Syllabus Edition

First teaching 2014

Last exams 2024

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Bond Enthalpy (DP IB Chemistry: SL)

Exam Questions

2 hours43 questions
1a2 marks

During chemical reactions, enthalpy changes occur as bonds are broken and formed.

i)
Thermal energy is needed to overcome the attractive forces between atoms. In terms of thermal energy, name the process where bonds are broken.

ii)
When bonds are formed, thermal energy is released to the surroundings. In terms of thermal energy, name the process where bonds are made.
1b5 marks

The energy level diagram for an endothermic reaction is shown below.

1-19

Complete the diagram by labelling parts A to E. 

1c1 mark

An element X undergoes complete combustion according to the following equation. The enthalpy change, ΔH, and activation energy, Ea, for this reaction are -520 kJ mol-1 and +630 kJ mol-1 respectively. Deduce whether this reaction is exothermic or endothermic. 

X + O2 → XO2

1d2 marks

Define the term average bond enthalpy.

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2a1 mark

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

2b
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4 marks

Use section 11 of the data booklet to calculate the enthalpy change, in kJ mol-1, for the following reaction.    

Cl2 + H2 → 2HCl 

2c1 mark

State whether the energy change for the reaction in part (b) is endothermic or exothermic.

2d
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4 marks

Using section 11 of the data booklet, calculate the enthalpy change of reaction, ΔHr, in kJ mol-1 for the following reaction.

CH4 + Cl2 → CH3Cl + HCl 

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3a2 marks

Draw the Lewis structure of an oxygen molecule, O2.

3b1 mark

State the type of energy in the stratosphere responsible for the break down of the oxygen molecule. 

3c2 marks

State the equation for the formation of ozone and whether this reaction is endothermic or exothermic.

3d1 mark

State the name of the type of compound that is responsible for the disruption of the temperature regulation in the stratosphere.

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4a2 marks

Using displayed formulae, write the equation for the reaction of ethene with water to form ethanol.

4b
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4 marks

Using section 11 in the data booklet calculate the enthalpy change of reaction, ΔHr, for the reaction of ethene with water. 

4c1 mark

Define bond dissociation energy.

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1a1 mark

Explain what is meant by the standard enthalpy change of reaction.

1b3 marks

An enthalpy level diagram for the reaction between solid ammonium nitrate and water is shown below.

Figure 15-3-ib-chemistry-sq-q1b-medium 

i)
Give the sign of ΔH for the reaction and state whether the reaction is endothermic or exothermic 

ii)
State the relative strength of the chemical bonds in the products and in the reactants.
1c3 marks

The enthalpy of combustion for propanol is, ΔHcϴ, is -2021 kJ mol-1. Draw a labelled energy level diagram for this reaction.

1d3 marks

Explain why the strength of the hydrogen halide bonds decreases down the group.

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2a3 marks

Enthalpy changes can be found using bond enthalpy data. Some bond enthalpy values are shown below in Table 1.

Table 1

Bond

Mean Bond Enthalpy ΔHϴ (kJ mol-1)

C-C

346

C-H

414

H-H

436

 

 The balanced equation for the reaction between methane and propane is

CH4 (g) +  CH3CH2CH3 (g) → CH3CH2CH2CH3 (g) + H2 (g)

Use the equation and bond enthalpy data to calculate the enthalpy change for the above reaction.                                  

2b1 mark

Define the term average bond enthalpy.

2c3 marks

Enthalpy changes can be found using bond enthalpy data. Some bond enthalpy values are shown below in Table 2. 

Table 2

Bond

Mean Bond Enthalpy ΔHϴ (kJ mol-1)

C=C

614

C-H

414

O-H

463

C=O

804

O=O

498

 

The balanced equation for the combustion of ethene is

C2H4 (g) + 3O2 (g) → 2CO2 (g) + 2H2O (l)

Use the equation and bond enthalpy data to calculate the enthalpy of combustion of ethene.

2d1 mark

Bond enthalpies can be found using Hess’s Law or from experimental data.

Outline the difference between the two ways of finding bond enthalpy.

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3a1 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 enthalpy of combustion of butane.

3b3 marks

Define the term standard enthalpy of combustion, ΔHϴc.

3c3 marks

Table 1 below contains bond enthalpy data for the reaction shown in part (a).

Table 1

 

C-C

C-H

O=O

C=O

O-H

Mean bond enthalpy (kJ mol-1)

346

414

498

804

463

 

Using the data in Table 1 and the equation in part (a), calculate the enthalpy change of combustion of butane.

3d1 mark

In the absence of sufficient oxygen, butane will undergo incomplete combustion.

Write an equation for the incomplete combustion of butane.

