IB Chemistry HL Reactivity 1 What Drives Chemical Reactions Questions

Practise IB Chemistry HL 1.4 with HL questions on bonding models, polarity and material properties.

Exam points

Distinguish endothermic and exothermic processes and calculate enthalpy changes from calorimetry data.
Use bond energies, Hess’s law, standard enthalpy data and Born–Haber cycles to calculate and compare reaction enthalpies.
Balance combustion equations, compare fossil and biofuels, and explain fuel-cell energy conversion.

Covered syllabus points

1.1 Measuring enthalpy changes 1.2 Energy cycles 1.3 Energy from fuels

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Study Reactivity 1. What drives chemical reactions? conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 3]

The increased concentration of carbon dioxide in the atmosphere is thought to result from the increased combustion of fossil fuels such as petroleum.

(a)

Determine the specific energy and energy density of petrol (gasoline), using data from sections 1 and 13 of the data booklet. Assume petrol is pure octane, $\mathrm{C}_{8} \mathrm{H}_{18}$ . Octane: molar mass $=114.26 \mathrm{~g} \mathrm{~mol}^{-1}$ , density $=0.703 \mathrm{~g} \mathrm{~cm}^{-3}$ .

Specific energy in $\mathrm{kJ} \mathrm{g}^{-1}$ :

Energy density in $\mathrm{kJ} \mathrm{cm}^{-3}$ :

[ 2 ]

specific energy « $=\frac{\text { energy released }}{\text { mass consumed }}=\frac{5470 \mathrm{~kJ} \mathrm{~mol}^{-1}}{114.26 \mathrm{~g} \mathrm{~mol}^{-1}}$ » =47.9 «kJ g-1»

energy density « $=\frac{\text { energy released }}{\text { volume consumed }}=$ specific energy × density $=47.9 \mathrm{~kJ} \mathrm{~g}{ }^{-1}$ $\times 0.703 \mathrm{~g} \mathrm{~cm}^{-3}$ » =33.7 « $\mathrm{kJ} \mathrm{cm}^{-3}$ »

Marking guidance:

Do not accept "-47.9 " $k J g^{-1}$ ".

Do not accept "-33.7 «kJ cm³»" unless "-47.9 «kJ g-1»" already penalized.

(b)

Outline why the energy available from an engine will be less than these theoretical values.

[ 1 ]

[Maximum number: 6]

Organic compounds have many industrial applications.

(a)

Ethene reacts with steam to produce ethanol.

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g})

[ 6 ]

(i)

Calculate the enthalpy, in kJ , of the reaction using section 12 of the data booklet.

[ 3 ]

ALTERNATIVE 1:

Bonds broken:

$1 \mathrm{C}=\mathrm{C}+4 \mathrm{C}-\mathrm{H}+2 \mathrm{O}-\mathrm{H} / 614+4(414)+2(463) / 3196$ «kJ»

Bonds formed:

1 C-C+1 C-O+5 C-H+1 O-H /

346+358+5(414)+463 /

3237 «kJ»
$\Delta H=\text { «3196-3237 }=\text { » }-41 \text { «kJ» » }$

ALTERNATIVE 2:

Bonds broken:
$1 \mathrm{C}=\mathrm{C}+1 \mathrm{O}-\mathrm{H} / 614+463 /$

1077 «kJ»

Bonds formed:
$\begin{aligned} & 1 \mathrm{C}-\mathrm{C}+1 \mathrm{C}-\mathrm{O}+1 \mathrm{C}-\mathrm{H} / 346+358+414 / & 1118\langle\mathrm{~kJ}\rangle{ }^{2} & \Delta H=\langle 1077-1118=\rangle-41\left\langle\mathrm{~kJ} \|{ }^{2} \right. \end{aligned}$

Award [3] for correct final answer.

Award [2 max] for +41 «kJ».

(ii)

Calculate the enthalpy of the reaction, in kJ . Use section 13 of the data booklet and $\Delta H_{f}^{\ominus}$ of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{g})=-235 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[ 2 ]

$\begin{aligned} \text { 《 } \Delta H^{\ominus}=\Sigma\left(\Delta H_{\mathrm{f}}^{\ominus} \text { products }\right)-\Sigma\left(\Delta H_{\mathrm{f}}^{\ominus} \text { reactants }\right) =-235-(+52+(-242)) =-45 \text { «kJ» } \end{aligned}$

Award [2] for correct final answer.

Award [1 max] for +45 «kJ»

(iii)

Outline why the enthalpies calculated in (i) and (ii) are different.

[ 1 ]

[Maximum number: 2]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

The combustion of urea produces water, carbon dioxide and nitrogen.

Formulate a balanced equation for the reaction.

$2\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}(\mathrm{~s})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 4 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+2 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{~N}_{2}(\mathrm{~g})$

[Maximum number: 1]

Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

In the first part of the project, the two groups had a sample of iodine monochloride (a corrosive brown liquid) prepared for them by their teacher using the following reaction.

\mathrm{I}_{2}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{ICl}(\mathrm{l})

The following data were recorded.

Using a digital thermometer, the students discovered that the reaction was exothermic. State the sign of the enthalpy change of the reaction, $\Delta H$ .

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Covered syllabus points

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Question 2

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Question (ii)

Question (iii)

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