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IB Chemistry HL 1.4.3 Hl Spontaneity Questions

Use real HL exam questions to calculate spontaneity thresholds, reason from ΔH and ΔS signs and interpret ΔG under stated conditions.

Syllabus
First assessment 2025
Course
Chemistry HL
Level
HL

Exam points

  • Use ΔG° = ΔH° − TΔS° and consistent units to calculate the temperature at which a reaction changes from spontaneous to non-spontaneous or vice versa.
  • Determine the signs of ΔH°, ΔS° and ΔG° and explain how increasing or decreasing temperature changes spontaneity, including reactions spontaneous only above or below a threshold.
  • Interpret a negative, zero or positive ΔG° as spontaneous, equilibrium-boundary or non-spontaneous behaviour under the stated conditions.

Learn this topic

Study 1.4.3 (HL)—Spontaneity conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

IB Chemistry HL 1.4.3 Hl Spontaneity Questions question 1

[Maximum number: 2]

Nitrous acid, HNO2\mathrm{HNO}_{2}, is a weak acid which can be used to make acidic buffers.

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2C2H6( g)+Cl2( g)+32O2( g)→C4H8O2( g)+2HCl( g)+H2O( g)2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

Explain the effect, if any, on the spontaneity of this reaction if the temperature was increased. Use section 1 of the data booklet.

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