Edexcel IAL Chemistry Unit 2 Energetics, groups and organic chemistry

Use the Unit 2 question bank to connect energy and rate models with group trends, redox equations, organic transformations and practical or spectroscopic data.

Covered syllabus points

Topic 6: Energetics Topic 7: Intermolecular Forces Topic 8: Redox Chemistry and Groups 1, 2 and 7 Topic 9: Introduction to Kinetics and Equilibria Topic 10: Organic Chemistry: Halogenoalkanes, Alcohols and Spectra

[Maximum number: 17]

Fuels

Fuels burn in oxygen to release a lot of energy.
Many hydrocarbons and alcohols are used as fuels. During complete combustion, they produce carbon dioxide and water.

Petrol contains 2,2,4-trimethylpentane, an isomer of octane, that promotes smooth combustion.

Figure for Question Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 1 — Edexcel A-Level Chemistry AS

2,2,4-trimethylpentane

Alcohols, such as methanol and ethanol, can be used as fuels either on their own or as additives in petrol.

(a)

The standard enthalpy change of combustion, $\Delta_{\mathrm{c}} H^{\ominus}$ , of 2,2,4-trimethylpentane is $-5461 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[ 6 ]

(i)

State the two standard conditions for this enthalpy change.

[ 1 ]

(ii)

Draw a labelled enthalpy level diagram for the complete combustion of 2,2,4-trimethylpentane.

Figure for Question (ii) — Edexcel A-Level Chemistry AS

[ 2 ]

(iii)

Calculate the heat energy released during the complete combustion of $1 \mathrm{dm}^{3}$ of 2,2,4-trimethylpentane.
[Density of 2,2,4-trimethylpentane $=0.692 \mathrm{~g} \mathrm{~cm}^{-3}$ ]

[ 3 ]

calculation of energy given out by 1 g

- calculation of energy given out by $1 \mathrm{~cm}^{3}$

- calculation of energy given out by $1 \mathrm{dm}^{3}$

Example of calculation:

Method 1 enthalpy change $/ \mathrm{g}=\frac{5461}{114}=47.904(\mathrm{~kJ})$

enthalpy change $/ \mathrm{cm}^{3}=47.904 \times 0.692=33.149(\mathrm{~kJ})$

TE on M1

enthalpy change $/ \mathrm{dm}^{3}=33.149 \times 1000$ $=33149 / 33.149 \times 10^{3}(\mathrm{~kJ})$

TE on M2

Method 2

mass of 2,2,4-trimethylpentane in $1 \mathrm{dm}^{3}$ $=0.692 \times 1000=692(\mathrm{~g})(1)$

mol in $1 \mathrm{dm}^{3}=\frac{692}{114}=6.0702(\mathrm{~mol})(1)$

TE on M1

enthalpy change $/ \mathrm{dm}^{3}=6.0702 \times 5461$ $=33149 / 33.149 \times 10^{3}(\mathrm{~kJ})(1)$

TE on M2

Allow alternative methods

Correct answer with some working scores (3)

Ignore SF except 1 SF

Ignore minus sign

Ignore units, even if incorrect

(b)

In an experiment to determine the enthalpy change of combustion of ethanol, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$ , a student used the apparatus shown.

Figure for Question (b) — Edexcel A-Level Chemistry AS

Results:
Mass of water $=100.0 \mathrm{~g}$
Mass of ethanol used $=0.305 \mathrm{~g}$
Temperature rise of water $=13.2^{\circ} \mathrm{C}$

[ 6 ]

(i)

Calculate the enthalpy change of combustion of ethanol.

Give your answer to an appropriate number of significant figures, and include a sign and units.
[Specific heat capacity of water $=4.18 \mathrm{~J} \mathrm{~g}^{-1} \mathrm{C}^{-1}$ ]

[ 4 ]

(ii)

The student looked in a data book and found the actual value for the standard enthalpy change of combustion of ethanol was more exothermic than the experimental value obtained.

Give two reasons for the difference between the data book value and the experimental value, other than referring to standard conditions.

