Introduction to electrolysis

By Francine Taylor-Campbell, Contributor

YOU SHOULD BE ABLE TO:

* Define the terms electrolyte, electrode, strong and weak electrolytes and electrolysis.

* Describe the factors which influence the discharge of the ions during electrolysis.

POINTS TO NOTE

* Electrolysis is the decomposition of an electrolyte by an electric current. This is carried out in an electrolytic cell.

* An electrolyte is a molten compound or the solution of a compound, which conducts an electric current and is decomposed by it.

* Electrolytes contain ions which are charged atoms, or charged radicles (groups of atoms) which carry the electric current. Eg Na+, OH-

Strong electrolytes consist totally of ions i.e. are fully ionised eg all salts, strong acids and alkalis. These have high electrical conductivity.

Weak electrolytes consist mainly of molecules with relatively few ions eg weak acids and weak bases such as ethanoic acid and aqueous ammonia. In solution a large proportion of the molecules remain undissociated.

NH3 (aq) + H2O (l) == NH4+(aq) + OH- (aq)

The electrodes in the electrolytic cell are the anode, which is the positive electrode and the cathode which is the negative electrode. During electrolysis the anode gains electrons, which flow from the anode to the cathode, which loses electrons.

Anions are negative ions, they travel towards the anode during electrolysis eg OH-, Cl- where they may give up electrons. (2Cl-(aq) = Cl2(g) + 2e)

Cations are positive ions, they travel towards the cathode during electrolysis eg H+, Na+, Cu2+ where they may gain electrons. (Cu2+ + 2e = Cu) II To identify the ions present in electrolytes

(i) Write the formula of the compound eg Al2(SO4)3

(ii) Write a balanced equation to separate it into atoms or radicles eg Al2(SO4)3 = 2Al + 3SO4

(iii) Put the appropriate charge on the atoms or radicles (numerically equal to the valency) ie Al2(SO4)3 = 2Al + 3SO42-

(iv) Check to see that the total charge on the ions is zero (2x3+) + (3x2-) = 0

In the case of aqueous solutions relatively very small numbers of H+ and OH- ions are present due to the extremely slight ionisation of water H2O == OH-(aq) + H+ (aq)

Q1 (a) Write equations for the ionisation of (i) H2SO4 (ii) CuSO4 (iii) FeCl3 (iv) Al2O3 (v) AlPO4 (vi) (NH4)3PO4 (b) Give the formulae of the ions and or molecules present in (i) CuSO4(aq) (ii) NaCl(aq) iii. NH3(aq) iv. CH3CO2H(aq).

III Factors affecting the formation of the products during electrolysis
(i) The degree of electropositiveness and electronegativeness of the ions. At the cathode if more than one type of positive ion arrives at the cathode the one which gives up its charge most readily ie the least electropositive ion is discharged eg H+ and Na+, H+ is preferentially discharged 2H+ + 2e = H2 Thus the ion derived from the element lower in the electrochemical series is discharged. At the anode, the least electronegative ion is discharged ie the ion which gives up its charge more readily. Thus in the electrolysis of dilute aqueous sodium chloride OH- ions are discharged in preference to Cl- ions

ie 4OH-(aq) = 2H2O(l) + O2(g) + 4e There are exceptions to this rule: (ii) Due to high concentration of an electrolyte its anion may be discharged in preference to a less electronegative ion eg with concentrated aqueous sodium chloride Cl- ions are discharged in preference to OH- ions (from water) 2Cl- = Cl2 + 2e (iii) The type of electrode. In some cases ie for active electrodes the anode may dissolve to provide electrons eg a copper electrode in the electrolysis of aqueous copper sulphate. Cu(s) = Cu2+(aq) + 2e (left at the anode) since this occurs more readily than the discharge of the anions. Platinum and carbon (graphite) are inert electrodes, they do not form ions during electrolysis.

* Francine Taylor-Campbell teaches at Munro College in St. Elizabeth and is an assistant examiner with CXC.