Enthalpy Changes (OCR A Level Chemistry)

Exam Questions

3 hours45 questions
1a3 marks

State the definition of enthalpy change of formation, ΔHθf.

1b3 marks

Exothermic reaction profiles show the energy change in the reaction.

Unlabelled reaction profile

i)
Complete the reaction profile for an exothermic reaction.
[2] 

ii)
Label the activation energy, Ea on the reaction profile. 
[1] 

1c1 mark

1 mole of butane is formed from carbon and hydrogen via the following equation


4C (s) + 5H2 (g) → C4H10 (g)  ΔHθf = -126 kJ mol-1

Calculate the enthalpy change for the following reaction.

8C (s) + 10H2 (g) → 2C4H10 (g)
1d2 marks

Write an equation for the complete combustion of butane.

Did this page help you?

2a1 mark

State Hess’s Law.

2b1 mark

Using the image below, construct an equation that can be used to determine ΔHr from ΔH1 and  ΔH2.

8bff5ea3-d355-492f-893a-89ad878d0f4e

2c3 marks

Complete the following Hess’s Law cycle for the decomposition of lithium carbonate. 

f01df39f-02a3-4f91-a550-5b5daa4b0486
2d2 marks

Define standard enthalpy of formation, ΔHθf

Did this page help you?

3a3 marks

A student set up a calorimetry experiment for the combustion of propan-1-ol. 50 cm3 of water was added to the beaker and the student lit the spirit burner and heated the water for 2 minutes. Their results are shown in the table below.

Mass of spirit burner before / g 22.07
Mass of spirit burner after / g  21.04
Initial temperature / °C  21.1
Final temperature / °C  47.8

Calorimetry

Calculate the energy change, q, for the reaction in kJ mol-1

(The heat capacity of water is 4.18 J K-1 g-1)

3b2 marks

Calculate the number of moles of propan-1-ol.

3c
Sme Calculator
2 marks

Calculate the overall energy change, ΔH, in kJ mol-1 for the reaction. 

3d1 mark

The data booklet value for the combustion of propan-1-ol is -2021 kJ mol-1. Suggest why the value from the experiment is different from the data booklet value.

Did this page help you?

4a2 marks

Define the term average bond enthalpy.

4b1 mark

Give the formula for calculating the standard enthalpy change of reaction, ΔHr, using bond energies.

4c3 marks

Use the data given in Table 1 to calculate the enthalpy change for the hydration of ethene to form ethanol. 

CH2CH2 (g) + H2O (g) → CH3CH2OH (l)

Table 1

Bond Bond energy / kJ mol-1
C-C 346
C=C 614
C-H 414
C-O 358
O-H 463
4d
Sme Calculator
4 marks

Use Table 2 to calculate the following enthalpy change for the reaction of ethane and bromine to form 1,2-dibromoethane and hydrogen bromide. 

Table 2

Bond Bond enthalpy / kJ mol-1
C-C 346
C-H 414
C-Br 285
H-Br 366
Br-Br 193



Did this page help you?

5a3 marks

Define the term of standard enthalpy of combustion, ΔHθc.

5b3 marks

In ΔHθc  state what is indicated by θ

5c4 marks

Table 1 shows some values for standard enthalpy changes of formation that can be used to calculate the enthalpy change for the reaction between propane and oxygen.

Table 1

Substance  ΔHθf / kJ mol-1
CH3CH2CH3 -105
O2 0
CO2 -393.5
H2O -285.8

i)
State the equation to calculate the enthalpy change for a reaction using enthalpy of formation data.
[1]
 
ii)
State the equation for the combustion of propane. 
[1]
 
iii)
Use the information in Table 1 and your answer to part i) and ii) to calculate the enthalpy change for the combustion of propane.
[2]

5d3 marks

Table 1 shows some values for standard enthalpy changes of formation that can be used to calculate the enthalpy change for the formation of propan-1-ol. This is shown in the following equation.

3C (s) + 4H2 (g) + ½O2 (g) → CH3CH2CH2OH (l) 


Table 2

Substance  ΔHθc / kJ mol-1
CH3CH2CH2OH (l)  -2021
H2 (g)  -286
C (s) -394

Use the information in Table 2 to calculate the enthalpy change for the formation of propan-1-ol. 

