A-LEVEL CHEMISTRY

ASSESSMENT POINT 1
PAPER 1
(TOPICS 10 and 11)
Answer all questions
Max 80 marks

	
Name		……………………………………………………………..

Mark		……../80	……....%	Grade ………

1.	This question is about bond dissociation enthalpies and their use in the calculation of enthalpy changes.
(a)     Define bond dissociation enthalpy as applied to chlorine.
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(2)
(b)     Explain why the enthalpy of atomisation of chlorine is exactly half the bond dissociation enthalpy of chlorine.
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(1)

(c)     The bond dissociation enthalpy for chlorine is +242 kJ mol−1 and that for fluorine is +158 kJ mol−1. The standard enthalpy of formation of ClF(g) is −56 kJ mol−1.
(i)      Write an equation, including state symbols, for the reaction that has an enthalpy change equal to the standard enthalpy of formation of gaseous ClF
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(1)
(ii)     Calculate a value for the bond enthalpy of the Cl – F bond.
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(2)
(iii)    Calculate the enthalpy of formation of gaseous chlorine trifluoride, ClF3(g). Use the bond enthalpy value that you obtained in part (c)(ii).
(If you have been unable to obtain an answer to part (c)(ii), you may assume that the Cl – F bond enthalpy is +223 kJ mol−1. This is not the correct value.)
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(3)
(iv)    Explain why the enthalpy of formation of ClF3(g) that you calculated in part (c)(iii) is likely to be different from a data book value.
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(1)
(d)     Suggest why a value for the Na – Cl bond enthalpy is not found in any data book.
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(1)
(Total 11 marks)

2.          Ammonia can be manufactured by the Haber Process.
The equation for the reaction that occurs is shown below.
N2(g) + 3H2(g)  2NH3(g)
(a)     The table below contains some bond enthalpy data.
 
 	N  N	H–H	N–H	
Mean bond enthalpy / kJ mol–1	944	436	388	
(i)      Use data from the table to calculate a value for the enthalpy of formation for one mole of ammonia.
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(3)
(ii)     A more accurate value for the enthalpy of formation of ammonia is
–46 kJ mol–1.
Suggest why your answer to part (a) (i) is different from this value.
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(1)
(b)     The table below contains some entropy data.
 
 	H2(g)	N2(g)	NH3(g)	
Sο / J K–1 mol–1	131	192	193	
Use these data to calculate a value for the entropy change, with units, for the formation of one mole of ammonia from its elements.
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(3)

(c)     The synthesis of ammonia is usually carried out at about 800 K.
(i)      Use the ΔH value of –46 kJ mol–1 and your answer from part (b) to calculate a value for ΔG, with units, for the synthesis at this temperature.
(If you have been unable to obtain an answer to part (b), you may assume that the entropy change is –112 J K–1 mol –1. This is not the correct answer.)
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(3)
(ii)     Use the value of ΔG that you have obtained to comment on the feasibility of the reaction at 800 K.
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(1)
(Total 11 marks)
 
3.      When a mixture of 0.345 mol of PCl3 and 0.268 mol of Cl2 was heated in a vessel of fixed volume to a constant temperature, the following reaction reached equilibrium.
PCl3(g)   +   Cl2(g)      PCl5(g)           DH = –93 kJ mol–1
At equilibrium, 0.166 mol of PCl5 had been formed and the total pressure was 225 kPa.
(a)     (i)      Calculate the number of moles of PCl3 and of Cl2 in the equilibrium mixture.
Moles of PCl3 ......................................................................................
Moles of Cl2 .........................................................................................
(ii)     Calculate the total number of moles of gas in the equilibrium mixture.
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(3)
(b)     Calculate the mole fraction and the partial pressure of PCl3 in the equilibrium mixture.
Mole fraction of PCl3 .....................................................................................
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Partial pressure of PCl3 ................................................................................
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(3)

(c)     (i)      Write an expression for the equilibrium constant, Kp, for this equilibrium.
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(ii)     The partial pressures of Cl2 and PCl5 in the equilibrium mixture were 51.3 kPa and 83.6 kPa, respectively, and the total pressure remained at 225 kPa. Calculate the value of Kp at this temperature and state its units.
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(4)
(d)     State the effect on the mole fraction of PCl3 in the equilibrium mixture if
(i)      the volume of the vessel were to be increased at a constant temperature,
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(ii)     the temperature were to be increased at constant volume.
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(2)
(Total 12 marks)
 
4.   (a)     Define the term electron affinity for chlorine.
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(2)

(b)     Complete this Born−Haber cycle for magnesium chloride by giving the missing species on the dotted lines. Include state symbols where appropriate.
The energy levels are not drawn to scale.

