JEE Main · 2020 · Shift-IImediumIEQ-024

For the following Assertion and Reason, the correct option is: Assertion: The pH of water increases with increase in…

Ionic Equilibrium · Class 11 · JEE Main Previous Year Question

Question

For the following Assertion and Reason, the correct option is:

Assertion: The pH of water increases with increase in temperature.

Reason: The dissociation of water into H+\mathrm{H^+} and OH−\mathrm{OH^-} is an exothermic reaction.

Options
  1. a

    Both assertion and reason are true, and the reason is the correct explanation for the assertion

  2. b

    Both assertion and reason are false

    ✓
  3. c

    Both assertion and reason are true, but the reason is not the correct explanation for the assertion

  4. d

    Assertion is not true, but reason is true

Correct Answerb

Both assertion and reason are false

Detailed Solution

Step 1 — Evaluate Assertion: At higher temperature, KwK_w increases (more dissociation of water). At 25 °C, pH of pure water = 7. At higher T, [H+][\mathrm{H^+}] increases →\rightarrow pH decreases (e.g., at 60 °C, pH ≈ 6.5).

However, the water is still neutral (pH = pOH) — just the neutral point shifts.

Assertion: pH of water DECREASES with temperature →\rightarrow Assertion is FALSE ✗

Step 2 — Evaluate Reason: The dissociation of water: H2O⇌H++OH−\mathrm{H_2O \rightleftharpoons H^+ + OH^-} is endothermic (requires energy to break O–H bonds), NOT exothermic. Reason is FALSE ✗

Answer: Option (2) — Both assertion and reason are false

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For the following Assertion and Reason, the correct option is: Assertion: The pH of water increases… (JEE Main 2020) | Canvas Classes