A LevelChemistryCIEExam Questions5. Chemical Energetics5.2 Hess’s Law

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Hess’s Law (CIE A Level Chemistry)

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5.2 Hess’s Law

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5.2 Hess’s Law

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11 mark

Enthalpy changes that are difficult to measure directly can often be determined using Hess’ Law to construct an enthalpy cycle.

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

1-5-e-2

  • +1 x Enthalpy of formation of water

  • -1 x Enthalpy of formation of water

  • +3 x Enthalpy of formation of water

  • -3 x Enthalpy of formation of water

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21 mark

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

  • ΔHr = ΣΔHf products + ΣΔHf reactants

  • ΔHr = ΣΔHf products - ΣΔHf reactants

  • ΔHr = ΣΔHf reactants - ΣΔHf products 

  • ΔHr = ΣΔHf reactants + ΣΔHf products 

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11 mark

Propanol has the molecular formula C3H8O. Use the following information to calculate the enthalpy change for the formation of propanol.

The enthalpy change of combustion of hydrogen is –286 kJ mol-1

The enthalpy change of combustion of carbon is –394 kJ mol-1

The enthalpy change of combustion of propanol is – 1816 kJ mol-1

  • -1816 kJ mol-1

  • -510 kJ mol-1

  • +510 kJ mol-1

  • +1136 kJ mol-1

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21 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 value of the standard enthalpy change ∆Hfθ 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

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31 mark

Titanium occurs naturally as the mineral rutile, TiO2. One possible method of extraction of titanium is to reduce the rutile 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 the extraction of titanium?

  • + 450 kJ mol–1

  • + 680 kJ mol–1

  • - 450 kJ mol–1

  • - 660 kJ mol–1

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41 mark

The combustion of ethanol (C2H5OH) is increasingly being used to fuel cars. 

The standard enthalpy change of formation of carbon dioxide is –382 kJ mol–1. 

The standard enthalpy change of formation of water is –275 kJ mol–1.

The standard enthalpy change of formation of ethanol is −266kJ mol–1. 

What is the standard enthalpy change of combustion of ethanol?

  • - 1367 kJ mol–1

  • - 1323 kJ mol–1

  • - 391 kJ mol–1

  • - 948 kJ mol–1

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11 mark

The equations below show the formation of sulfur oxides from sulfur and oxygen.

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

S (s) + 1½ O2 (g) → SO3 (g)    ∆Hfθ = –395 kJ mol–1

What is the enthalpy change of reaction, ∆Hrθ, of 2SO2 (g) + O2 (g) → 2SO3 (g)?

  • + 98 kJ mol–1

  • - 98 kJ mol–1

  • + 196 kJ mol–1

  • - 196 kJ mol–1

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21 mark

A student calculated the standard enthalpy change of formation of propane, C3H8, using a method based on standard enthalpy changes of combustion.

He used correct values for the standard enthalpy change of combustion of propane (–2220 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol–1.

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

  • -1234 kJ mol-1 

  • -411.3 kJ mol-1 

  • -2084 kJ mol-1 

  • -694.7 kJ mol-1 

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31 mark

Given the following 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 correct value for ∆Hfθ of iodine trichloride, ICl3 (s)?

  • –214 kJ mol–1

  • –176 kJ mol–1

  • –88 kJ mol–1

  • +176 kJ mol–1

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41 mark

Using the following information:

                       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 enthalpy change, ∆Hθ, for the following reaction? 

CO2 (g) + H2 (g) → CO (g) + H2O (g)

  • -41 kJ mol-1

  • +41  kJ mol-1

  • -525  kJ mol-1

  • +525  kJ mol-1

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51 mark

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

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

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

By using the data above, what is the enthalpy change of the formation for solid iodine trichloride?

  • –162 kJ mol–1

  • –81 kJ mol–1

  • –74 kJ mol–1

  • –60 kJ mol–1

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