IB Chemistry SL Reactivity 1 What Drives Chemical Reactions Questions

Practise IB Chemistry SL 1.4 with evidence-led questions on ionic, covalent and metallic bonding, structures and properties.

Exam points

Distinguish endothermic and exothermic processes and calculate enthalpy changes from calorimetry data.
Use bond energies, Hess’s law and standard enthalpy data 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: 1]

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

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

[Maximum number: 6]

Alkenes, such as A (shown below), are important intermediates in the petrochemical industry because they undergo addition reactions to produce a wide variety of products, such as the conversion shown below.

(a)

In the gas phase, A reacts with hydrogen to form D.

Figure for Question (a) — IB Chemistry SL

[ 6 ]

(i)

Determine the enthalpy change, in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the reaction of A with hydrogen, using Table 10 of the Data Booklet, and state whether the reaction is exothermic or endothermic.

[ 4 ]

bonds broken: $(\mathrm{E}(\mathrm{C}=\mathrm{C})+\mathrm{E}(\mathrm{H}-\mathrm{H})=612+436=) 1048\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;
Accept (6956+436=) 7392 if all bonds in alkene broken.
bonds formed: $\mathrm{E}(\mathrm{C}-\mathrm{C})+2 \times \mathrm{E}(\mathrm{C}-\mathrm{H})=347+(2 \times 413)=1173\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

Marking guidance:

Accept 7517 if all the bonds in the product are summed.
$\Delta H=1048-1173 / 7392-7517=-125\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$
Award [3] for correct final answer.
Award [2] for +125 .
exothermic;

Apply ECF if sign of $\Delta H$ incorrect.
Do not award a mark for "exothermic" if $\Delta H$ given as positive.

(ii)

The standard enthalpy change of combustion of A is $-4000 \mathrm{~kJ} \mathrm{~mol}^{-1}$ . Calculate the amount of A, in mol , that would have to be burned to raise the temperature of $1 \mathrm{dm}^{3}$ of water from $20^{\circ} \mathrm{C}$ to $100^{\circ} \mathrm{C}$ .

[ 2 ]

energy required to heat water $(=m \times s \times \Delta T=1 \times 4.18 \times(100-20))=334.4(\mathrm{~kJ})$ ;
Ignore sign of energy change.

Marking guidance:

Award [2] for correct final answer.

[Maximum number: 5]

Ethyne, $\mathrm{C}_{2} \mathrm{H}_{2}$ , reacts with oxygen in welding torches.

(a)

Write an equation for the complete combustion of ethyne.

[ 1 ]

$\mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{~g})+2.5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

OR
$2 \mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 4 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

(b)

Ethyne reacts with steam.

Two possible products are:

Figure for Question (b) — IB Chemistry SL

[ 4 ]

(i)

Determine the enthalpy change for the reaction, in kJ , to produce A using section 11 of the data booklet.

[ 3 ]

Reactant bonds:

$2(\mathrm{C-H})+\mathrm{C\equiv C}+2(\mathrm{O-H}):\quad2\times414+839+2\times463=2593\,\mathrm{kJ}$

Product A bonds:

$3(\mathrm{C-H})+\mathrm{C=C}+\mathrm{C-O}+\mathrm{O-H}:\quad3\times414+614+358+463=2677\,\mathrm{kJ}$

Enthalpy change:

$\Delta H=2593-2677=-84\,\mathrm{kJ}$

Award [3] for correct final answer.

(ii)

The enthalpy change for the reaction to produce B is -213 kJ .

Predict, giving a reason, which product is the most stable.

[ 1 ]

[Maximum number: 4]

Chlorine occurs in Group 7, the halogens.

(a)

Chloroethene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCl}$ , the monomer used in the polymerization reaction in the manufacture of the polymer poly(chloroethene), PVC, can be synthesized in the following two-stage reaction pathway.

Stage 1: $\quad \mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}(\mathrm{g})$

Stage 2: $\quad \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{C}=\mathrm{CHCl}(\mathrm{g})+\mathrm{HCl}(\mathrm{g})$

[ 4 ]

(i)

Determine the enthalpy change, $\Delta H$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for stage 1 using average bond enthalpy data from Table 10 of the Data Booklet.

[ 3 ]

Bonds breaking:
$1 \times(C=C)+4 \times(C-H)+1 \times(C l-C l)$ $=(1)(612)+(4)(413)+(1)(243) /=(+) 2507\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$
Bonds forming:
$1 \times(C-C)+4 \times(C-H)+2 \times(C-C l)$ $=(1)(347)+(4)(413)+(2)(346) /=-2691\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$
Enthalpy change:
$(2507-2691=)-184\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

OR

OR
Bonds breaking:
$1 \times(C=C)+1 \times(C l-C l)$ $=(1)(612)+(1)(243) /=(+) 855\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$
Bonds forming:
$1 \times(C-C)+2 \times(C-C l)$ $=(1)(347)+(2)(346) /=-1039\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;
Enthalpy change:
$(855-1039=)-184\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

Marking guidance:

Award [3] for correct final answer.

(ii)

State whether the reaction given in stage 1 is exothermic or endothermic.

[ 1 ]

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