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CAIE A-Level Chemistry AS 5.1 Enthalpy Change H Questions

Practise interpreting ΔH signs, standard enthalpy definitions, pathway diagrams and energy calculations.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • distinguish exothermic negative ΔH from endothermic positive ΔH using system energy change
  • write standard enthalpy equations with correct stoichiometry, states and one mole of the defined product or process
  • read reaction profiles and calculate energy changes from bond energies or enthalpy data

Covered syllabus points

5.1.1—Chemical reactions are accompanied by enthalpy5.1.2—Construct/interpret a reaction pathway diagram5.1.3—Terms5.1.4—Energy transfers occur during chemical5.1.5—Bond energies and ΔHr5.1.6—Some bond energies are exact and some bond5.1.7—Enthalpy changes from appropriate experimental

Question 1

[Maximum number: 1]

Which statement about enthalpy changes is correct?

A

Enthalpy changes of reaction are always negative.

B

Enthalpy changes of combustion are always positive.

C

Enthalpy changes of formation are always positive.

D

Enthalpy changes of neutralisation are always negative.

Question 2

[Maximum number: 1]

For a particular reversible reaction the backward reaction is endothermic.
The activation energy of the backward reaction is 160 kJ mol−1160 \mathrm{~kJ} \mathrm{~mol}^{-1}.
It can be assumed that the backward reaction proceeds by a mechanism that is the exact reverse of the mechanism for the forward reaction.

Which statement about the activation energy of the forward reaction is correct?

A

The activation energy of the forward reaction is equal to −160 kJ mol−1-160 \mathrm{~kJ} \mathrm{~mol}^{-1}.

B

The activation energy of the forward reaction is 0 kJ mol−10 \mathrm{~kJ} \mathrm{~mol}^{-1} but less than +160 kJ mol−1+160 \mathrm{~kJ} \mathrm{~mol}^{-1}.

C

The activation energy of the forward reaction is equal to +160 kJ mol−1+160 \mathrm{~kJ} \mathrm{~mol}^{-1}.

D

The activation energy of the forward reaction is greater than +160 kJ mol−1+160 \mathrm{~kJ} \mathrm{~mol}^{-1}.

Question 3

[Maximum number: 1]

Which equation represents the reaction whose standard enthalpy change is the standard enthalpy change of formation of water?

A

2H2( g)+O2( g)→2H2O(l)2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

B

H2( g)+12O2( g)→H2O(l)\mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

C

H+(aq)+OH−(aq)→H2O(l)\mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

D

2H(g)+O(g)→H2O(l)2 \mathrm{H}(\mathrm{g})+\mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

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