Electrolytes and electrolysis (General and Credit)
(General)
An electrolyte is a liquid which can carry an electric current through it. Ionic solutions and ionic melts are all electrolytes. Electrolytes can be very useful - see Revision bite on the electrochemical series.
Electrolysis describes the process which takes place when an ionic solution or melt has electricity passed through it.
The essential features of electrolysis are:
direct current electricity (DC) is passed into the substance through electrodes which are often made of carbon
- ions are attracted to the electrodes,
- negative non-metal ions are attracted to the positive electrode
- positive metal ions are attracted to the negative electrode
- at the electrodes, electrons are gained or lost by the ions involved
- products are formed at each electrode.
Let us look at two examples to see how this works:
Example 1 - The electrolysis of molten lead(II) bromide
 The products of this electrolysis are:
- lead metal at the negative electrode
- bromine (Br2) at the positive electrode
Happening Hint
Direct current electricity (DC) has to be used for electrolysis so that the positive ions all travel in one direction and the negative ions all travel in the opposite direction. Otherwise you would not be able to obtain separate products.
At the negative electrode:
- lead (II) ions (Pb2+) are attracted
- the Pb2+ ions are forced to accept two electrons
The ion-electron half equations for this reaction is:
 The lead will form as a liquid at the bottom of the reaction vessel.
At the positive electrode
- bromide ions (Br--) are attracted to the positive electrode
- the bromide ions are forced to give away their extra electron to form bromine atoms
- the bromine atoms join up in pairs to form diatomic bromine gas (Br2).
The ion-electron half equation for this reaction is:
 The bromine appears as a brown gas at the positive electrode.
Example 2 - The electrolysis of copper(II) chloride solution
 The products of this electrolysis are:
- brown copper metal at the negative electrode
- chlorine gas (Cl2) at the positive electrode.
Happening Hint
At the negative electrode:
- copper (II) ions (Cu2+) are attracted
- the blue Cu2+ ions are forced to accept two electrons
The ion-electron half equations for this reaction is:
 The copper forms as a brown solid on the negative electrode.
At the positive electrode:
- colourless chloride ions (Cl-) are attracted to the positive electrode
- the chloride ions are forced to give away their extra electron to form chlorine atoms
- the chlorine atoms join up in pairs to form diatomic chlorine gas (Cl2).
The ion-electron half equation for this reaction is:
 The chlorine appears as a gas with a characteristic smell at the positive electrode.
Worked examples
Here are some typical Standard Grade questions. Use the information on the previous pages to help answer them if you are unsure. Look at the answer after you have tried to do them.
The first 2 questions are at General level
Question 1
Look at the substances in this grid:
 a) Select the substance which conducts by means of a flow of electrons.
b) Which substance conducts by means of a flow of ions?
c) Select the solution which is the non-conductor and explain why it does not conduct.
d) Select the substance which would electrolyse to give a metal at one electrode and a non-metal at the other. Name the products of this electrolysis.
The Answera) A- solid magnesium - Only metals (and graphite) conduct electricity by means of a flow of electrons
b) F - copper bromide solution. - Ionic substances conduct by means of a flow of ions
c) E - glucose - glucose is covalent and has no ions to carry an electric current.
d) F - copper bromide solution - ionic solutions and melts can be electrolysed. The products are brown copper metal at the negative electrode and bromine at the positive electrode.
Examiner's Note
Q1 is a grid question which you usually find in the multiple choice section of the exam paper. In the multiple choice section you don't usually have to explain anything - just choose the correct answer, but it will help if you are more aware of why answers are correct.
Remember - sometimes there is more than one answer, but never more than two!
Question 2
Look at this data about four different substances
 a) Which substance is the metal?
b) Which substance is ionic?
c) Which one could be octane (C8H18)?
d) If the ionic substance was nickel iodide, what products would you expect when you electrolysed it when molten?
The Answera) C is the metal because solid metals can conduct electricity (also liquid metals will be conductors). Metals are insoluble in water.
b) A is ionic because ionic substances do not conduct when solid, but they will conduct in the molten state. (Many ionic substances are soluble in water)
c) D is octane because it is covalent and cannot conduct electricity under any circumstances. As a double check to your answer, it is also insoluble in water.
d) Nickel metal at the negative electrode and iodine at the positive electrode.
Examiner's Note
In Q2 you have to use your knowledge of ionic and covalent conductors to select the correct answers.
The next two questions test your knowledge at Credit level
Question 3
Copper bromide solution can be electrolysed.
a) write down the products you would expect at
i) the positive electrode and
ii) the negative electrode.
b) Write ion-electron half equations for the reactions at each electrode.
The Answer
At the positive electrode you would obtain bromine:
 At the negative you would obtain copper metal
Examiner's Note
In Q3, being able to select the products of electrolysis is a General level activity, but writing the ion-electron half equations is Credit level.
Remember - you can find ion-electron half equations in your data booklet.
Question 4
Shona was given 3 bottles labelled X, Y and Z. The bottles contained aqueous solutions of hydrochloric acid, silver(I) nitrate and alcohol.
She tested their conductivities using a low voltage DC current. A lamp was used to indicate whether or not there was conduction. Here are her results.
 a) Identify
i) the solution Z
ii) the grey metal
>iii) the two gases observed when Y was electrolysed.
b) What kind of bonding does Z have?
The Answer
a) i) Z is alcohol solution. Alcohol is not ionic or a metal so it cannot conduct electricity at all.
ii) The grey metal is silver. Silver(I) nitrate is ionic and can conduct in solution with water and gives silver metal at the negative electrode during electrolysis.
iii) The two gases are hydrogen (at the negative electrode) and chlorine (at the positive electrode). These are the products from the electrolysis of hydrochloric acid solution.
b) Alcohol is covalent
Examiner's Note
In Q4 you use the knowledge at General level to solve more a more difficult Credit problem solving situation.
Now try a Test Bite
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