MC test: Hess's Law

Multiple choice test on 5.2 Hess's Law

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which statements about Hess's Law are correct?

I. It is an expression of the Law of Conservation of Energy

II. It can be used to indirectly determine the enthalpy of formation of a substance.

III. The overall enthalpy change for a reaction is independent of the reaction pathway.

I and II only

I, II and III

I and III only

II and III only

Provided the initial and final conditions are the same, Hess's Law states that the overall enthalpy change of a chemical reaction is independent of the reaction pathway. This is a statement both of the Law of Conservation of Energy and the First Law of Thermodynamics.

The energy cycle shows how Hess's Law can be used to find the standard enthalpy of formation of methane if the enthalpies of combustion of carbon, hydrogen and methane are known.

Which expression will give the correct value for the enthalpy of formation of methane?

ΔH_c^⦵(methane) + ΔH_c^⦵(carbon) + ΔH_c^⦵(hydrogen)

ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen) − ΔH_c^⦵(methane)

ΔH_c^⦵(methane) − ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen)

ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen) + ΔH_c^⦵(methane)

C + 2H₂ can react directly to give CO₂ and 2H₂O and the enthalpy change will be ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen). C + 2H₂ can also react to form methane first, which can then be combusted to give CO₂ and 2H₂O with an enthalpy change equal to ΔH_f^⦵(methane) + ΔH_c^⦵(methane). By Hess's Law these two enthalpy changes are equal.

2-methylpropene can be obtained by dehydrating 2-methylpropan-1-ol.

(CH₃)₂CHCH₂OH(l) → (CH₃)₂C=CH₂(g) + H₂O(l)

The standard enthalpies of formation of 2-methylpropan-1-ol, 2-methylpropene and water are −335, −17 and −286 kJ mol⁻¹ respectively.

What is the standard enthalpy change for the dehydration of 2-methylpropan-1-ol?

+ 638 kJ mol^-1

− 32 kJ mol^-1

− 638 kJ mol^-1

+ 32 kJ mol^-1

ΔH^⦵ = − 17 + (− 286) − (− 335) = + 32 kJ mol^-1

Which species has a standard enthalpy of formation value equal to zero?

I(l)

I₂(g)

I₂(s)

I₂(l)

ΔH_f^⦵ refers to the formation of a substance in its standard state from its elements in their standard states so for all elements in their standard state the value of ΔH_f^⦵ is zero.

Ethanol can be oxidised to ethanal,

C₂H₅OH(l) + ½O₂(g) → CH₃CHO(l) + H₂O(l) ΔH^⦵ = a

which can then be further oxidized to ethanoic acid.

CH₃CHO(l) + ½O₂(g) → CH₃COOH(l) ΔH^⦵ = b

Ethanol can also be oxidized directly to ethanoic acid

C₂H₅OH(l) + O₂(g) → CH₃COOH(l) + H₂O(l) ΔH^⦵ = c

Which is the correct relationship between a, b and c?

a = b − c

c = a − b

b = a − c

b = c − a

The direct route has an enthalpy change of c and the indirect route has an enthalpy change of a + b. By Hess's law c = a + b.

The copper thermite reaction is used for creating electrical joints:

3Cu₂O(s) + 2Al(s) → 6Cu(s) + Al₂O₃(s)

The enthalpies of formation of Cu₂O and Al₂O₃ are −170 and −1676 kJ mol-¹ respectively.

What is the standard enthalpy change of the copper thermite reaction in kJ?

− 2186

− 1506

− 1166

− 1846

ΔH = − 1676 − (− 510) = − 1166 kJ

Diborane, B₂H₆has been suggested as a possible rocket fuel. The equation for its combustion is

B₂H₆(g) + 3O₂(g) → B₂O₃(s) + 3H₂O(l) ΔH_c^⦵ = − 2168 kJ mol^-1

The enthalpies of formation of boron oxide and water are −1274 and −286 kJ mol⁻¹ respectively.

What is the standard enthalpy of formation of diborane?

+ 36 kJ mol⁻¹

− 608 kJ mol⁻¹

+ 608 kJ mol⁻¹

− 36 kJ mol⁻¹

ΔH = − 1274 + (3 x − 286) − (− 2168) = + 36 kJ mol⁻¹

What is the standard enthalpy change for the reduction of magnesium oxide by hydrogen to produce water and magnesium metal given that the standard enthalpies of formation of magnesium oxide and water are −602 and −286 kJ mol⁻¹ respectively.

− 316 kJ mol⁻¹

+ 316 kJ mol⁻¹

− 888 kJ mol⁻¹

+ 888 kJ mol⁻¹

ΔH^⦵ = − 286 − (− 602) = + 316 kJ mol⁻¹

The ore magnetite, Fe₃O₄, can be reduced by carbon monoxide

Fe₃O₄(s) + 4CO(g) → 3Fe(s) + 4CO₂(g)

The standard enthalpies of formation of magnetite, carbon monoxide and carbon dioxide are −1118, −110 and −394 kJ mol⁻¹ respectively.

What is the standard enthalpy change for the reduction of magnetite by carbon monoxide?

+ 3134 kJ

− 18 kJ

+ 18 kJ

− 3134 kJ

ΔH^⦵ = (4 x − 394) − [− 1118 + (4 x − 110)] = − 18 kJ

Synthesis gas is a mixture of hydrogen and carbon monoxide. It is produced by passing steam over carbon.

H₂O(g) + C(s) → H₂(g) + CO(g)

The standard enthalpies of formation of water and carbon monoxide are –286 and –110 kJ mol⁻¹ respectively.

What additional information is required to calculate the standard enthalpy change for the production of synthesis gas?

The stoichiometry of the reaction

The pressure under which the process is carried out

The standard enthalpy of formation of hydrogen

The enthalpy of vaporisation of water

The standard enthalpy of formation of water is for liquid water. Energy needs to be put in to turn liquid water into steam before Hess's law can be applied.

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MC test: Hess's Law

Multiple choice test on 5.2 Hess's Law

MC test: Hess's Law