IB CHEMISTRY

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HL Assessment Bank

Reactivity 1.4 Exam Practice

Section A: Multiple Choice

1. Which reaction has a positive entropy change (ΔS>0\Delta S > 0)?

A. N2(g)+3H2(g)→2NH3(g)N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)
B. CaCO3(s)→CaO(s)+CO2(g)CaCO_3(s) \rightarrow CaO(s) + CO_2(g) (Correct)
C. H2O(l)→H2O(s)H_2O(l) \rightarrow H_2O(s)
D. 2SO2(g)+O2(g)→2SO3(g)2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)

Section B: Structured Questions

Q1: Calculating Spontaneity [4 marks]

For a reaction:

  • ΔHθ=+100kJmol−1\Delta H^\theta = +100 kJ mol^{-1}
  • ΔSθ=+200JK−1mol−1\Delta S^\theta = +200 J K^{-1} mol^{-1}

Calculate the temperature above which this reaction becomes spontaneous.

Examiner's Markscheme

1. Condition: ΔG<0\Delta G < 0, so find ΔG=0\Delta G = 0.

2. T=ΔH/ΔST = \Delta H / \Delta S

3. Units! ΔS=0.200kJK−1mol−1\Delta S = 0.200 kJ K^{-1} mol^{-1}.

4. T=100/0.200=500KT = 100 / 0.200 = \mathbf{500 K}.