Electrolysis and Making salts

Making salts
1. Most chlorides, sulfates and nitrates are soluble in water (with the exception of lead chloride, lead sulfate and silver chloride)
2. Making soluble salts using a metal or an insoluble base
  1. Pick the right acid plus a metal or an insoluble base (metal oxide or metal hydroxide)
    1. E.g. if you want copper chloride mix hydrochloric acid and copper oxide
      1. CuO (s) + 2HCl (aq) to CuCl2 (aq) + H2O (l)
  2. Add the metal, metal oxide or hydroxide to the acid. The solid will dissolve in the acid as it reacts. Excess solid will sink to the bottom
    1. Then filter out the excess metal, metal oxide or hydroxide to get the salt solution. To get pure solid crystals of the salt evaporate some of the water (to make the solution more concentrated. Then leave the rest to evaporate. This is called crystallisation
3. Making soluble salts using an Alkali
  1. You cant use the method above with alkalis (soluble bases) like sodium, potassium or ammonium hydroxides because you cant tell whether the reaction has finished
  2. You have to add exactly the right amount of alkali to just neutralise the acid. You need to use an indicator to show when the reaction is finished. Then repeat using exactly the same volumes of alkali and acid so the salt isnt contaminated so the salt isnt contaminated with indicator. Then just evaporate of the water to crystallise it
4. Making insoluble salts. Precipitation reactions
  1. If the salt you want to make is insoluble you can use a precipitation reaction
  2. You just need two solutions that contain the ions you need
    1. E.g. to make lead chloride you need a solution which contains lead ions and one which contains chloride ions. So you can mix lead nitrate solution with sodium chloride solution ( all group 1 compounds are soluble)
      1. Pb(NO3)2 (aq) + 2NaCl (aq) to PbCl2 (s) + 2NaNO3 (aq)
      2. Once the salt has precipitated out (and is at the bottom of the flask) all you have to do is filter it from the solution wash it then dry it on filter paper. Precipitation reactions can be used to remove poisonous ions from drinking water. Calcium and magnesium ions can also be removed from water this way they make water hard. Another use of precipitation is in treating effluent (sewage) unwanted ions can be removed
Electrolysis
1. Electrolysis means splitting up with electricity
  1. If you pass an electric current through an ionic substance thats molten or in solution it breaks down into the elements its made of.
    1. Electrolytes contain free ions there usually the molten or dissolved ionic substance
      1. Its the free electrons that carry electricity
        For an artificial circuit to be complete theres got to be a flow of electrons. Electrons are taken away from ions at the positive electrode and given to other ions at the negative electrode. As ions gain or lose electrons they become atoms or molecules and are released
  2. It requires liquid to conduct the electricity called the electrolyte
2. Electrolysis reactions always involve oxidation and reduction
  1. Reduction is the gain of electrons or the loss of oxygen
  2. Oxidation is a gain of oxygen or a loss of electrons
  3. Doesnt have to involve oxygen
3. Electrolysis of molten lead bromide
Electrolysis of sodium Chloride solution
1. Reactivity affects the products formed by electrolysis
  1. Sometimes there are more than two free ions in the electrolyte
    1. E.g. if a salt is dissolved in water there will be some H+ and OH- ions
  2. At the negative electrode if metal ions and H+ ions are present the metal ions will stay in solution if the metal is more reactive than hydrogen. This is because the more reactive an element the keener it is to stay as ions. So hydrogen will be produced unless the metal is less reactive in it
  3. At the positive electrode if OH- and halide ions (Cl- , Br-, I-) are present then molecules of chlorine bromine or iodine will be formed. If no halide is present then oxygen will be formed
2. When common salt (sodium chloride) is dissolved in water and electrolysed it produces three products: hydrogen, chlorine and sodium hydroxide
3. At the negative two hydrogen ions accept two electrons to become one hydrogen molecule
4. At the positive electrode two chloride ions lose their electrons and become one Chlorine molecule
5. The sodium ions stay in solution because they are more reactive than hydrogen. Hydroxide ions from water are also left behind. This means sodium hydroxide is left in one solution (NaOH)
6. The Half equation
  1. Negative electrode: 2H+ + 2e- to H2
  2. Positive electrode: 2Cl- to Cl2 +2e- or 2Cl- - 2e- to Cl2
7. Useful Products
  1. Chlorine for the production of bleach and plasitics
  2. Sodium hydroxide is a very strong alkali and is used widely in the chemical industry e.g. soap
Extraction of Aluminium and Electroplating
1. Electrolysis is used to remove Aluminium from its ore
  1. Aluminium is a very abundant metal but it is always found naturally in compounds
    1. Its main ore is bauxite and after mining and purifying a white powder is left
      1. This is pure aluminium oxide Al2O3
2. Cryolite is used to lower the temperature (and cost)
  1. Al2O3 has a very high melting point at 2000 degrees. So melting would be expensive
  2. Instead the aluminium oxide is dissolved in molten cryolite ( a less common ore of aluminium)
    1. This brings the temperature down to 900 degrees which makes it easier and cheaper
  3. The electrodes are made of carbon (graphite) a good conductor of electriciry
    1. Aluminium forms at the negative electrode and oxygen forms at the positive electrode
      1. Negative electrode: Al 3+ + 3e- to Al
      2. Positive electrode: 2O 2- to O2 + 4e-
      3. The oxygen then reacts with the carbon in the electrode to produce carbon dioxide. This means that the positive electrodes gradually get eaten away and have to be replaced every now and again
3. Electroplating uses electrolysis
  1. Electroplating uses electrolysis to coat the surface of one metal with another metal e.g. you might want to put electroplate silver onto a brass cup to make it look nice
  2. The negative electrode is the metal object you want to plate and the positive electrode is the pure metal you want it to be plated with. You also need the electrolyte to contain ions of the plating metal. (The ions that plate the metal object come from the solution while the positive electrode keeps the solution topped up)
  3. Uses for electroplating
    1. Decoration: Silver is attractive but very expensive. Its much cheaper to plate a boring brass cup with silver than it is to make the cup out of solid silver
    2. Conduction: Metals like copper conduct electricity because of this they are often used to plate metals for electronic circuits and computers

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Electrolysis and Making salts

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	Created by sian.allison about 10 years ago