(6)
(c)     Table 1 contains some enthalpy data.
Table 1
 
 	 	Enthalpy change / kJ mol−1	
 	Enthalpy of atomisation of magnesium	+150	
 	Enthalpy of atomisation of chlorine	+121	
 	First ionisation energy of magnesium	+736	
 	Second ionisation energy of magnesium	+1450	
 	Enthalpy of formation of magnesium chloride	−642	
 	Lattice enthalpy of formation of magnesium chloride	−2493	
Use your Born−Haber cycle from part (b) and data from Table 1 to calculate a value for the electron affinity of chlorine.
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 (3)

(d)     Table 2 contains some more enthalpy data.
Table 2
 
 	 	Enthalpy change / kJ mol−1	
 	Enthalpy of hydration of Mg2+ ions	−1920	
 	Enthalpy of hydration of Na+ ions	−406	
 	Enthalpy of hydration of Cl− ions	−364	

(i)      Explain why there is a difference between the hydration enthalpies of the magnesium and sodium ions.
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(2)
(ii)     Use data from Table 1 and Table 2 to calculate a value for the enthalpy change when one mole of magnesium chloride dissolves in water.
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(2)
(Total 15 marks)

5.      When potassium nitrate (KNO3) dissolves in water the value of the enthalpy change
ΔH = +34.9 kJ mol–1 and the value of the entropy change ΔS = +117 J K–1 mol–1.
(a)     Write an equation, including state symbols, for the process that occurs when potassium nitrate dissolves in water.
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(1)
(b)     Suggest why the entropy change for this process is positive.
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(1)
(c)     Calculate the temperature at which the free-energy change, ΔG, for this process is zero.
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(3)
(d)     (i)      Deduce what happens to the value of ΔG when potassium nitrate dissolves in water at a temperature lower than your answer to part (c).
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(1)
(ii)     What does this new value of ΔG suggest about the dissolving of potassium nitrate at this lower temperature?
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(1)
(Total 7 marks)

6.	Hydrogen can be manufactured from the reaction of steam with methane.
CH4(g)  +  H2O(g)    CO(g)  +  3H2(g)
(a)     The table contains some enthalpy of formation and entropy data.
 
 	Substance	ΔHf / kJ mol−1	S / J K−1 mol−1	
 	CH4(g)	−75	186	
 	H2O(g)	−242	189	
 	CO(g)	−111	198	
 	H2(g)	0	131	
 	CO2(g)	−394	214	
 
(i)      Use data from the table to calculate the enthalpy change, ΔH, for the reaction of steam with methane.
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(3)
(ii)     Use data from the table to calculate the entropy change, ΔS, for the reaction of steam with methane.
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(2)

(b)     Use your values of ΔH and ΔS from parts (a)(i) and (a)(ii) to calculate the temperature above which this reaction is feasible.
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 (4)
(c)     The temperature used for this manufacture of hydrogen is usually about 1300 K.
Suggest one reason, other than changing the position of equilibrium, why this temperature is used rather than the value that you calculated in part (b).
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(1)
(d)     Hydrogen can also be obtained by reaction of carbon monoxide with steam.
CO(g) + H2O(g)  CO2(g) + H2(g)   ΔH = −41 kJ mol−1,   ΔS = −42 J K−1 mol−1
(i)      Explain, using a calculation, why this reaction should not occur at 1300 K.
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 (3)
(ii)     Explain how the conditions for the reaction could be changed to allow this reaction to take place.
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(2)
(Total 15 marks)

7.	Consider the following process that represents the melting of ice
 
H2O(s)		H2O(I)	∆Hϴ = +6.03 kJ mol–1,	∆Sϴ = +22.1 J K–1 mol–1	
(a)     State the meaning of the symbol ϴ in ∆Hϴ .
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(1)
(b)     Use your knowledge of bonding to explain why ∆Hϴ is positive for this process.
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(2)
(c)     Calculate the temperature at which ∆Gϴ = 0 for this process. Show your working.
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(3)
(d)     The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 °C when it freezes.
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(1)
(e)     Pure ice can look pale blue when illuminated by white light. Suggest an explanation for this observation.
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(2)
(Total 9 marks)