Syllabus Edition

First teaching 2014

Last exams 2024

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Hess's Law (DP IB Chemistry: SL)

Exam Questions

3 hours44 questions
1a1 mark

State Hess’s Law.

1b1 mark

State the type of system in which the total amount of matter present is always constant. 

1c1 mark

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

5-2-ib-sl-sq-easy-q1c-hesss-law-cycle

1d3 marks

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

5-2-ib-sl-sq-easy-q1d-hesss-law-cycle-cuco3

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2a2 marks

Define standard enthalpy of formation, ΔHf

2b
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8 marks

Write an equation to show the enthalpy of formation of 1 mole of the following compounds. Include state symbols in your equations. 

Methanol, CH3OH ………………………………..

Carbon dioxide, CO………………………………..

Ethane, C2H………………………………..

2c3 marks

Using the equations given, construct a Hess’s Law cycle for the following reaction. Include the values for ΔHf  in your cycle.  

BaCl2 (s) + Zn (s) → Ba (s) + ZnCl(s)

Ba (s) + Cl2 (g) → BaCl(s)      ΔHf = -858.6 kJ mol-1

Zn (s) + Cl2 (g) → ZnCl2 (s)         ΔHf = -415.1 kJ mol-1

2d
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3 marks

Calculate the enthalpy of reaction, ΔHrfor the reaction given in part (c).

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3a4 marks

Aluminium oxide reacts with magnesium to form magnesium oxide and aluminium in a displacement reaction via the following reaction.Construct a Hess’s Law cycle for this reaction

Al2O3 (s) + 3Mg (s) → 3MgO (s) + 2Al (s)

 

Enthalpy of formation Enthalpy of formation (kJ mol-1)
ΔHf (Al2O3) -1675.7 
ΔHf (MgO) -601.7 
ΔHf (Mg)  
ΔHf (Al)  

3b1 mark

Outline why no values are listed for Al (s) and Mg (s) in the table given in part (a).

3c
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2 marks

Calculate the enthalpy change of reaction, ΔHr, for the reaction in part (a).

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4a3 marks

Determine the enthalpy change of reaction, ΔHr, for the following equations if they are reversed. 

2Na + Cl2 → 2NaCl           ΔHr = -790 kJ ……………………….

C2H+ H2 → C2H6               ΔHr = -65.6 kJ ………………………

2H2O → 2H+ O2             ΔH= +571 kJ ……………………….

4b
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1 mark

Using the information given in part (a), determine the enthalpy change for the following reaction.

2C2H4 + 2H2 → 2C2H6

4c
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1 mark

Using the information in the table, deduce which equation should be reversed to determine the enthalpy change for the following reaction. 

SiO2 + 3C → SiC + 2CO

Equation number  Equation Enthalpy change (kJ)
1 Si + O2 → SiO2 -911
2 2C + O2 → 2CO -211
3 Si + C → SiC -65.3

4d
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2 marks

Use the information in part (c) to produce an overall cancelled down equation which can be used to determine the overall enthalpy change for the following reaction. 

SiO2 + 3C → SiC + 2CO

4e
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2 marks

Deduce the overall enthalpy change, in kJ, using the information in part (c) for the reaction SiO2 + 3C → SiC + 2CO

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5a1 mark

State the equation required to calculate the enthalpy change of reaction, ΔHr, given enthalpy of formation, ΔHf, data. 

5b
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3 marks

Using section 12 in the data booklet and the data in the table calculate the enthalpy change of reaction, ΔHr, for the following reaction.

SO2 (g) + 2H2S (g) → 3S (s) + 2H2O (l) 

  SO2 (g) H2S (g)
ΔHf (kJ mol-1 -297 -20.2
5c2 marks

Show how the equations can be used to produce an alternative route for this reaction.

C2H4 + H2 → C2H6

  ΔH (kJ mol-1)
C2H4 + 3O2 → 2CO2 + 2H2O -1411
C2H6 + 3½O2 2CO(g) + 3H2O -1560
H2 + ½O2 → H2O -285.8

5d
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1 mark

Calculate ΔH

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1a3 marks

Define the term standard enthalpy of formation, ΔHϴf.

1b2 marks

State Hess’s Law.

1c2 marks

The following equation represents the second step in the extraction of titanium, using the Kroll process:

TiCl4 (g) + 4Na (l) → 4NaCl (s) + Ti (s)

Use the standard formation data shown in Table 1 to calculate the enthalpy change for the reaction, ΔHϴr.

Table 1 

 

TiCl4 (g)

Na (l)

NaCl (s)

Ti (s)

ΔHϴf (kJ mol-1)

-720

+3

-411

0

 

1d3 marks

Construct a Hess’s Law cycle for the reaction of calcium fluoride, CaF2 (s) , and sulfuric acid, H2SO4 (aq).

