IB Chemistry SL Reactivity 1: Reaction Drivers

[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: 4]

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.

Figure for Question Reactivity 1. What drives chemical reactions? question 2 — IB Chemistry SL

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

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.

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.

[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 ]

«sum of bond enthalpies of reactants = » $2(\mathrm{C}-\mathrm{H})+\mathrm{C} \equiv \mathrm{C}+2(\mathrm{O}-\mathrm{H})$
OR
$2 \times 414 « \mathrm{~kJ} \mathrm{~mol}^{-1}$ » +839 « $\mathrm{kJ} \mathrm{mol}^{-1}$ » $+2 \times 463$ « $\mathrm{kJ} \mathrm{mol}^{-1}$ »
OR
2593 «kJ»
«sum of bond enthalpies of $\mathrm{A}=» 3(\mathrm{C}-\mathrm{H})+\mathrm{C}=\mathrm{C}+\mathrm{C}-\mathrm{O}+\mathrm{O}-\mathrm{H}$
OR
$3 \times 414 « \mathrm{~kJ} \mathrm{~mol}^{-1} »+614<\mathrm{kJ} \mathrm{mol}^{-1}>+358<\mathrm{kJ} \mathrm{mol}^{-1} \gg+463<\mathrm{kJ} \mathrm{mol}^{-1}><$
OR
2677 «kJ»
«enthalpy of reaction $=2593 \mathrm{~kJ}-2677 \mathrm{~kJ}$ » =-84 «kJ»

Marking guidance:

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 ]

[Maximum number: 3]

(a)

Magnesium sulfate can exist in either the hydrated form or in the anhydrous form. Two students wished to determine the enthalpy of hydration of anhydrous magnesium sulfate. They measured the initial and the highest temperature reached when anhydrous magnesium sulfate, $\mathrm{MgSO}_{4}(\mathrm{~s})$ , was dissolved in water. They presented their results in the following table.

Table for Question (a) — IB Chemistry SL

[ 2 ]

(i)

Calculate the enthalpy change, $\Delta H_{1}$ , for anhydrous magnesium sulfate dissolving in water, in $\mathrm{kJ} \mathrm{mol}^{-1}$ . State your answer to the correct number of significant figures.

[ 2 ]

energy released $=50.0 \times 4.18 \times 9.7=2027(\mathrm{~J}) / 2.027(\mathrm{~kJ})$ ;
$\Delta H_{1}=-81\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$

Marking guidance:

Award [2] for correct answer.
Award [2] if 53.01 is used giving an answer of $-86\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .
Award [1 max] for $+81 / 81 /+86 / 86\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .
Award [1 max] for -81000/-86000 if units are stated as $\mathrm{J} \mathrm{mol}^{-1}$ .
Allow answers to 3 significant figures.

(b)

The students repeated the experiment using 6.16 g of solid hydrated magnesium sulfate, $\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}(\mathrm{s})$ , and $50.0 \mathrm{~cm}^{3}$ of water. They found the enthalpy change, $\Delta H_{2}$ , to be $+18 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The enthalpy of hydration of solid anhydrous magnesium sulfate is difficult to determine experimentally, but can be determined using the diagram below.

\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}(\mathrm{~s}) \xrightarrow[\text { water }]{\Delta H_{2}} \mathrm{Mg}^{2+}(\mathrm{aq})+\mathrm{SO}_{4}^{2-}(\mathrm{aq})

[ 1 ]

(i)

Determine the enthalpy change, $\Delta H$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the hydration of solid anhydrous magnesium sulfate, $\mathrm{MgSO}_{4}$ .

[ 1 ]

$\quad \Delta H\left(=\Delta H_{1}-\Delta H_{2}\right)=-99\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

Marking guidance:

Award [1] if-86 is used giving an answer of $-104\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ .

[Maximum number: 1]

Which products may be formed during incomplete combustion of octane?

A

$\mathrm{CO}_{2}$ and $\mathrm{H}_{2}$

B

CO and $\mathrm{H}_{2}$

C

CO and $\mathrm{H}_{2} \mathrm{O}$

D

C, CO and $\mathrm{H}_{2}$

IB Chemistry SL Reactivity 1: What Drives Chemical Reactions?

Covered syllabus points

Covered syllabus points

Reactivity 1. What drives chemical reactions? question 1

Reactivity 1. What drives chemical reactions? question 2

Reactivity 1. What drives chemical reactions? question 3

Question (a)

Question (b)

Question (i)

Question (ii)

Reactivity 1. What drives chemical reactions? question 4

Question (a)

Question (i)

Question (ii)

Reactivity 1. What drives chemical reactions? question 5

Question (a)

Question (i)

Question (b)

Question (i)

Reactivity 1. What drives chemical reactions? question 6