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4a1 mark

Use the energy level diagram to determine the activation energy, Ea, for the given reaction in Figure 1.

Figure 1

5-3-ib-chemistry-sq-q4a-medium

4b3 marks

Ethene can be hydrated via the following reaction:

C2H4 (g) + H2O (g) → C2H5OH (g)

Table 1

Bond

C-C

C=C

C-H

C-O

O-H

Mean bond enthalpy (kJ mol-1)

346

614

414

358

463

 

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

4c2 marks

Explain why the value to your answer to part (b) is different from the data book value for the hydration of ethene.

C2H4 (g) + H2O (g) → C2H5OH (g)

4d3 marks

Table 2 below has some enthalpy data for a different chemical reaction. Hydrazine, N2H4 can react with hydrogen peroxide in an exothermic reaction, as shown below.

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

The structure of hydrazine is shown in Figure 1.

Figure 1

5-3-ib-chemistry-sq-q4d-medium 

Table 2

Bond

Mean Bond Enthalpy ΔHϴ (kJ mol-1)

N-N

+158

N≡N

+945

O-H

+463

O-O

+144

 

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

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5a2 marks

The bond enthalpies, in kJmol-1, of oxygen-oxygen single and double bonds are shown below in Table 1.

Table 1

O=O

O-O

498

144

Predict, with a reason, the bond enthalpy of the oxygen-oxygen bond in ozone, O3.

5b2 marks

Ozone is formed in the upper atmosphere in a two-step process as shown below:

            Step A:            O2 →  2O
            Step B:            O2    +     O
    →   O3

Ozone is naturally lost through the decomposition of ozone:

            Step C:            O3   O2    +     O   

i)
Identify, with a reason, which of the three steps is exothermic. 

ii)
Identify which of the steps is endothermic. Explain with reference to the bonding 
5c2 marks

Draw an enthalpy level diagram to represent the three steps A, B and C shown in part (b). Clearly label the position of oxygen, ozone, and the oxygen radical.

5d2 marks

What can be deduced from the fact that ozone absorbs UV radiation in the region of 340 nm and molecular oxygen in the region of 242 nm?

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1a2 marks

Define the term average bond enthalpy.

1b
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3 marks

Determine the bond dissociation energy, in kJ mol-1, for one mole of O−F bonds using the following equation and section 11 of the data booklet. Give your answer to 3 significant figures. 

F(g) + ½O(g) → OF2 (g)    ΔHr = +28 kJ mol-1

[3]

1c2 marks

The reaction of ethanoyl chloride, CH3COCl , and ethanol form an ester. State the equation for this reaction.

1d
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2 marks

Use section 11 in the data booklet to deduce the energy required, in kJ mol-1, to break the bonds.

1e
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3 marks

Deduce the energy released, in kJ mol-1, when the bonds are formed and therefore the enthalpy change for the reaction.

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2a
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2 marks

Methane reacts violently with fluorine to form carbon tetrafluoride and hydrogen fluoride

Formulate the equation for this reaction. 

2b
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4 marks

Use your answer to part a) and section 10 of the data booklet to calculate the following:

i)
The energy required, in kJ, to break the bonds for the reaction between methane and fluorine.
[1]
ii)
The energy released, in kJ, to form the bonds for the reaction between methane and fluorine.
[1]
iii)
The enthalpy change, ΔHr, in kJ mol-1 for this reaction. 
[2]

2c2 marks

A student suggested that one reason for the high reactivity of fluorine is a weak F−F bond. State whether the student is correct. Justify your answer. 

2d3 marks

Sketch a labelled energy diagram for the reaction of methane and fluorine.

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3a2 marks

Hydrazine has the formula N2H4 and is used as a rocket fuel (e.g. for the Apollo moon rockets). It burns in the following reaction for which the enthalpy change is -583 kJ mol-1.

N2H4 (g) + O2 (g) → N2 (g) + 2H2O (g)


Sketch the Lewis structure of hydrazine, N2H4.

3b
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3 marks

Use section 11 of the Data booklet and the information in part a) to deduce the bond enthalpy, in kJ mol-1, for the N-N bond.

3c1 mark

Outline why the value of enthalpy of reaction calculated from bond enthalpies is less accurate.

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4a3 marks

Ozone prevents UV radiation emitted from the Sun reaching the surface of the Earth. Draw the resonance Lewis structures of ozone. 

4b4 marks

By using equations, state the environmental impact of CFCs on the ozone layer. 

4c3 marks

The destruction of ozone is a significant environmental issue as ozone can absorb frequencies of ultraviolet radiation that oxygen can not. 

Explain with reference to the structure and bonding of oxygen and ozone why this occurs. 

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