[ 2 ]

(c)

The enthalpy changes for the conversion of four compounds in the gas phase into their constituent atoms are shown.

\begin{aligned} \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \rightarrow 2 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+928 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{CH}_{4}(\mathrm{~g}) & \rightarrow \mathrm{C}(\mathrm{~g})+4 \mathrm{H}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+1740 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{CH}_{3} \mathrm{OH}(\mathrm{~g}) & \rightarrow \mathrm{C}(\mathrm{~g})+4 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+2105 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g}) & \rightarrow 2 \mathrm{C}(\mathrm{~g})+6 \mathrm{H}(\mathrm{~g})+\mathrm{O}(\mathrm{~g}) & \Delta_{\mathrm{r}} H & =+3322 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}

Calculate the bond enthalpy of the C-C bond, in $\mathrm{kJ} \mathrm{mol}^{-1}$ .
You must show your working.

[ 3 ]

calculation of bond energies of O-H and C-H

- calculation of bond energy of C-O

- calculation of bond energy of C-C

Example of calculation:
Method 1 bond energy $\mathrm{O}-\mathrm{H}=928 / 2=(+) 464\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$
and
bond energy C-H $=1740 / 4=(+) 435\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$
bond energy $\mathrm{C}-\mathrm{O}=2105-(3 \times 435)-464$ $=(+) 336\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$
TE on M1
bond energy $\mathrm{C}-\mathrm{C}=3322-(5 \times 435)-464-336$ $=(+) 347\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$
Method 2
$3322-2105=1217=\mathrm{C}-\mathrm{C}+2 \times \mathrm{C}-\mathrm{H}(1)$
bond energy $2 \times \mathrm{C}-\mathrm{H}=1740 / 2=(+) 870\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)(1)$ $\mathrm{C}-\mathrm{C}=1217-870=(+) 347\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)(1)$
M3 TE on M1 and M2 in both methods
Correct answer with some working scores

(d)

Ethanol can be manufactured by reacting ethene with steam.

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g}) \quad \Delta_{\mathrm{r}} H=-45 \mathrm{~kJ} \mathrm{~mol}^{-1}

This reaction is usually carried out in industry at $300^{\circ} \mathrm{C}$ and 70 atm pressure using a catalyst.

Explain the effect on the equilibrium position and the equilibrium yield of ethanol if the reaction is carried out at $300^{\circ} \mathrm{C}$ and 200 atm pressure.

[ 2 ]

[Maximum number: 12]

Some diesel cars contain an extra catalytic converter for the reduction of nitrogen oxides $\left(\mathrm{NO}_{\mathrm{x}}\right)$ in exhaust gases.
A solution of urea is used for this process.

Figure for Question Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 2 — Edexcel A-Level Chemistry AS

urea

(a)

Urea has a melting temperature of $133^{\circ} \mathrm{C}$ .

Explain why this value is higher than expected for a relatively small molecule.

[ 3 ]

(b)

An infrared spectrum of urea is shown.

Refer to your Data Booklet.

$stretching of the \(\mathrm{N$

stretching of the \(\mathrm{N

[ 2 ]

(i)

Draw a circle around an absorption in the spectrum that could be due to the

[ 1 ]

(ii)

Identify the bond responsible for the absorption at $1683 \mathrm{~cm}^{-1}$ .

ption in

[ 1 ]

(c)

In a diesel car exhaust system, the urea reacts with water to form ammonia and carbon dioxide. The enthalpy change for this reaction is $+133 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[ 3 ]

(i)

Sketch the reaction profile for the forward reaction on the axes provided. Include labels for $\Delta H$ and the activation energy $\left(E_{\mathrm{a}}\right)$ .

Figure for Question (i) — Edexcel A-Level Chemistry AS

[ 3 ]

(d)

The catalytic converter contains metal oxides. When the exhaust gases pass through the catalytic converter, ammonia reacts with $\mathrm{NO}_{\times}$ gases to form nitrogen and water.

[ 4 ]

(i)

Explain why it is not correct to state that urea is acting as a catalyst in the reaction.

[ 1 ]

(ii)

Explain how a catalyst increases the rate of a chemical reaction.

Use the Maxwell-Boltzmann distribution shown and refer to the collision theory.

Number of particles with energy, E

[ 3 ]

[Maximum number: 15]

Ammonium nitrate, $\mathrm{NH}_{4} \mathrm{NO}_{3}$ , is used in the manufacture of fertilisers and explosives. It is produced on a large scale using only methane, water and air.
The process has four stages.

