The puzzle
In a typical chemistry textbook, you learn two facts:
- Zinc reacts with dilute sulfuric acid and releases hydrogen gas.
- Copper does not ordinarily react with dilute sulfuric acid—but does react with hot concentrated sulfuric acid.
The usual explanation invokes acidity for the first reaction and oxidizing power for the second. Those words describe different properties. Let's make the difference precise.
An acid donates protons (H⁺). An oxidizing agent accepts electrons. A substance can do both, but the two roles are not identical.
1. What does “strong acid” actually mean?
In water, a strong acid ionizes to a large extent. Sulfuric acid's first proton is released essentially completely in ordinary aqueous solutions; the second dissociation is not complete under all conditions.
Being a strong acid does not mean being a concentrated solution. “Strong” describes how extensively the acid ionizes; “concentrated” describes how much acid is present relative to the solution. Dilute sulfuric acid is still a strong acid in the usual high-school classification.
Crucially, a high concentration of H⁺ does not guarantee that those ions can oxidize every metal.
2. Zinc reacts because hydrogen ions accept electrons
Remove the spectator ions, and the electron transfer becomes clearer:
Zn → Zn²⁺ + 2e⁻Zinc loses two electrons.2H⁺ + 2e⁻ → H₂Hydrogen ions gain them.So zinc plus dilute sulfuric acid is itself a redox reaction. Calling it a reaction due to the acid's “acidic properties” is high-school shorthand: here the important oxidizing species is H⁺, rather than sulfur(VI) in the sulfate-containing acid.
3. Why copper doesn't do the same thing
Recall the simplified reactivity order taught in high school:
Zinc can reduce H⁺ to H₂, but under ordinary conditions, copper cannot do so spontaneously to a useful extent. That is why dropping copper into ordinary dilute sulfuric acid does not produce the familiar stream of hydrogen bubbles.
This does not mean copper can never lose electrons. It means H⁺ under these conditions is not an effective enough oxidizing partner for copper.
4. Hot concentrated sulfuric acid changes the oxidizing partner
Here, copper goes from oxidation state 0 to +2. Sulfur in sulfuric acid goes from +6 to +4 in sulfur dioxide (SO₂). The sulfur-containing species accepts electrons; it is reduced as copper is oxidized.
Temperature and concentration matter. The hot, concentrated conditions make this reaction possible; the equation does not imply that cold, dilute sulfuric acid reacts with copper in the same way. The gas is SO₂, not H₂.
5. Compare the two reactions
| Question | Zn + dilute H₂SO₄ | Cu + hot conc. H₂SO₄ |
|---|---|---|
| What is oxidized? | Zn → Zn²⁺ | Cu → Cu²⁺ |
| What accepts electrons? | H⁺ | S(VI) in sulfuric acid |
| Gas produced? | H₂ | SO₂ |
| What matters most? | Acidic H⁺ | Oxidizing ability under hot, concentrated conditions |
6. A useful check: what about nitric acid?
Copper also reacts with dilute nitric acid, because nitrate in acidic solution can act as an oxidizing agent. One typical equation is:
The nitrogen is reduced from +5 to +2 in NO. The lesson: “dilute acid” does not automatically mean “no oxidizing power.”
Quick check
1. In Zn + dilute H₂SO₄, which species is reduced?
H⁺ is reduced to H₂. Zinc is oxidized to Zn²⁺.
2. In the reaction with hot concentrated H₂SO₄, how does sulfur's oxidation state change?
From +6 in sulfuric acid to +4 in SO₂.
3. Does “copper is below hydrogen in the reactivity series” mean copper reacts with no acids?
No. It explains why copper does not liberate H₂ from ordinary non-oxidizing dilute acids. Oxidizing acids can still dissolve copper.
When a metal meets an acid, don't just ask “Is the acid strong?” Ask: Which species can accept the electrons?
Safety note: Concentrated sulfuric and nitric acids are hazardous. SO₂ and nitrogen oxides are toxic gases. These are textbook reactions, not at-home experiments.
Further reading: OpenStax Chemistry 2e (acid-base and oxidation–reduction fundamentals).