Electrolysis

YOU SHOULD be able to:

■ Predict chemical reactions making use of electrochemical series.

■ Distinguish between metallic and electrolytic conduction.

■ Classify electrolytes as strong and weak based on their conductivity.

■ Define the terms electrolyte, electrode, strong and weak electrolytes, electrolysis, cathode, anode, cation and anion.

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

QUICK POINTS

Electrolysis is the decomposition of an electrolyte by an electric current 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. The ions are only free to move when molten or in solution.

Electrolytes contain ions which are charged atoms, or charged radicles (groups of atoms) that carry the electric current, e.g., Na+, OH- Strong electrolytes are fully ionized and have high electrical conductivity. Weak electrolytes consist mainly of molecules with relatively few ions, e.g., weak acids and weak bases, and have low electrical conductivity.

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 and the cathode loses electrons. Electrons flow from the anode to the cathode in an electrolytic cell. Electrodes which react and take part in electrolysis are called active electrodes. Those which do not are called inert electrodes. Anions travel towards the anode during electrolysis, e.g., OH-and Cl-, where they may give up electrons. (2Cl(aq) –– Cl2(g) + 2e)

Cations travel towards the cathode during electrolysis, e.g., H+, Na+, Cu2+, where they may gain electrons. (Cu2+ + 2e –– Cu)

ELECTROLYSIS OF MOLTEN COMPOUNDS

In the electrolysis of molten substances, only two kinds of ions are present, and both are discharged. The cation is discharged at the cathode, where it gains electrons (reduced) and form atoms. The molten metal generally coats the cathode. The anion is discharged at the anode by losing electrons (oxidized) and form atoms.

EXAMPLE: MOLTEN LEAD BROMIDE

At the anode: 2 Br- = Br2 + 2e, bromine gas is given off

At the cathode: Pb2+ + 2e = Pb (s), solid lead forms at the cathode

ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE

In a molten substance such as NaCl (l), two ions are present; a positive ion (cation), Na+, and a negative ion (anion), Cl-.

At the cathode: Na+ migrates towards the cathode (negative electrode) and gains electrons to form Na which is deposited on the electrode.

Reaction: Na+(l) + e –– Na (s)

At the anode: Cl- ions migrate towards the anode (positive electrode) and lose electrons to form chlorine gas.

Reaction: 2Cl-(l) –– Cl2(g) + 2e

QUICK REMINDER

The more reactive metals in the reactivity series (such as sodium, potassium, magnesium, calcium, aluminium) are obtained from their ores by the process of electrolysis. These ores cannot be reduced using carbon as the reducing agent, as carbon is less reactive than these metals.

FURTHER PRACTICE