What is the gas evolved when zinc reacts with sulphuric acid?
- (a)Carbon dioxide
- (b)Oxygen
- (c)Hydrogen sulphide
- (d)Hydrogen
Answer
Why
Correct — D. Zinc stands above hydrogen in the reactivity series, so it displaces hydrogen from dilute sulphuric acid.
Zn + H₂SO₄ → ZnSO₄ + H₂↑
The gas evolved is hydrogen, identified by the pop sound a burning splint makes at the mouth of the tube. The salt left in solution is zinc sulphate.
Why the others are wrong
- (a)Carbon dioxide — Carbon dioxide comes from an acid meeting a carbonate or bicarbonate, as sodium carbonate does with dilute acid. There is no carbon anywhere in zinc plus sulphuric acid.
- (b)Oxygen — Oxygen is not a product of a metal-acid reaction. It comes from decomposing an oxygen-rich compound, such as heating potassium chlorate, or from water at the anode during electrolysis.
- (c)Hydrogen sulphide — Hydrogen sulphide needs a sulphide to begin with — iron sulphide with dilute sulphuric acid gives H₂S. A sulphate ion does not hand its sulphur over to hydrogen.
Concept
A metal placed above hydrogen in the reactivity series displaces hydrogen from a dilute acid, giving a salt and hydrogen gas.
metal + dilute acid → salt + hydrogen
Zinc, magnesium, aluminium and iron all do this. Copper, silver and gold sit below hydrogen and do not, which is why copper is unaffected by dilute sulphuric acid.
The salt takes its name from the acid's anion: sulphuric acid gives a sulphate, hydrochloric acid a chloride.
The rule holds for dilute acid. Hot concentrated sulphuric acid acts as an oxidising agent and gives sulphur dioxide with zinc instead of hydrogen, so a stem that specifies concentration is asking different chemistry.
This stem leaves it unqualified and the key takes the textbook dilute case.
Key facts
- Zinc with dilute sulphuric acid gives zinc sulphate and hydrogen gas.
- Hydrogen is confirmed when a burning splint produces a pop sound.
- Metals below hydrogen in the reactivity series, such as copper, do not displace hydrogen from dilute acids.
- Hot concentrated sulphuric acid oxidises zinc and releases sulphur dioxide instead.
Study next
Common traps
- Reading the sulphur in sulphuric acid as a route to hydrogen sulphide
- Forgetting that the dilute-acid rule fails for hot concentrated sulphuric acid
The same reaction is asked for its salt rather than its gas at 12 Sep 2024, 12:30, General Awareness Q.19, and the same hydrogen answer comes from iron reacting with water at 24 Sep 2024, 12:30, General Awareness Q.10.
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