Acidified water is electrolysed and the volumes of gas collected at each electrode are recorded in Table 17.1. Table 17.1 Time / min Volume of gas at cathod...

This question is about the electrolysis of acidified water, which decomposes water into hydrogen and oxygen. The data show hydrogen collecting at the cathode at exactly twice the rate of oxygen at the anode.

(a) The ratio of cathode gas to anode gas is 2:1 [1] (for example 12:6, 24:12).

(b) Water is H₂O, containing two hydrogen atoms for every one oxygen atom [1], so twice the volume of hydrogen is produced compared with oxygen [1].

(c) The gas collected in the larger volume is hydrogen [1].

(d) The larger volume is collected at the cathode (negative electrode) [1].

(e) The cathode gas rises by 12 cm³ every 2 minutes (6 cm³ per minute), so after 10 minutes: \( 6 \times 10 = 60\,\text{cm}^3 \) = 60 cm³ [1].

(f) Test for the anode gas (oxygen): a glowing splint relights [1].

(g) A direct current is used so the polarity of the electrodes stays fixed and the two gases are produced separately at the correct electrodes [1]; with alternating current the electrodes would keep swapping and the gases would mix.

(h) A suitable electrode material: platinum or carbon (graphite) [1], because they are inert.

(i) Word equation:

water → hydrogen + oxygen [2] (1 mark for correct reactant and products, 1 for the correct direction).

(j) A hazard of the cathode gas (hydrogen): it is flammable/explosive [1].

(k) A little acid is added because pure water hardly conducts electricity, so the acid provides ions that let the current flow [1].

Exam tip: the 2:1 volume ratio comes straight from the formula of water; hydrogen (which relights nothing but pops with a lit splint) is at the cathode, oxygen (which relights a glowing splint) is at the anode.