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CAIE A-Level Chemistry AS 5.2 Hesss Law Questions

Practise constructing energy cycles and combining formation, combustion or bond-energy data to obtain indirect ΔH values.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • state that enthalpy change is independent of pathway because enthalpy is a state function
  • align equations or cycle arrows with the target reaction and reverse signs when reversing a route
  • multiply enthalpy values by stoichiometric coefficients before summing the complete path

Covered syllabus points

5.2.1—Hess’s law to construct simple energy cycles5.2.2—Calculations: cycles and relevant energy terms

Question 1

[Maximum number: 1]

An energy cycle is shown.

Figure for Question 1 — 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 2

[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)

Question 3

[Maximum number: 1]

ΔH1⊖\Delta H_{1}^{\ominus} is the standard enthalpy of formation of methane.
ΔH2⊖\Delta H_{2}^{\ominus} is the standard enthalpy of combustion of carbon.
ΔH3⊖\Delta H_{3}^{\ominus} is the standard enthalpy of combustion of hydrogen.

Which expression is equivalent to ΔHc⊖\Delta H_{\mathrm{c}}^{\ominus} ?

A

ΔH1⊖−ΔH2⊖+ΔH3⊖\Delta H_{1}^{\ominus}-\Delta H_{2}^{\ominus}+\Delta H_{3}^{\ominus}

B

ΔH1⊖−2ΔH3⊖−ΔH2⊖\Delta H_{1}^{\ominus}-2 \Delta H_{3}^{\ominus}-\Delta H_{2}^{\ominus}

C

ΔH2⊖−ΔH3⊖+ΔH1⊖\Delta H_{2}^{\ominus}-\Delta H_{3}^{\ominus}+\Delta H_{1}^{\ominus}

D

ΔH2⊖+2ΔH3⊖−ΔH1⊖\Delta H_{2}^{\ominus}+2 \Delta H_{3}^{\ominus}-\Delta H_{1}^{\ominus}

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