Slutions-ions from water are present too; more than one type of ion which is attracted to an electrode. E.g.:the positively charged metal ion and H+ ions will be attracted to the negative electrode.
    • The less reactive of the two ions is reduced at the negative electrode, as they sort of 'want' to be ions but the more reactive one will be better at staying as an ion.
    • E.g: if the metal ion is potassium, that is more reactive than H+ and so H+ is the one reduced, forming hydrogen.
    • At the positive electrode-hydroxide ions from water are often discharged, unless the solution has a high conentration of a halide, group 7 ion, where the halide ion is discharged instead.
    • order of discarge at positive electrode: halide ion>hydroxide>all other negatively charged ions. Discharged ions-oxygen gas given off.
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C2.5.6 Extraction of Aluminium

  • Uses: pans, overhead power cables, planes, cooking foil.
  • Less reactive than magnesium, more so than zinc, iron & carbon.
  • Aluminium compound electrolysed-Al2O3 - from the bauxite ore, which has to be purified and then melted. Very high melting point-250 degrees celsius, but mixing it with molten cryolite means that it works at around 850 degrees instead-electrical energy transferred to electrolysis cells keeps the mixture molten.
  • Aluminium oxide--> Aluminium and oxygen.
  • Half Equations:
    • Negative electrode: Al3+ + 3e-à Al
    • Positive electrode: 2O2-à O2+4e-
  • Carbon electrodes have to be replaced a lot as they react with oxugen and gradually burn away.
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C2.5.7 Brine

  • This is salt water- sodium chloride solution.
  • Chlorine gas forms at the positive electrode, hydrogen gas at the negative electrode and there is a Sodium hydroxide solution left.
  • NaCl(aq)à H2(g)+Cl(g)+NaOH (aq)
  • Half equations:
    • Positive: 2Cl-(aq)à Cl2(g) + 2e-
    • Negative: 2H+(aq) +2e-àH2(g)
  • H+ ions in brine are formed when water breaks down. It is attracted to the negative electrode as well as the sodium ion, but is less reacitive than sodium and so is discharged.
  • Chlorine can be used in bleach, with NaOH, for other disinfectants and plastics like PVC.
  • Hydrogen-food-margarine and hardening.
  • Sodium hydroxide-soap and paper.
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C2.5.8 Electroplating

  • An electroplated object-coated with a thin layer of metal by electrolysis.
  • Why?
    • To protect metal beneath from corrosion or to make it look more attractive.
    • To increase hardness of a surface and increase scratch-resistance.
    • To save money-using thin layers of precious metal instead of pure expensive metal.
    • Helps people allergic to nickel-often used for cheap jewellery.
  • The metal to be plated is used as the negative electrode. E.g.: copper foil.
  • The positive electrode is made of the plating metal. E.g.:nickel.
  • Electrolysis takes place in a solution with nickel ions, like and Ni2SO4 solution.
  • Positive electrode:
    • Ni(s)àNi2+(aq) + 2e-
    • Nickel atoms in the electrode are oxidised, losing 2 electrongs and forming Ni2+ ions which go into the solution.
  • Negative:
    • Ni2+(aq) + 2e- àNi(s)
    • Ni2+ ions from the solution are reduced, and gain 2 electrons from the nickel atoms deposited on the other electrode.
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