AS LEVEL CHEMISTRY

2.3.1 ENTHALPY CHANGES

ASSESSED HOMEWORK

Answer all questions
Max 64 marks

	
Name		……………………………………………………………..

Mark		……../64	……....%	Grade ………

1.	The standard enthalpy change of formation of hexane is –199 kJ mol–1.
Using the axes below, show the enthalpy profile diagram for the formation of hexane.
	On your diagram label the enthalpy change of reaction, DH, and the activation energy, Ea.

[Total 3 marks]

2.	The combustion of butane is shown in the equation below.
C4H10(g) + 6 O2(g) ® 4CO2(g) + 5H2O(l)
(i)	The standard enthalpy change of combustion of butane is –2877 kJ mol–1. What does standard mean in this context?
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[1]
(ii)	Define the term enthalpy change of combustion.
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[2]
(iii)	Complete the enthalpy profile diagram for the combustion of butane. Label the activation energy, Ea, and the enthalpy change, DH.

[3]
[Total 6 marks]
 
3.	Define the term standard enthalpy change of combustion.
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[Total 3 marks]

4.	Write an equation, including state symbols, representing the standard enthalpy change of formation of PbO.
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[Total 2 marks]

5.	Alkanes are important hydrocarbons since they are used as fuels in homes and in industry. It is important that the enthalpy changes involved in alkane reactions are known.
(i)	Define the term enthalpy change of formation of a compound.
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[2]
(ii)	Write the equation, including state symbols, that accompanies the enthalpy change of formation of hexane, C6H14(l).
[2]
(iii)	What conditions of temperature and pressure are used when measuring the standard enthalpy change of formation?
temperature ...............................................................
pressure .........................................................
[1]
[Total 5 marks]
 
6.	Propane, C3H8, is a gas at room temperature and pressure. It is used in blow torches to melt the bitumen needed to apply the felt to flat roofs.
Write the equation for the complete combustion of propane.

[Total 2 marks]

7.	This question is about the relationship between bond enthalpies and enthalpy changes of combustion.
Define the term bond enthalpy.
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[Total 2 marks]

8.	There are several oxides of lead. This question is about the enthalpy changes that occur during the reactions of some of these oxides.
(i)	Define the term enthalpy change of formation.
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[2]
(ii)	What are the standard conditions of temperature and pressure used in enthalpy calculations?
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[1]
[Total 3 marks]

 9.	In the vapour state, hydrogen and iodine undergo the following reaction.
		H2(g) + I2(g)  2HI(g)	reaction 2.1
Write an equation, including state symbols, for the bond enthalpy of I - I.
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[Total 2 marks]

10.	Enthalpy changes of reaction can be determined indirectly from average bond enthalpies and standard enthalpy changes.
	The table below shows the values of some average bond enthalpies. 
bond	average bond
enthalpy /kJ mol–1	
C–H	+410	
O–H	+465	
O=O	+500	
C=O	+805	
C–O	+336	
(i)	Why do bond enthalpies have positive values?
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[1]
(ii)	The equation below shows the combustion of methanol, CH3OH, in the gaseous state.
CH3OH(g) + 1½O2(g) ® CO2(g) + 2H2O(g)
	Using the average bond enthalpies in the table above, calculate the enthalpy change of combustion, ΔHc, of gaseous methanol.

ΔHc = ............................................. kJ mol–1
[3]
[Total 4 marks]

11.	Methane burns in air according to the following equation.
		CH4(g) + 2O2(g) ® CO2(g) + 2H2O(g)
(i)	Use the average bond enthalpies given in the table below to calculate a value for the enthalpy change of combustion of methane, DHC.
bond	average bond enthalpy /
kJ mol–1	
C–H	+410	
O–H	+465	
O=O	+500	
C=O	+805	
DHC = ...................................... kJ mol–1
[3]
(ii)	The standard enthalpy change of combustion of methane is -890 kJ mol–1
Suggest why your calculated value differs from this value.
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[1]
[Total 4 marks]

12.	The equations for the combination of gaseous atoms of carbon and hydrogen to form methane, CH4, and ethane, C2H6, are shown below.
C(g) + 4H(g) ® CH4(g)	DH = −1652 kJ mol−1
2C(g) + 6H(g) ® C2H6(g)	DH = −2825 kJ mol−1
Use these data to calculate:
(i)	the bond enthalpy of a C−H bond,
bond enthalpy = ........................................... kJ mol−1
[1]
(ii)	the bond enthalpy of a C−C bond.

bond enthalpy = ........................................... kJ mol−1
[2]
[Total 3 marks]

 13.	The Table below gives values for the standard enthalpy changes of combustion for the first three alkanes.
alkane	formula	DHcq/kJ mol-1	
methane	CH4	–890	
ethane	C2H6	–1560	
propane	C3 H8	–2220	
(i)	Write a balanced equation, including state symbols, illustrating the standard enthalpy change of combustion of propane gas.
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[2]
(ii)	As the chain length in the alkanes increases, the value of DHcq becomes more negative. Use your understanding of bond breaking and bond making to explain this trend.
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[2]

(iii)	Propane can be cracked to form ethene and methane.
	C3H8(g) ® C2H4(g) + CH4(g)
The standard enthalpy change of combustion of ethene, DHcq = –1410 kJ mol-1.
	Use this value together with relevant values in Table above to calculate the enthalpy change of this reaction.

DHcq = ......................... kJmol-1
[3]
[Total 7 marks]

 14.	Metal priming paints often contain ‘red lead’, Pb3O4. Red lead can be made by heating PbO in the presence of air.
	3PbO(s) +  O2 (g) ® Pb3O4(s)
(i)	Use the  values in Table below to calculate the standard enthalpy change for the above reaction.
compound	 /kJ mol–1	
PbO(s)	–217	
Pb3O4(s)	–718	
 = ............................................... kJ mol–1
[3]
(ii)	Red lead can also be obtained by reacting PbO2 with PbO.
	PbO2(s) + 2PbO(s) ® Pb3O4(s)	 = –10 kJ mol–1
	Use the value of  for this reaction, together with the values of  in the table above, to calculate a value for the enthalpy change of formation of PbO2(s).

 = .................................... kJ mol–1
[3]
[Total 6 marks]
15.	Enthalpy changes can be calculated using enthalpy changes of combustion. The table below shows some values for standard enthalpy changes of combustion.
substance	DHcӨ/ kJ mol−1	
C(s)	−394	
H2(g)	−286	
CH4(g)	−890	
Use these values to calculate the standard enthalpy change of the reaction below.
	C(s) + 2H2(g) ® CH4(g)
standard enthalpy change = ........................................... kJ mol−1
[Total 3 marks]

16.	(a)	Methane reacts with steam to produce carbon monoxide and hydrogen. The equation for this process is given below.
CH4(g) + H2O(g) ® CO(g) + 3H2(g)
	The table below shows the standard enthalpy changes of formation for CH4, H2O and CO.
compound	ΔHfo/kJ mol–1	
CH4	–75	
H2O	–242	
CO	–110	
(i)	Define the term enthalpy change of formation.
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[2]
(ii)	In ΔHfo, what are the conditions indicated by the symbol o?
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[1]

(iii)	Write the equation, including state symbols, that represents the standard enthalpy change of formation for carbon monoxide, CO.
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[2]
(iv)	Using the ΔHfo values in the table above, calculate the enthalpy change for the reaction of methane with steam.
CH4(g) + H2O(g) ® CO(g) + 3H2(g)

ΔH = .............................................. kJ mol–1
[3]
 (b)	State one important manufacturing process in which hydrogen is used.
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[1]
[Total 9 marks]