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CAIE A-Level Chemistry 23.4.4 Temperature Effects on Feasibility

Practise predicting how temperature changes ΔG from the signs of ΔH and ΔS and locating crossover temperatures.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • use the sign of −TΔS to decide whether raising temperature makes ΔG more or less negative
  • for ΔH and ΔS with the same sign, solve ΔG=0 to find the feasibility threshold temperature
  • explain a solubility or reaction trend by the change in ΔG, not by temperature alone

23.4.4—Temperature effect on feasibility question 1

[Maximum number: 1]

Some electrode potentials are shown in Table 3.1.

Table 3.1

Table 3.1

Iron(II) chloride, FeCl2\mathrm{FeCl}_{2}, is oxidised by chlorine to form iron(III) chloride, FeCl3\mathrm{FeCl}_{3}, under standard conditions.

2FeCl2( s)+Cl2( g)→2FeCl3( s)2 \mathrm{FeCl}_{2}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{FeCl}_{3}(\mathrm{~s})
ΔH⊖=−128 kJ mol−1\Delta H^{\ominus}=-128 \mathrm{~kJ} \mathrm{~mol}^{-1}
Table 3.2

Table 3.2

Predict whether this reaction becomes more or less feasible at a higher temperature.

Explain your answer.
The reaction becomes feasible.
explanation

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