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CAIE A-Level Chemistry AS 5 Chemical Energetics Questions

Practise using ΔH signs, equations, diagrams, calorimetry, bond energies and Hess cycles to interpret and calculate energy changes.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Covered syllabus points

5.1 Enthalpy change, ΔH5.2 Hess’s law

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]

An energy cycle is shown.

Figure for Question 2 — CAIE A-Level Chemistry AS

The energy changes involved are X, Y and Z .
The numerical value of energy change Y is either -890 or +890 .
The numerical value of energy change Z is either -964 or +964 .
Which of the three values are negative?

A

X and Z

B

X only

C

Y and Z

D

Y only

Question 3

[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 4

[Maximum number: 1]

The enthalpy change for a reaction can be calculated from values of:
- enthalpies of formation, ΔHf⊖\Delta H_{\mathrm{f}}^{\ominus}
- enthalpies of combustion, ΔHc⊖\Delta H_{\mathrm{c}}^{\ominus}
- bond energies, E.

The enthalpy change of the reaction given =ΔHr⊖=\Delta H_{\mathrm{r}}^{\ominus}.

2C2H6( g)+3O2( g)→2CH4( g)+2CO2( g)+2H2O(l)2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Which expression could be used to calculate ΔHr⊖\Delta H_{\mathrm{r}}^{\ominus} ?

A

ΔHce(C2H6( g))\Delta H_{\mathrm{c}}^{e}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)

B

2ΔHc⊖(C2H6( g))−2ΔHc⊖(CH4( g))2 \Delta H_{\mathrm{c}}^{\ominus}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)-2 \Delta H_{\mathrm{c}}^{\ominus}\left(\mathrm{CH}_{4}(\mathrm{~g})\right)

C

E(C−C)+2E(C−H)−4E(C=O)−4E(H−O)E(\mathrm{C}-\mathrm{C})+2 E(\mathrm{C}-\mathrm{H})-4 E(\mathrm{C}=\mathrm{O})-4 E(\mathrm{H}-\mathrm{O})

D

ΔHf⊖(CH4( g))+ΔHf⊖(CO2( g))+ΔHf⊖(H2O(l))−ΔHf⊖(C2H6( g))\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{CH}_{4}(\mathrm{~g})\right)+\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{CO}_{2}(\mathrm{~g})\right)+\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right)-\Delta H_{\mathrm{f}}^{\ominus}\left(\mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})\right)

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