DEPARTAMENTO DE 
CIENCIAS NATURALES 	
Year 11 IB Chemistry
Term test    
Topics 5 and 15 (60 minutes)	
Name and surnames 		
Date 	_________________________     	 	

1.	Methanol is made in large quantities as it is used in the production of polymers and in fuels.
The enthalpy of combustion of methanol can be determined theoretically or experimentally.
		CH3OH(l) + 1O2(g) → CO2(g) + 2H2O(g)
 
(a)	Using the information from Table 10 of the Data Booklet, determine the theoretical enthalpy of combustion of methanol.
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(3)

(b)	The enthalpy of combustion of methanol can also be determined experimentally in a school laboratory. A burner containing methanol was weighed and used to heat water in a test tube as illustrated below.
		
	The following data were collected.

Initial mass of burner and methanol / g	80.557	
Final mass of burner and methanol / g	80.034	
Mass of water in test tube / g	20.000	
Initial temperature of water / °C	21.5	
Final temperature of water / °C	26.4	
 
(i)	Calculate the amount, in mol, of methanol burned.
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(2)

(ii)	Calculate the heat absorbed, in kJ, by the water.
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(3)
 
(iii)	Determine the enthalpy change, in kJ mol–1, for the combustion of 1 mole of methanol.
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(2)
 
(c)	The Data Booklet value for the enthalpy of combustion of methanol is –726 kJ mol–1.
Suggest why this value differs from the values calculated in parts (a) and (b).
(i)	Part (a)
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(1)
(ii)	Part (b)
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(1)
(Total 12 marks)

2.	If white anhydrous copper(II) sulfate powder is left in the atmosphere it slowly absorbs water vapour giving the blue pentahydrated solid.
	CuSO4(s) + 5H2O(l) → CuSO4•5H2O(s)
(anhydrous)	(pentahydrated)
	It is difficult to measure the enthalpy change for this reaction directly. However, it is possible to measure the heat changes directly when both anhydrous and pentahydrated copper(II) sulfate are separately dissolved in water, and then use an energy cycle to determine the required enthalpy change value, ∆Hx, indirectly.

(a)	To determine ∆H1 a student placed 50.0 g of water in a cup made of expanded polystyrene and used a data logger to measure the temperature. After two minutes she dissolved 3.99 g of anhydrous copper(II) sulfate in the water and continued to record the temperature while continuously stirring. She obtained the following results.

(i)	Calculate the amount, in mol, of anhydrous copper(II) sulfate dissolved in the 50.0 g of water.
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(1)
 
(ii)	Determine what the temperature rise would have been, in °C, if no heat had been lost to the surroundings.
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(2)
 
(iii)	Calculate the heat change, in kJ, when 3.99 g of anhydrous copper(II) sulfate is dissolved in the water.
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(2)
 
(iv)	Determine the value of ∆H1 in kJ mol–1.
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(1)
 
(b)	To determine ∆H2, 6.24 g of pentahydrated copper(II) sulfate was dissolved in 47.75 g of water. It was observed that the temperature of the solution decreased by 1.10 °C.
(i)	Calculate the amount, in mol, of water in 6.24 g of pentahydrated copper(II) sulfate.
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(2)
 
(ii)	Determine the value of ∆H2 in kJ mol–1.
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(2)
 
(iii)	Using the values obtained for ∆H1 in (a) (iv) and ∆H2 in (b) (ii), determine the value for ∆Hx in kJ mol–1.
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(1)
 
(c)	The magnitude (the value without the + or – sign) found in a data book for ∆Hx is 78.0 kJ mol–1.
(i)	Calculate the percentage error obtained in this experiment. (If you did not obtain an answer for the experimental value of ∆Hx then use the value 70.0 kJ mol–1, but this is not the true value.)
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(1)
 
(ii)	The student recorded in her qualitative data that the anhydrous copper(II) sulfate she used was pale blue rather than completely white. Suggest a reason why it might have had this pale blue colour and deduce how this would have affected the value she obtained for ∆Hx.
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(Total 14 marks)

3.	One important property of a rocket fuel mixture is the large volume of gaseous products formed which provide thrust. Hydrazine, N2H4, is often used as a rocket fuel. The combustion of hydrazine is represented by the equation below.
		N2H4(g) + O2(g) → N2(g) + 2H2O(g)	∆HcO = –585 kJ mol–1
 
(a)	Hydrazine reacts with fluorine to produce nitrogen and hydrogen fluoride, all in the gaseous state. State an equation for the reaction.
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(2)
 
(b)	Draw the Lewis structures for hydrazine and nitrogen.

(2)
 
(c)	Use the average bond enthalpies given in Table 10 of the Data Booklet to determine the enthalpy change for the reaction in part (a) above.
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(3)

(d)	Based on your answers to parts (a) and (c), suggest whether a mixture of hydrazine and fluorine is a better rocket fuel than a mixture of hydrazine and oxygen.
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(2)
(Total 9 marks)

4.	Consider the following reaction.
		2CH3OH(g) + H2(g) → C2H6(g) + 2H2O(g)
 
(a)	The standard enthalpy change of formation for CH3OH(g) at 298 K is –201 kJ mol–1 and for H2O(g) is –242 kJ mol–1. Using information from Table 11 of the Data Booklet, determine the enthalpy change for this reaction.
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(2)
 
(b)	The standard entropy for CH3OH(g) at 298 K is 238 J K–1 mol–1, for H2(g) is
131 J K–1 mol–1 and for H2O(g) is 189 J K–1 mol–1. Using information from Table 11 of the Data Booklet, determine the entropy change for this reaction.
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(2)

(c)	Calculate the standard change in free energy, at 298 K, for the reaction and deduce whether the reaction is spontaneous or non-spontaneous.
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(3)
(Total 7 marks)

5.	(i)	Define the terms lattice enthalpy and electron affinity.

(2)
 
(ii)	Use the data in the following table and from the data booklet to construct the Born-Haber cycle for sodium chloride, NaCl, and determine the lattice enthalpy of NaCl(s).
Na(s) + Cl2(g) → NaCl(g)	∆Hο = –411 kJ mol–1
Na(s) → Na(g)	∆Hο = +108 kJ mol–1
(4)
 
(iii)	Describe the structure of sodium chloride.
(2)
(Total 8 marks)