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JEE Main 2025 Jan 24 Shift 2, Chemistry Q6: Calorimetry

JEE Main2025Jan 24, Shift 2Chemistry
Q.

Which of the following mixing of 1M base and 1M acid leads to the largest increase in temperature?

  1. A

    30 mL HCl and 30 mL NaOH

  2. B

    30 mL and 30 mL NaOH

  3. C

    50 mL HCl and 20 mL NaOH

  4. D

    45 mL and 25 mL NaOH

Solution

The temperature rise depends on (i) the millimoles of acid-base actually neutralised (limited by the smaller of the two) and (ii) the heat released per mole of neutralisation, which is highest for a strong-acid + strong-base pair (about ) and lower for a weak acid since some energy is consumed in ionising it.

Limiting millimoles in each option (volume 1 M):

OptionPairLimiting mmol
(1)HCl + NaOH (SA + SB)30
(2) + NaOH (WA + SB)30
(3)HCl + NaOH (SA + SB)20
(4) + NaOH (WA + SB)25

Options (1) and (2) both neutralise 30 mmol, but (2) releases less heat per mole because acetic acid must first ionise. Options (3) and (4) involve smaller amounts of reaction.

Hence (1) produces the largest temperature rise.

Correct option: (1)

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