Did this page help you?

1a
Sme Calculator
1 mark

Alkanes can be used as fuels in internal combustion engines. When sufficient oxygen is present, they undergo complete combustion reactions. 

Write an equation for the complete combustion of butane.

1b
Sme Calculator
3 marks

Define the term standard enthalpy of combustion, ΔcHϴ.

1c
Sme Calculator
5 marks

The table below shows the bond enthalpy data for the reaction shown in part (a).

  C-C C-H O=O C=O O-H
Mean bond enthalpy, kJ mol-1  348 412 496 805 463

i)

Using the data and the equation in part (a), calculate the enthalpy change of combustion of butane.

ii)
State whether this reaction is endothermic or exothermic. Explain your answer.
1d
Sme Calculator
2 marks

The values given in part (c) are mean bond enthalpies State the meaning of the term mean bond enthalpies.

Did this page help you?

2a
Sme Calculator
3 marks

Define the term standard enthalpy of formation, ΔfHϴ.

2b
Sme Calculator
1 mark

Ammonia is a colourless gas which has a very pungent smell. It also has a number of important chemical applications. 

Ammonia reacts with hydrogen chloride, to produce solid ammonium chloride. 

Write the equation, including state symbols, for this reaction.

2c
Sme Calculator
3 marks

Using the enthalpy of formation data provided, calculate the enthalpy change for the reaction between ammonia and hydrogen chloride in part (b). Show your working.

  NH3  HCl NH4Cl
Enthalpy of formation, ΔfHϴ (kJ mol-1) -46 -92 -314

2d
Sme Calculator
1 mark

Ammonia is produced via the Haber process, where hydrogen and nitrogen gas are reacted together. 

State why the enthalpy of formation of hydrogen would be zero.

Did this page help you?

3a
Sme Calculator
3 marks

A teacher instructs a class to complete a calorimetry practical, to calculate the enthalpy change that occurs when hydrochloric acid and sodium hydroxide react together. 

Each student was given roughly 60 cm3 of 0.35 mol dm-3 hydrochloric acid, roughly 60 cm3 of 0.35 mol dm-3 sodium hydroxide, a polystyrene cup and access to all standard laboratory equipment.  

Draw a diagram to demonstrate the practical set up that the students would need to use to determine the enthalpy change during this neutralisation reaction, and state the key measurements that the students would have to make.

3b
Sme Calculator
5 marks

The students then completed the practical from part (a), using their own method and measurements that they had chosen.

One student found that when they reacted 35.0 cm3 of the hydrochloric acid with 35.0 cm3 of the sodium hydroxide, the temperature rose from 19.6 oC to 22.3 oC. 

Calculate the enthalpy change for this reaction in kJ mol-1

Assume that both solutions have a density of 1.00 g cm-3 and a specific heat capacity of 4.18 J g-1 K-1.

3c
Sme Calculator
1 mark

Explain why the value that you have calculated for the students’ practical in part (b), might be different from the correct value given in a data book.

3d
Sme Calculator
1 mark

State how the students' practical could be improved to allow the students to calculate a more accurate value which is closer to the correct value given in data books.

Did this page help you?

4a
Sme Calculator
1 mark

Propane gas is commonly used as a fuel for outdoor cooking. It can be produced in a number of ways, including from the addition reaction of propyne gas with hydrogen. Propyne has the formula CHCCH3 and includes a triple bond.  

Write an equation for the formation of propane from propyne.

4b
Sme Calculator
3 marks

The table below contains some key enthalpy of combustion data. 

 

Propyne

Hydrogen

Propane

Enthalpy of combustion, ΔcHϴ (kJ mol-1)

-1940

-285.8

-2220


Calculate the enthalpy change in kJ mol
-1 for the reaction in part (a) using the data provided. Show all working.

4c
Sme Calculator
3 marks

hydrazine-displayed-formula

Hydrazine, N2H4 can react with hydrogen peroxide in an exothermic reaction, according to the following equation. 

N2H4 (g) + 2H2O2 (l) → N2 (g) + 4H2O (g)   ΔrHϴ = -789 kJ mol-1 

Some enthalpy data for a different chemical reaction is shown below.

