Home / IB DP / IB DP Chemistry 2026, 2027 & 2028 / IBDP Chemistry HL 2026, 2027 & 2028

IB DP Chemistry -Reactivity 1.4 - Entropy and spontaneity - IB Style Questions For HL Paper 1A -FA 2025

IBDP Chemistry HL Paper 1A & 1B - All Chapters

Question

Which of the following reactions will show the greatest positive change in entropy?
(A) CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
(B) CaO(s) + CO2(g) → CaCO3(s)
(C) I2(g) + H2(g) → 2HI(g)
(D) 2H2(g) + O2(g) → 2H2O(g)
▶️ Answer/Explanation
Detailed solution

Entropy increases with:
• More gas moles produced
• Solid → liquid or gas
(A) 0 gas moles in reactants (solid + aq), 1 gas mole in products → large ΔS > 0
(B) Gas consumed → ΔS < 0
(C) 2 gas moles → 2 gas moles → ΔS ≈ 0
(D) 3 gas moles → 2 gas moles → ΔS < 0

✅ Answer: (A)

Question

Which process leads to a reduction in entropy?
(A) \( \text{N}_2\text{H}_4(\text{I}) \rightarrow \text{N}_2(\text{g}) + 2\text{H}_2(\text{g}) \)
(B) \( \text{K}_2\text{Cr}_2\text{O}_7(\text{s}) \rightarrow \text{K}_2\text{Cr}_2\text{O}_7(\text{aq}) \)
(C) \( \text{MgCO}_3(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{CO}_2(\text{g}) \)
(D) \( 2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{g}) \)
▶️ Answer/Explanation
Detailed solution

Entropy decreases when the system becomes more ordered.
(D) \( 2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{g}) \): 3 moles of gas → 2 moles of gas, fewer gas particles means lower entropy.
✅ Answer: (D)

More DP Chemistry HL Paper 1 Questions..
Scroll to Top