Electrolysis Revision



Electrolysis means 'splitting up with electricity':

1) If you pass an electric current through an ionic substance that's molten or in solution, it breaks down into the elements it's made of. This process is called electrolysis.

2) The process requires a liquid to conduct the electricity, which is called the electrolyte. Electroytes contain free ions that are usually the molten or dissolved ionic substance, and they conduct the electricity.

3) There must be a flow of electrons, so electrons are taken away from ioans 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.

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Electrolysis reactions involve Oxidation and Reduction:

  • Reductions is the gain of electrons and the loss of oxygen.
  • Oxidation is a loss of electrons and a gain of oxygen.
  • Electrolysis always involves an oxidation and a reduction.



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Reactivity affects the products formed by electrolysis:

1) Sometimes there are more than two free ions in the electrolyte. For example, if a salt is dissolved in water, there will also be some H+ and OH- ions. 

2) In a mixture of positive ions, the least reactive one will be made at the negative electrode. 

3) 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 more likely it is to stay as ions. Hydrogen will be produced unless the metal is less reactive than it.

4) At the positive electrode, if OH- and halide ions (Cl-, Br-, I-) are present then molecules of chlorine, brommine or iodine will be formed. If no halide is present, then oxygen will be formed. 

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Electrolysis of Brine - NaCl (aq) Sodium Chloride solution:  

1) When sodium chloride is dissolved in water and electrolysed, it produces three products:

Chlorine gas: 

  • Made at the positive electrode 
  • Can see it's made when colour is removed around the positive electrode (chlorine is a bleach)
  • Uses - Disinfectant, Bleach

Hydrogen gas:

  • Made at the negative electrode
  • Can see its made when the liquid is bubbling/fizzing
  • Uses - margarine 

Sodium Hydroxide:

  • Made in the solution
  • Can see its made when the liquid turns blue (using universal indicator)
  • Uses - Lab alkali
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Electrolysis is used to remove Aluminium from it's ore:

1) Aluminium is a very abundant metal, but it is always found naturally in compounds. It's main ore is bauxite, and, after mining and purifying, a white powder is left. This is pure aluminium oxide, Al2O3. The aluminium has to be extracted using electrolysis.

Cryolite is used to lower the temperature (and costs):

1) Aluminium Oxide has a very high melting point (over 2000'C), so melting it is very expensive. Instead, the aluminuim oxide is dissolved in molten cryolite, a less common ore of aluminium. This brings the temperature down to about 900'C, which makes it much cheaper and easier.7

2) The electrodes are made of carbon (graphite), a good conductor of electricity.

3) Aluminium is formed at the negative electrode and oxygen forms at the positive electrode. 

4) The oxygen then reacts with the carbon in the electrode to produce carbon dioxide. This means that the positive electrode gradually wears away and has to be replaced regularly.

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