IGCSE Chemistry 0620: Electrolysis of Dilute Sulfuric Acid with Platinum Electrodes

A student sets up an electrolysis cell containing dilute sulfuric acid, using two platinum electrodes connected to a d.c. power supply. Gas bubbles form steadily at both electrodes, and the student collects each gas in an inverted measuring cylinder filled with water.

(a) State the name of the gas collected at the cathode, and describe a test that would confirm its identity. [2]

(b) State the name of the gas collected at the anode, and describe a test that would confirm its identity. [2]

(c) After several minutes, the volume of gas collected at the cathode is $24\text{ cm}^3$ . Hydrogen and oxygen are always produced in the ratio $V(\text{H}_2) : V(\text{O}_2) = 2 : 1$ Calculate the volume of gas collected at the anode in the same time. [1]

(d) Suggest why platinum electrodes, rather than reactive metal electrodes, are used in this electrolysis. [1]

Part (a): Gas at the cathode

Dilute sulfuric acid supplies both $\text{H}^+$ ions (from the acid) and $\text{OH}^-$ ions (from the water). The cathode is the negative electrode, so it attracts the positive $\text{H}^+$ ions. These gain electrons (reduction) and are discharged as hydrogen gas.

Test: hold a lighted splint at the mouth of the gas jar. Hydrogen burns rapidly with a squeaky pop.

Part (b): Gas at the anode

The anode is the positive electrode, so it attracts the negative $\text{OH}^-$ ions. These are discharged and react further to release oxygen gas.

Test: insert a glowing splint into the gas jar. Oxygen relights the glowing splint.

Part (c): Volume ratio calculation

Hydrogen and oxygen are always produced in a fixed ratio by volume: $V(\text{H}_2) : V(\text{O}_2) = 2 : 1$

Since $V(\text{H}_2) = 24\text{ cm}^3$ : $V(\text{O}_2) = \frac{V(\text{H}_2)}{2} = \frac{24}{2} = 12$

So the volume of gas collected at the anode is $\boxed{12}\text{ cm}^3$ .

Part (d): Why platinum electrodes are used

Platinum is inert. It does not react with the sulfuric acid, with either gas produced, or dissolve into the solution during electrolysis. Using a reactive metal electrode instead would risk the electrode itself reacting or dissolving, which would contaminate the results and stop only the electrolyte’s own ions being discharged.

Final answers

(a) Hydrogen; lighted splint gives a squeaky pop.
(b) Oxygen; glowing splint relights.
(c) $12\text{ cm}^3$
(d) Platinum is inert, so it does not react with the acid or the products.