(a)

The first two reactions in Stage 1 involve the production of hydrogen.

At temperature $T_{1}$ , methane reacts with excess steam to give hydrogen.

\mathrm{CH}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons 3 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{CO}(\mathrm{~g}) \quad \Delta H=+206 \mathrm{~kJ} \mathrm{~mol}^{-1}

At a different temperature, $T_{2}$ , the carbon monoxide reacts with more steam.

\mathrm{CO}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g})+\mathrm{CO}_{2}(\mathrm{~g}) \quad \Delta H=-42 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 2 ]

(i)

Give the reason why excess steam is used in the first reaction.

[ 1 ]

An answer that makes reference to one of the following points:

- shifts position of equilibrium to the right

OR

increases the (equilibrium) yield (of $\mathrm{H}_{2}$ )

Ignore to increase rate (of forward reaction) Ignore cheaper to have steam in excess Ignore to react with most of the $\mathrm{CH}_{4}$

Allow to increase yield (of CO / products)

Do not award so all of the $\mathrm{CH}_{4}$ reacts / so reaction goes to completion

Do not award to increase the moles of gas/pressure

(ii)

Predict which of $T_{1}$ and $T_{2}$ is the higher temperature.

Justify your answer.

[ 1 ]

An answer that makes reference to the following points:

- $T_{1}$ (is higher)

and

(first reaction is) endothermic

Accept reverse argument

Allow positive enthalpy change for endothermic Allow (first reaction) absorbs (heat) energy for endothermic

Ignore just +206 for endothermic

Ignore correct reference to effect of temperature on equilibrium yields

Do not award absorbs more energy to break (reactant) bonds

(b)

In Stage 2, the hydrogen from Stage 1 reacts with nitrogen (from the air) to produce ammonia. The conditions for this reaction are:
- a temperature of 700 K
- a pressure in the range $100-200 \mathrm{~atm}$
- an iron catalyst

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} \mathrm{~mol}^{-1}

Give one advantage and one disadvantage of using a pressure of 200 atm, compared to a pressure of 100 atm , in Stage 2.

[ 2 ]

An answer that makes reference to the following points:

- advantage of using high pressure

- disadvantage of using high pressure

Examples of advantage: shifts position of equilibrium to right / products

OR increases (equilibrium) yield (of $\mathrm{NH}_{3}$ )

OR increases rate

OR increases occupation of catalyst active sites

Ignore any reference to collisions

(c)

The reaction in Stage 2 has an activation energy, $E_{\text {cat }}=+70 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The uncatalysed reaction between $\mathrm{N}_{2}$ and $\mathrm{H}_{2}$ has an activation energy, $E_{\mathrm{a}}=+290 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

[ 4 ]

(i)

Complete the profile for the catalysed and uncatalysed reactions.

Label the activation energies and the enthalpy change of reaction, $\Delta H$ .
Your diagram must match the scale shown for the production of $\mathrm{NH}_{3}$ .

[ 3 ]

An answer that makes reference to the following
points:

- $\Delta H$ labelled

and

arrow pointing downwards

(1)

- labelled reaction profiles for uncatalysed and catalysed reactions

(1)

- correct scale for activation energies

(1)

Example of labelled reaction profile:
$\mathrm{H} / \mathrm{kJ} \mathrm{mol}^{-1}$
reaction progress
Allow arrows to start/end within one small square of correct
placement and penalise incorrect placement once only
Allow -92 / 'enthalpy change' for $\Delta H$
Do not award double headed arrow
Allow any form of unambiguous labelling, eg values
Allow double headed arrows
Do not award downward arrows
Do not award $E_{\text {cat }}>E_{\text {a }}$
Accept accuracy of ± one small square
Ignore scale shown on y-axis

Figure for Answer to Question (i) — Edexcel A-Level Chemistry AS

(ii)

Suggest why the use of the catalyst makes Stage 2 more sustainable.

[ 1 ]

(d)

In Stage 3, nitrogen monoxide, NO, is produced in the reaction between $\mathrm{NH}_{3}$ (from Stage 2) and $\mathrm{O}_{2}$ (from the air). The conditions used are a temperature of 1100 K in the presence of a platinum-rhodium catalyst.

