A LevelChemistryCambridge (CIE)Exam Questions5. Chemical EnergeticsHess’s Law

Hess’s Law (Cambridge (CIE) A Level Chemistry): Exam Questions

Exam code: 9701

58 mins17 questions

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Hess’s Law

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1
1 mark

Which enthalpy change is indicated by X in the enthalpy cycle shown?

Hess's law enthalpy cycle for the combustion of ethane (C~2~H~6~), with X indicating the enthalpy change for the formation of 3 mol of H~2~O (l) from its elements in their standard states

  • +ΔHfθ(H2O)

  • −ΔHfθ(H2O)

  • +3ΔHfθ(H2O)

  • −3ΔHfθ(H2O)

Choose your answer

2
1 mark

Which is the correct equation to calculate the enthalpy of reaction using enthalpy of formation data? 

  • ΔHrθ = ΣΔHfθ(products) + ΣΔHfθ(reactants)

  • ΔHrθ = ΣΔHfθ(products) − ΣΔHfθ(reactants)

  • ΔHrθ = ΣΔHfθ(reactants) − ΣΔHfθ(products)

  • ΔHrθ = −ΣΔHfθ(products) − ΣΔHfθ(reactants)

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3
1 mark

The diagram shows a Hess's law cycle for the conversion of graphite to diamond.

Hess's law enthalpy cycle for the conversion of graphite to diamond, showing ΔH~1~ for combustion of graphite, ΔH~2~ for combustion of diamond, and ΔH~r~ for the graphite to diamond conversion

Which equation correctly represents ΔHr?

  • ΔHr = ΔH1 + ΔH2

  • ΔHr = ΔH1 − ΔH2

  • ΔHr = ΔH2 − ΔH1

  • ΔHr = −(ΔH1 + ΔH2)

Choose your answer

1
1 mark

What is the standard enthalpy change of formation, ΔHfθ, of propan-1-ol, C3H8O?

Substance

ΔHcθ / kJ mol-1

C (s)

−394

H2 (g)

−286

C3H8O (l)

−1816

  • −1136 kJ mol−1

  • −510 kJ mol−1

  • +510 kJ mol−1

  • +1136 kJ mol−1

Choose your answer

2
1 mark

The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation.

4NH3 (g) + 5O2 (g) ⇌ 4NO (g) + 6H2O (g)

Using the following standard enthalpy change of formation data, what is the standard enthalpy change of reaction, ΔHrθ, for this reaction?

Compound

∆Hfθ /kJ mol–1

NH3 (g)

−51.3

NO (g)

+92.2

H2O (g)

−239.6

  • + 863.6 kJ mol–1

  • - 1601.2 kJ mol–1

  • - 863.6 kJ mol–1

  • - 1274.0 kJ mol–1

Choose your answer

3
1 mark

Titanium occurs naturally as the mineral rutile, TiO2. One possible method of extraction of titanium is to reduce it by heating with carbon.

TiO2 (s) + 2C (s) → Ti (s) + 2CO (g)

The standard enthalpy changes of formation of TiO2 (s) and CO (g) are –890 kJ mol−1 and –105 kJ mol−1 respectively.

What is the standard enthalpy change of reaction, ΔHrθ, for this reaction?

  • −785 kJ mol−1

  • −680 kJ mol−1

  • +680 kJ mol−1

  • +785 kJ mol−1

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4
1 mark

The combustion of ethanol is increasingly being used to fuel cars.

Substance

ΔHfθ / kJ mol-1

CO2 (g)

−382

H2O (g)

−275

C2H5OH (l)

−266

What is the standard enthalpy change of combustion, ΔHcθ, of ethanol?

  • −1323 kJ mol−1

  • −391 kJ mol−1

  • +391 kJ mol−1

  • +1323 kJ mol−1

Choose your answer

1
1 mark

The equations for the formation of two sulfur oxides are shown.

S (s) + O2 (g) → SO2 (g)     ∆Hfθ = –297 kJ mol–1

S (s) + 1.5O2 (g) → SO3 (g)    ∆Hfθ = –395 kJ mol–1

What is the standard enthalpy change of reaction, ΔHrθ, for 2SO2 (g) + O2 (g) → 2SO3 (g)?

  • −196 kJ mol–1

  • −98 kJ mol–1

  • +98 kJ mol–1

  • +196 kJ mol–1

Choose your answer

2
1 mark

A student calculated the standard enthalpy change of formation of propane, C3H8, using the standard enthalpy changes of combustion shown.

Substance

ΔHcθ / kJ mol-1

C3H8 (g)

−2220

H2 (g)

−286

C (s)

Y

The student used an incorrect value, Y, for the standard enthalpy change of combustion of carbon but performed the calculation correctly. The final answer obtained was −158 kJ mol−1.

What value did the student use for the standard enthalpy change of combustion of carbon?

  • −1234 kJ mol−1

  • −697 kJ mol−1

  • −411 kJ mol−1

  • −306 kJ mol−1

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3
1 mark

Given the following standard enthalpy changes,

I2 (s) → I2 (g) ∆HƟ = +38 kJ mol–1

I2 (g) + 3Cl2 (g) → 2ICl3 (s) ∆HƟ = –214 kJ mol–1

What is the standard enthalpy change of formation, ΔHfθ, of ICl3 (s)?

  • –214 kJ mol– 1

  • –176 kJ mol– 1

  • –107 kJ mol– 1

  • −88 kJ mol– 1

Choose your answer

4
1 mark

The following standard enthalpy changes are given.

CO (g) + 1 half O2 (g)   → CO2 (g) ∆Hθ  = –283 kJ mol–1

H2 (g) + 1 half O2 (g)   → H2O (I) ∆Hθ  = –286 kJ mol–1

H2O (g)   → H2O (I) ∆Hθ = –44 kJ mol–1

What is the standard enthalpy change, ΔHθ, for the following reaction?

CO2 (g) + H2 (g) → CO (g) + H2O (g)

  • −41 kJ mol– 1

  • −3 kJ mol– 1

  • +3 kJ mol– 1

  • +41 kJ mol– 1

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5
1 mark

Iodine trichloride, ICl3, is made by reacting iodine with chlorine.

I2 (s) + Cl2 (g) → 2ICl (s) ∆Ho = +14 kJ mol–1

ICl (s) + Cl2 (g) → ICl3 (s) ∆Ho = –88 kJ mol–1

What is the standard enthalpy change of formation, ΔHfθ, of solid iodine trichloride?

  • –162 kJ mol–1

  • –81 kJ mol–1

  • –74 kJ mol–1

  • –60 kJ mol–1

Choose your answer