Chemical bond

ΔHϴ (kJ mol-1)

N-N

+163

N≡N

+944

O-H

+463

O-O

+146


Using the reaction equation and the data in the table above, calculate the value of
the N-H bond in hydrazine.

4d
Sme Calculator
1 mark

Often when a bond enthalpy is calculated, it is a different value to the one which is quoted in a data book. State a reason for this.

Did this page help you?

5a
Sme Calculator
5 marks

Alcohols are often used as fuels. They can be used to fill spirit burners, which can be used in calorimetry practicals by students.
A teacher set a class a calorimetry practical to calculate the enthalpy of combustion of ethanol, propan-1-ol and butan-1-ol. The set up that the student used for the experiment is shown

spirit-burner-calorimetry

The table below shows the change in mass when using the ethanol spirit burner. 

Mass of spirit burner before (g)

Mass of spirit burner after (g)

197.6

195.1


The temperature increased by 34.5 oC during the reaction.

The specific heat capacity of the water is 4.18 J g-1 K-1.

Using the students results calculate the enthalpy of combustion for ethanol in this experiment.

5b
Sme Calculator
2 marks

Suggest two reasons, excluding heat transfer, why the value which you have calculated from the student’s data in part (a), might not be the same as a value provided in a data book.

5c
Sme Calculator
1 mark

The student repeated the experiment in part (a) with propan-1-ol and butan-1-ol.  

Write an equation for the complete combustion of one mole of butan-1-ol.

5d
Sme Calculator
3 marks

Enthalpy data for butanol, carbon dioxide and water is shown in the table below.

 

C4H9OH

CO2

H2O

Enthalpy of formation, ΔfHϴ (kJ mol-1)

-327.4

-393.5

-285.8

Use the data in the table and your equation in part (c), to calculate the enthalpy of combustion of butan-1-ol.

Did this page help you?

1a2 marks

This question is about the determination of enthalpy changes. 

A student carries out an experiment to determine the enthalpy change of reaction, ΔHr, for the reaction between sodium carbonate and hydrochloric acid. 

Write a balanced symbol equation for this reaction. 

1b6 marks

During this experiment, the student adds 3.50 g of sodium carbonate to 25 cm3 of 2.00 mol dm-3 hydrochloric acid.

Outline a method that the student will use to determine an accurate temperature change for the reaction. 

1c
Sme Calculator
5 marks

The student carried out the method describe in part (b) under standard conditions.

They found that the temperature rose to 31 oC . 

Calculate ΔHr, in kJ mol−1, for this reaction.

The specific heat capacity, c, of the reaction mixture is 4.18 J g−1 K−1.

1d
Sme Calculator
1 mark

Another student completed the same investigation.  The temperature rose from 25.1 oC to 29.5  oC. 

The uncertainty of each reading of the thermometer is plus-or-minus 0.1.

Calculate the percentage uncertainty in the temperature change. 

Did this page help you?

2a
Sme Calculator
4 marks

Nitrogen and oxygen react under certain conditions.

The following reaction forms, N2O, and is a reversible reaction.

2N2 (g)   +   O2 (g)  rightwards harpoon over leftwards harpoon   2N2O (g)

The enthalpy profile diagram for the reaction is shown below. 

3-4_q2a-ocr-a-as--a-level-hard-sq

i)
Calculate the enthalpy change when 480 dm3 of N2O, measured at room temperature and pressure, is formed from nitrogen and oxygen. 
[2]
ii)
What is the enthalpy change of formation? 
[1]
iii)
Calculate the activation energy for the reverse reaction.
[1]
2b
Sme Calculator
3 marks

Another oxide of nitrogen can be used to release large amounts of energy in rockets. 

The equation for this reaction is:

4CH3NHNH2 (l) + 5N2O4 (l) → 4CO2 (g) + 12H2O (g) + 9N2 (g)

Use the data to calculate the enthalpy change of reaction.

Substance ΔH/ kJ mol-1
CH3NHNH2 +53
N2O4 -20
CO2 -395
H2 -240

2c2 marks

Explain, in terms of bond breaking and forming, why this reaction is exothermic.