4 \mathrm{NH}_{3}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 4 \mathrm{NO}(\mathrm{~g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta H=-905 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 2 ]

(i)

Give one reason why a high temperature is needed in this reaction.

[ 1 ]

An answer that makes reference to one of the following points:

- increase rate

OR

rate is slow at low temperature

OR

catalyst does not work at low temperature

OR

so more reactants/collisions have $E \geq E_{\mathrm{a}}$

OR

to break O=O / N-H bonds

Do not award to increase yield

Do not award to shift position of equilibrium (to left / right)

Do not award reverse reaction is endothermic

Allow to increase the number of successful collisions Ignore to increase collision frequency

Allow catalyst more efficient at high temperature

Allow to activate the catalyst

Accept (to reach) high activation energy

Allow to break bonds in oxygen/ammonia/reactants

(ii)

Suggest why only a small amount of energy is used to maintain the temperature at 1100 K .

[ 1 ]

An answer that makes reference to the following points:

- (forward reaction is highly) exothermic

OR

(forward reaction) releases (a lot of) heat (energy)

Ignore any reference to catalysis

Allow thermal energy for heat

Do not award $\mathrm{NH}_{3}$ from Stage 2 is hot Do not award 1100 K is not very high

(e)

The NO from the first reaction in Stage 3 is cooled and then converted to nitrogen dioxide, $\mathrm{NO}_{2}$ , by reaction with more $\mathrm{O}_{2}$ .

2 \mathrm{NO}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NO}_{2}(\mathrm{~g})

Nitric acid, $\mathrm{HNO}_{3}(\mathrm{aq})$ , is produced by the addition of water.

3 \mathrm{NO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{NO}(\mathrm{~g})

Explain how adding water in the second reaction affects the yield of $\mathrm{NO}_{2}$ in the first reaction.

[ 2 ]

An explanation that makes reference to the following points:

- $\mathrm{NO}_{2}$ removed (in second reaction)

- shifting position of equilibrium (in first reaction) to right and increasing the yield (of $\mathrm{NO}_{2}$ )

(1)

Allow (as) NO formed (in second reaction)

Ignore $\mathrm{HNO}_{3}$ is formed (in second reaction) Ignore reaction is irreversible Ignore $\mathrm{NO}_{2}$ dissolves

(f)

In Stage 4, a solution of $\mathrm{NH}_{4} \mathrm{NO}_{3}$ is produced by reacting $\mathrm{NH}_{3}$ (from Stage 2) with $\mathrm{HNO}_{3}$ (from Stage 3).

\mathrm{NH}_{3}(\mathrm{~g})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{NH}_{4} \mathrm{NO}_{3}(\mathrm{aq})

Data

Table for Question (f) — Edexcel A-Level Chemistry AS

[ 3 ]

(i)

Complete the enthalpy cycle.

Enthalpy cycle

[ 2 ]

(ii)

Calculate the enthalpy change, $\Delta_{\mathrm{r}} H$ , in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the reaction of $\mathrm{NH}_{3}(\mathrm{~g})$ with $\mathrm{HNO}_{3}(\mathrm{aq})$ .

[ 1 ]

An answer that makes reference to the following point:

- calculation of $\Delta_{\mathrm{r}} H$

Example of calculation:
$\begin{aligned} \Delta_{\mathrm{r}} H & =-(-32.6)-(-220.2)+(-365.6)+25.6 & =-87.2 /-87\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) \end{aligned}$

Allow omission of units

Allow kJ

TE on cycle in (g)(i)

[Maximum number: 2]

Propan-1-ol may be converted into propene.

\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \rightarrow \mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}

(a)

Which reagent is used for this reaction?

50% sulfuric acid

ethanolic potassium hydroxide

phosphoric(V) acid

red phosphorus

[ 1 ]

(b)

The reaction is best classified as

elimination

hydrolysis

reduction

substitution

[ 1 ]

All question bank results loaded

Covered syllabus points

Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (b)

Question (i)

Question (ii)

Question (c)

Question (d)

Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

Question (i)

Question (d)

Question (i)

Question (ii)

Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 3

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (i)

Question (ii)

Question (d)

Question (i)

Question (ii)

Question (e)

Question (f)

Question (i)

Question (ii)

Unit 2: Energetics, Group Chemistry, Halogenoalkanes and Alcohols question 4

Question (a)

Question (b)