Q.Assertion: $Cu$ is less reactive than hydrogen.
Reason: $E^\circ_{Cu^{2+}/Cu}$ is negative.

The key idea is Galvanic Corrosion (or the electrochemical series). The standard reduction potential $E^\circ$ determines reactivity — a more negative $E^\circ$ means a metal is more reactive (easily oxidised).

Reasoning:

1. The assertion: $Cu$ is less reactive than hydrogen. In the reactivity series, metals with $E^\circ < 0$ (like Zn, Fe) displace $H^+$; those with $E^\circ > 0$ (like Cu, Ag) do not. Cu does not react with dilute acids, so the assertion is true. …

This is a classic assertion-reason question from electrochemistry. Let’s unpack it properly.

The core idea: Reactivity and electrode potentials

The reactivity of a metal with hydrogen (or acids) is directly linked to its standard reduction potential ($E^\circ$). A more negative $E^\circ$ means the metal is more easily oxidised — i.e., it is more reactive. Hydrogen has an $E^\circ$ of $0.00\ \text{V}$ by definition (for the reaction $2H^+ + 2e^- \rightarrow H_2$).

If a metal has $E^\circ < 0$, it can displace hydrogen from acids (it is more reactive than hydrogen). If $E^\circ > 0$, it cannot — it is less reactive.

This is a standard value you must memorise for exams.

Step-by-step reasoning

1. Check the assertion: Is Cu less reactive than hydrogen?

Yes. Copper does not react with dilute acids like HCl or H₂SO₄ to produce H₂ gas. This is because its reduction potential is positive (+0.34 V), meaning the reverse reaction (oxidation of Cu to Cu²⁺) is less favourable than the oxidation of H₂ to H⁺. So the assertion is true.

2. Check the reason: Is $E^\circ_{Cu^{2+}/Cu}$ negative?

No. The standard reduction potential for copper is +0.34 V, not negative. The reason given is factually incorrect. So the reason is false.

3. Relate assertion and reason …

Method: Standard Electrode Potential Comparison

Concept: The reactivity of metals with hydrogen is determined by their standard reduction potentials ($E^\circ$). A more negative $E^\circ$ means the metal is more reactive (easily oxidized), while a more positive $E^\circ$ means it is less reactive.

Steps:

1. Recall the standard reduction potential for hydrogen The reference half-reaction is:

2. Recall the standard reduction potential for copper

3. Compare the values
   • $E^\circ_{Cu^{2+}/Cu} = +0.34 \, \text{V}$ is positive, not negative. …

Here’s a breakdown of the common mistakes students make on this assertion-reason question and how to avoid each.

Mistake 1: Thinking $E^\circ_{Cu^{2+}/Cu}$ is negative

Why students do this:

They confuse the sign convention for standard reduction potentials. Many remember that zinc has a negative $E^\circ$ and is reactive, so they assume copper, being less reactive, must also have a negative value.

Correct fact:

$E^\circ_{Cu^{2+}/Cu} = +0.34\ \text{V}$ (positive).

A positive reduction potential means $Cu^{2+}$ is easily reduced — copper is a less reactive metal (it does not readily lose electrons).

How to avoid:

• Memorise the standard reduction potential series for common metals:
  • $Zn^{2+}/Zn$: $-0.76\ \text{V}$ (reactive)
  • $Fe^{2+}/Fe$: $-0.44\ \text{V}$
  • $H^+/H_2$: $0.00\ \text{V}$ (reference)
  • $Cu^{2+}/Cu$: $+0.34\ \text{V}$ (noble)
• Remember: more negative $E^\circ$ = more reactive metal (easier to oxidise).
• For copper, the positive value tells you it is below hydrogen in the reactivity series.

Mistake 2: Assuming the reason correctly explains the assertion

Why students do this:

They see “Assertion true” and “Reason true” and tick option (i) without checking if the reason actually causes the assertion.

Correct logic:

• Assertion: “Cu is less reactive than hydrogen” → True (copper does not displace $H^+$ from acids).
• Reason: “$E^\circ_{Cu^{2+}/Cu}$ is negative” → False (it is positive).
• So the correct answer is (iii): Assertion true, reason false.

How to avoid:

• Always check the truth value of each statement separately first.
• Then, only if both are true, ask: “Does the reason directly cause the assertion?”
• In this case, the reason is factually wrong, so no further analysis is needed.

Mistake 3: Misinterpreting “less reactive” as “more easily reduced”

Why students do this:

“Less reactive” means the metal does not lose electrons easily. But students sometimes think “less reactive” means it gains electrons easily — which is actually the same as “more noble.” They get the direction wrong.

Correct understanding:

• Reactivity refers to the tendency to lose electrons (be oxidised).
• Copper has a high reduction potential (+0.34 V), meaning $Cu^{2+}$ is easily reduced — so copper metal is hard to oxidise, i.e., less reactive.
• Hydrogen has $E^\circ = 0.00\ \text{V}$, so it is more easily oxidised than copper.

How to avoid:

• Draw a simple reactivity series: K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > H > Cu > Ag > Au
• The more negative the $E^\circ$, the higher the metal is in this series (more reactive).
• Copper is below hydrogen → less reactive.

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