Electrolysis: Question 10

Worked solution

Step 1: Recall what a hydrogen-oxygen fuel cell produces

A hydrogen–oxygen fuel cell combines hydrogen and oxygen to generate electrical energy, with water as the only chemical product. No carbon dioxide or other polluting gases are formed.

Step 2: Write and balance the equation

Starting from hydrogen and oxygen forming water: $\text{H}_2(g) + \text{O}_2(g) \rightarrow \text{H}_2\text{O}(l)$

This is not balanced: there are 2 oxygen atoms on the left but only 1 on the right. Doubling the water balances the oxygen atoms: $\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$

Now there are 4 hydrogen atoms on the right but only 2 on the left, so the hydrogen must also be doubled: $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$

Checking: 4 H atoms and 2 O atoms appear on both sides, so this equation is balanced.

Step 3: Check each option

• Option A is exactly this balanced equation, correct.
• Option B shows hydrogen peroxide, $\text{H}_2\text{O}_2$, as the product instead of water, the wrong substance.
• Option C is missing the “2” in front of $\text{H}_2$: it has only 2 hydrogen atoms on the left but 4 on the right, so it is not balanced.
• Option D doubles the hydrogen without correctly adjusting the water: it has 8 hydrogen atoms and 2 oxygen atoms on the left, but 8 hydrogen atoms and only 4 oxygen atoms on the right, so it is not balanced.

Final answers

• $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$, $\boxed{\text{option A}}$.