Which of the following mixing of 1 M base and 1 M acid leads to the largest increase in temperature?
- (A)30 mL HCl and 30 mL NaOH✓ Correct
- (B)30 mL CH 3 COOH and 30 mL NaOH
- (C)50 mL HCl and 20 mL NaOH
- (D)45 mL CH 3 COOH and 25 mL NaOH
Answer: (A)
Solution
The temperature rise in a neutralization reaction depends on the total heat released, , where is the number of millimoles of the limiting reagent that actually reacts. For a strong acid–strong base pair the enthalpy of neutralization is the full value (about , from complete ionization), while a weak acid such as loses part of this heat in ionizing itself, giving a smaller net .
Comparing the millimoles reacting: 30 mL HCl with 30 mL NaOH reacts completely as 30 mmol of strong acid with strong base; 30 mL with 30 mL NaOH also reacts fully to 30 mmol but releases less heat per mole; 50 mL HCl with 20 mL NaOH is capped by the limiting NaOH at 20 mmol; 45 mL with 25 mL NaOH is capped at 25 mmol of a weak acid.
The mixture with both the largest quantity reacting and the highest heat released per mole is 30 mL HCl with 30 mL NaOH, giving the largest temperature rise.
Comparing the millimoles reacting: 30 mL HCl with 30 mL NaOH reacts completely as 30 mmol of strong acid with strong base; 30 mL with 30 mL NaOH also reacts fully to 30 mmol but releases less heat per mole; 50 mL HCl with 20 mL NaOH is capped by the limiting NaOH at 20 mmol; 45 mL with 25 mL NaOH is capped at 25 mmol of a weak acid.
The mixture with both the largest quantity reacting and the highest heat released per mole is 30 mL HCl with 30 mL NaOH, giving the largest temperature rise.
JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Single correct option