CaF2 (s) + H2SO4 (aq) → 2HF (g) + CaSO4 (s)

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2a3 marks

Define the term standard enthalpy of combustion, ΔHϴc.

2b2 marks

Write an equation for the complete combustion of propanol, CH3CH2CH2OH (l).

2c3 marks

Construct a Hess’s Law cycle for the complete combustion of propanol.

Table 1

 

CH3CH2CH2OH (l)

O2 (g)

CO(g)

H2O (l)

ΔHϴf (kJ mol-1)

-303

0

-393.5

-285.8

 

2d3 marks

Use the data given in Table 1 in part (d) to calculate the enthalpy change of the reaction, ΔHϴr.

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3a2 marks

Urea can be used as a fertiliser and is manufactured by the reaction of ammonia and carbon dioxide via the following equation. 

2NH3 (g) + CO2 (g) → NH2CONH2 (s) + H2O (l)

Using the data in Table 1 calculate the enthalpy change for the formation of urea, ΔHrө.

Table 1

 

NH3 (g)

NH2CONH2 (s)

CO2  (g)

H2O (l)

ΔHϴf (kJ mol-1)

-46.2

-333.2

-393.5

-285.8

3b3 marks

Ammonia reacts with oxygen to produce steam and nitrogen(II) oxide. Draw a Hess’s Law cycle which could be used to calculate the enthalpy change of the reaction using formation data.

3c3 marks

Use Hess’s Law and the information below to calculate the enthalpy change, ΔHϴr, for the conversion of one mole of ethene and one mole of hydrogen to one mole of ethane. 

C2H4 (g) + 3O2 (g) → 2CO2 (g) + 2H2O (l)                  ΔHϴr = -1411 kJ mol-1

C2H6 (g) + 3.5O2 (g) → 2CO2 (g) + 3H2O (l)               ΔHϴr = -1560 kJ mol-1

H2 (g) + 0.5O2 (g) → H2O (l)                                       ΔHϴr = –286 kJ mol-1

3d3 marks

Use Hess’s Law and the information below to calculate the enthalpy change for the conversion of one mole of solid carbon into carbon monoxide.

C (s) + O2 (g) → CO(g)                    ΔHϴr = - 393.5 kJ mol-1

CO (g) + ½O2 (g)  → CO(g)            ΔHϴr = - 283.5 kJ mol-1

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4a2 marks

Define the term standard enthalpy of reaction, ΔHϴr.

4b4 marks

Use Hess’s Law and the information below to calculate the enthalpy change, ΔHϴr, for the conversion of methane and ammonia to form hydrogen cyanide and hydrogen.

N2 (g) + 3H2 (g) → 2NH3 (g)                           ΔHϴr = -91.8 kJ

C (s) + 2H2 (g) → CH4 (g)                                        ΔHϴr = -74.9 kJ

H2 (g) + 2C (g) + N2 (g) → 2HCN (g)              ΔHϴr = 270.3 kJ

4c3 marks

Using your answer to part (b) draw a reaction profile diagram for the reaction outlined.

4d1 mark

Draw the Lewis structure for hydrogen cyanide, HCN.

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5a1 mark

Butane, C4H10,  is typically used as fuel for cigarette lighters and portable stoves, a propellant in aerosols, a heating fuel, a refrigerant, and in the manufacture of a wide range of products.

Write an equation for the complete combustion of butane.

5b4 marks

Determine the enthalpy of formation of butane, C4H10 , using the enthalpy of combustion data below.

C (s) + O2 (g) → CO2 (g)                                            ΔHϴf = -394 kJ           

H2 (g) + 0.5O2 (g) → H2O (l)                                       ΔHϴf = -286 kJ

C4H10 (g) + 6.5O2 (g) → 4CO2 (g) + 5H2O (l)             ΔHϴf = -2878 kJ

5c3 marks

Butane can be formed from the hydrogenation of butene. Using the data in Table 1, determine a value for the enthalpy of formation.

Table 1

Bond

Mean Bond Enthalpy ΔHϴ (kJ mol-1)

C-C

346

C-H

414

H-H

436

C=C

614

 

5d1 mark

The data book value for the hydrogenation of butene is -126 kJ mol-1. Suggest why your answer to part (c) may be different to this value.

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1a2 marks

Vanadium is commonly found in different ores such as magnetite, vanadinite and patronite. The vanadium is commonly extracted from these ores by reduction and displacement.

Vanadium can be extracted by the reduction of vanadium pentoxide, V2O5, with calcium at high temperatures, according to the following equation.