Did this page help you?

3a
Sme Calculator
4 marks

This question is about enthalpy of combustion. 

A student carried out the following method to determine the enthalpy of combustion of cyclohexane. 

  • Place a pure sample of cyclohexane in a spirit burner
  • Place the spirit burner under a beaker containing 50 g of water 
  • Ignite the cyclohexane 
  • Extinguish the flame after a few minutes 

The results for the table are below: 

 Initial temperature of the water / oC 19.2
 Initial mass of spirit burner and cyclohexane / g 184.730
 Final mass of spirit burner and cyclohexane / g 184.100

The student determined from this experiment that the enthalpy of combustion of cyclohexane is -1216 kJ mol-1.

Use the data to calculate the final temperature of the water in this experiment.

The specific heat capacity of water = 4.18 J K–1 g–1

The relative molecular mass (Mr) of cyclohexane = 84.0

3b2 marks

A 50.0 g sample of water was used in the experiment. 

Explain how you could measure out this mass of water without using a balance. 

3c
Sme Calculator
3 marks

The table gives some standard enthalpies of combustion.

 Substance

C(s)

H2(g)

C6H12(l)

 Standard enthalpy of combustion, ∆Hc / kJ mol–1

−394

−286

−3920

Use these to calculate the enthalpy change for the following reaction:

6C (s)   +   6H2 (g) →    C6H12 (l)

3d3 marks

Other than heat loss, identify three major sources of error in the experiment. 

You must not refer to the precision of equipment in your answer.

Did this page help you?

4a2 marks

A student investigated the temperature change for the neutralisation of malonic acid,  HOOCCH2COOH, and sodium hydroxide solution according to the following method.

  1. Measure 25.0 cm3 of 0.400 mol dm–3 of malonic acid into a beaker.
  2. Record the temperature every minute for three minutes.
  3. On the fourth minute, add 50.0 cm3 of 0.500 mol dm–3 sodium hydroxide solution and stir.
  4. Record the temperature every minute for eight minutes.

The results are shown below. 

3-4_q4a-ocr-a-as--a-level-hard-sq

Explain why the student’s temperature change of 3.5 oC is not an accurate temperature change.

4b
Sme Calculator
5 marks

Another student completed the same investigation and recorded a maximum temperature of 22.4 oC. The student calculated the heat energy, q, for the reaction to be 9.719 x 10-1 kJ.

i)
Use the information in part (a) to estimate the initial temperature for this student’s investigation. 
Assume that both solutions have the same initial temperature, a density of 1.0 g cm-3
and a specific heat capacity of 4.18 J K-1 g-1

[4]

ii)
Use the information in part (a) to explain why your calculated value for the initial temperature is an estimate.

[1]

4c1 mark

Write a balanced symbol equation for the neutralisation of malonic acid with sodium hydroxide solution. 

4d
Sme Calculator
3 marks

Calculate the enthalpy change in this reaction per mole of water. 

Use the information in part (a) and (c) to help you.

Did this page help you?

5a
Sme Calculator
3 marks

This question is about the reactions of ethyne, C2H2.

Ethyne is also known as acetylene, and can be used to cut through steel when used in an oxy-acetylene flame. 

The equation for the complete combustion of ethyne is:

3-4_q5a-ocr-a-as--a-level-hard-sq

Calculate the enthalpy change of combustion of ethyne using the mean bond enthalpies in the table below. 

Bond Mean bond enthalpy / kJ mol-1
C=O +805
O-H +464
C-H +412
O=O +498
Cidentical toC +837

5b1 mark

Give one reason why the value of enthalpy of combustion calculated from bond enthalpies in part (a) is not entirely accurate. 

5c
Sme Calculator
3 marks

Ethyne can also react with hydrogen fluoride, shown in the equation below. 

3-4_q5c-ocr-a-as--a-level-hard-sq

Bond C-H Cidentical toC H-F C-C
Mean bond enthalpy / kJ mol-1  +412 +837 +562 +348

Use the data in the table to calculate a value for the bond enthalpy of a C-F bond in difluoroethane.

5d1 mark

Draw the dot and cross diagram for ethyne, C2H2.

Did this page help you?