IB Chemistry 1.2 Energy cycles Questions

IB Chemistry syllabusChemistry SL/HLFirst assessment 2025

Work through Hess's law, bond enthalpy, and lattice enthalpy questions by combining data-booklet values with careful sign changes and clear chemical reasoning.

Exam points

calculate reaction ΔH by adding Hess cycle equations, reversing steps, and changing ΔH signs correctly
compare Hess and bond enthalpy values for ethanol + O2, noting average gaseous bonds cause differences
explain lower lattice enthalpy in CaCl2 than MgCl2 by the larger Ca2+ radius and greater ion separation

[Maximum number: 4]

Nitrous acid, $\mathrm{HNO}_{2}(\mathrm{aq})$ and hydrochloric acid, HCl(aq) are both inorganic acids.

(a)

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

[ 4 ]

(i)

Calculate the enthalpy change for the reaction, $\Delta H$ . Use section 12 of the data booklet.

[ 3 ]

(ii)

One student used bond enthalpy data and correctly calculated the enthalpy change for the reaction between ethanol and ethanoic acid as $0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ . Another student used Hess's law and correctly calculated the enthalpy change for the same reaction as $-4 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Explain how the two students can carry out a calculation for the same reaction and obtain different results when both calculations are correct.

[ 1 ]

[Maximum number: 1]

Consider these equations:

\begin{array}{ll} \mathrm{S}(\mathrm{~s})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{SO}_{2}(\mathrm{~g}) & \Delta H^{\ominus}=x \mathrm{~kJ} \mathrm{~mol}^{-1} \\ 2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{SO}_{3}(\mathrm{~g}) & \Delta H^{\ominus}=y \mathrm{~kJ} \mathrm{~mol}^{-1} \end{array}

What is the value of $\Delta H^{\ominus}$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for this reaction?

\mathrm{S}(\mathrm{~s})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{SO}_{3}(\mathrm{~g})

x+y

$x+\frac{1}{2} y$

$x-\frac{1}{2} y$

x-y

[Maximum number: 1]

What is $\Delta H$ , in kJ , in the energy cycle below?

+52-176

+52+176

-52+124

-5 2-1 7 6

[Maximum number: 1]

What is the enthalpy change for the reaction in $\mathrm{kJ} \mathrm{mol}^{-1}$ ?

\mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{~g})+2 \mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})

-1561-2(-286)-1301

-1561+2(-286)-1301

-1301+2(-286)+1561

-1301-2(-286)+1561

[Maximum number: 6]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

(a)

The overall reaction for the synthesis of ethyl ethanoate from ethane is:

2 \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})+\frac{3}{2} \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{HCl}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

[ 6 ]

(i)

Calculate the enthalpy change for the reaction, $\Delta H$ . Use section 12 of the data booklet.

[ 3 ]

(ii)

One student used bond enthalpy data and correctly calculated the enthalpy change for the reaction between ethanol and ethanoic acid as $0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ . Another student used enthalpy of formation data and correctly calculated the enthalpy change for the same reaction as $-4 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}(\mathrm{l})+\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}(\mathrm{l}) \rightleftharpoons \mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}(\mathrm{l})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Explain how the two students can carry out a calculation for the same reaction and obtain different results when both calculations are correct.

[ 1 ]

(iii)

Calculate the standard enthalpy change of formation, $\Delta H^{\theta}{ }_{\mathrm{f}}$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , of ethyl ethanoate, $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}$ . Use sections 1 and 13 of the data booklet and the value of $-4 \mathrm{~kJ} \mathrm{~mol}^{-1}$ for the standard enthalpy change of reaction, $\Delta H^{\theta}{ }_{\mathrm{r}}$ .

[ 2 ]

[Maximum number: 1]

Which equation represents the enthalpy change of atomisation, $\Delta H^{\theta}{ }_{a t}$ , of bromine?

$\mathrm{Br}_{2}(\mathrm{l}) \rightarrow \mathrm{Br}_{2}(\mathrm{~g})$

$1 / 2 \mathrm{Br}_{2}(\mathrm{~g}) \rightarrow \mathrm{Br}(\mathrm{g})$

$1 / 2 \mathrm{Br}_{2}(\mathrm{l}) \rightarrow \mathrm{Br}(\mathrm{g})$

$\mathrm{Br}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{Br}(\mathrm{g})$

[Maximum number: 4]

Bromoethane, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}$ , is produced by reacting ethene, $\mathrm{C}_{2} \mathrm{H}_{4}$ , with hydrogen bromide, HBr .

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{HBr}(\mathrm{~g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}(\mathrm{l})

(a)

Determine the standard enthalpy change of the reaction, in $\mathrm{kJ} \mathrm{mol}^{-1}$ . Use section 12 of the data booklet.

[ 2 ]

(b)

State two reasons why the result using bond enthalpies is less accurate than one calculated from enthalpies of formation.

[ 2 ]

0 selected

Exam points

Question 2

Question 2(c)

Question 2(c)(i)

Question 2(c)(ii)

Question 16

Question 17

Question 21

Question 3

Question 3(d)

Question 3(d)(i)

Question 3(d)(ii)

Question 3(d)(iii)

Question 22

Question 4

Question 4(b)(i)

Question 4(b)(ii)