V2O5 (s) + 5Ca (s) → 2V (s) + 5CaO (s)

The enthalpy of formation of vanadium pentoxide is -1560 kJ mol-1 and the standard enthalpy change for the reaction is -1615 kJ mol-1


Construct a Hess’s Law cycle for this reaction.

1b
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3 marks

Use the data in part a) to calculate the enthalpy of formation, ΔHf, of calcium oxide in kJ mol-1

1c2 marks
Define standard enthalpy of neutralisation, ΔHneut

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2a3 marks

The compound diborane, B2H6, is used as a rocket fuel. The equation for the combustion of diborane is shown below.


B2H6 (g) + 3O2 (g) → B2O3 (s) + 3H2O (l)

Calculate the standard enthalpy change of this reaction using the following data

I. 2B (s) + 3H2 (g) → B2H6 (g)          ΔH = 36 kJ mol-1
II. H2 (g) + ½O2 (g) → H2O (l)          ΔH = -286 kJ mol-1

III. 2B (s) + 1½O2 (g) → B2O3 (s)       ΔH = -1274 kJ mol-1

2b1 mark

Ethyne, C2H2, is a useful gas as it gives a high temperature flame when burnt with oxygen. State the equation for the combustion of ethyne gas.

2c3 marks

Use your answer to part b) to construct a Hess's Law cycle for the combustion of ethyne gas.

2d
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3 marks

Use sections 12 and 13 in the data booklet to determine the enthalpy of formation, ΔHf, of ethyne gas. 

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3a3 marks

Coal gasification converts coal into a combustible mixture of carbon monoxide and hydrogen known as coal gas, in a gasifier. 

H2O (l) + C (s) → CO (g) + H2 (g) 

Using the following equations, calculate the enthalpy change of reaction, ΔHr, in kJ for cola gasification. 

I. 2C (s) + O2 (g) → 2CO (g)            ΔH = -222 kJ

II. 2H(g) + O2 (g) → 2H2O (g)        ΔH = -484 kJ

III. H2O (l) → H2O (g)                       ΔH = +44 kJ

[3]
3b
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3 marks

This coal gas can be used as a fuel as the following equation shows.

CO (g) + H2 (g) + O2 (g) → CO2 (g) + H2O (g) 

Calculation the enthalpy change of reaction, ΔHr, in kJ for this combustion reaction from the following equations.

I. 2C (s) + O2 (g) → 2CO (g)       Δ= -222 kJ

II. C (s) + O2 (g) → CO2 (g)         Δ= -394 kJ

III. 2H2 (g) + O2 (g) → 2H2O (g) Δ= -484 kJ

3c2 marks

Blending amounts of alternative fuel with conventional fuel is one way to replace petroleum. A fuel blend of 51% to 83% ethanol and the remaining being gasoline is known as E85.

If the fuel blend is vaporised before combustion, predict whether the amount of energy released would be greater, less or the same. Explain your answer.

3d
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3 marks

Use sections 6 and 13 of the Data booklet to calculate the following.

i)
The amount, in moles, of ethanol in 1 kg of E85 containing 60% ethanol.

[2]

ii)
The energy released, in kJ, by ethanol if 1 kg of E85 is burnt.

[1]

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4a3 marks

Strontium salts have a number of applications such as fireworks, flares, glow in the dark paint and toothpaste for sensitive teeth. The strontium required for these salts can be extracted from the ore strontia, SrO, by displacement with powdered aluminium in a vacuum.

i)
Write a balanced symbol equation, including state symbols, for the reaction of strontia with aluminium.
[2]

ii)
State the role of the aluminium in this reaction.
[1]
4b
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3 marks

The standard enthalpy change for this extraction of strontium is 99.3 kJ mol-1 and the standard enthalpy of formation of aluminium oxide is -1676.7 kJ mol-1

Use this information to calculate the standard enthalpy of formation, ΔHf, in kJ mol-1 of strontia.

4c
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3 marks

Manganese is too brittle for use as a pure metal, so it is often alloyed with other metals. Manganese is used in steel to increase the strength and resistance to wear. Manganese steel (13% Mn) is extremely strong and used for railway tracks, safes and prison bars. Alloys of 1.5% manganese with aluminium are used to make drinks cans due to the improved corrosion resistance of the alloy.

Manganese is extracted from different ores by reduction with carbon monoxide.

Mn2O3 (s) + 3CO (g) → 2Mn (s) + 3CO2 (g)

 

The enthalpy of formation, ΔHf, of Mn2O(s) is −971 kJ mol-1. Use this information and section 12 of the data booklet to calculate the enthalpy change of reaction, ΔHr, in kJ mol-1

4d3 marks

The reaction in part c) reaches equilibrium at high temperatures.

Use your answer to part c) to explain how temperature can be altered to increase the yield of
the reaction and explain the effect that this would have on the rate of reaction.

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