A-LEVEL CHEMISTRY

ASSESSMENT POINT 2
PAPER 1
(TOPICS 10, 11 and 12)
Answer all questions
Max 80 marks

	
Name		……………………………………………………………..

Mark		……../80	……....%	Grade ………

1.     (a)     Ammonia, NH3, reacts with sodium to form sodium amide, NaNH2, and hydrogen.
(i)      Write an equation for the reaction between ammonia and sodium.
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(ii)     Draw the shape of an ammonia molecule and that of an amide ion, NH2-
In each case show any lone pairs of electrons.
NH3                                                  NH2-

(iii)     State the bond angle found in an ammonia molecule.
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(iv)    Explain why the bond angle in an amide ion is smaller than that in an ammonia molecule.
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(6)
(b)     A salt, X, contains 16.2% by mass of magnesium, 18.9% by mass of nitrogen and 64.9% by mass of oxygen.
(i)      State what is meant by the term empirical formula.
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(ii)     Determine the empirical formula of X.
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(3)
(Total 9 marks)

 2.        The table below gives some values of standard enthalpy changes. Use these values to answer the 	questions.
 
Name of enthalpy change	∆Hο /kJ mol–1	
Enthalpy of atomisation of chlorine	+121	
Electron affinity of chlorine	–364	
Enthalpy of atomisation of silver	+289	
First ionisation enthalpy of silver	+732	
Enthalpy of formation of silver chloride	–127	

(a)     Calculate the bond enthalpy of a Cl–Cl bond.
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(1)
(b)     Explain why the bond enthalpy of a Cl–Cl bond is greater than that of a Br–Br bond.
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(2)
(c)     Suggest why the electron affinity of chlorine is an exothermic change.
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(1)

(d)    The diagram below is an incomplete Born–Haber cycle for the formation of silver chloride. The diagram is not to scale.

(i)      Complete the diagram by writing the appropriate chemical symbols, with state symbols, on each of the three blank lines.
(3)
(ii)     Calculate a value for the enthalpy of lattice dissociation for silver chloride.
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(2)

(e)    The enthalpy of lattice dissociation for silver chloride can also be calculated theoretically assuming a perfect ionic model.
(i)      Explain the meaning of the term perfect ionic model.
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(1)
(ii)     State whether you would expect the value of the theoretical enthalpy of lattice dissociation for silver chloride to be greater than, equal to or less than that for silver bromide. Explain your answer.
Theoretical lattice enthalpy for silver chloride .....................................
Explanation .........................................................................................
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(3)
(iii)     Suggest why your answer to part (d) (ii) is greater than the theoretical value for the enthalpy of lattice dissociation for silver chloride.
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(2)
(Total 15 marks)

3.       The sketch graph below shows how the entropy of a sample of water varies with temperature.
	
(a)     Suggest why the entropy of water is zero at 0 K.
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(1)
(b)     What change of state occurs at temperature T1?
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(1)
(c)     Explain why the entropy change, ∆S, at temperature T2 is much larger than that at temperature T1.
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(2)

(d)     It requires 3.49 kJ of heat energy to convert 1.53 g of liquid water into steam at 373 K and 100 kPa.
(i)      Use these data to calculate the enthalpy change, ∆H, when 1.00 mol of
liquid water forms 1.00 mol of steam at 373 K and 100 kPa.
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(ii)     Write an expression showing the relationship between free-energy change, 
∆G, enthalpy change, ∆H, and entropy change, ∆S.
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(iii)     For the conversion of liquid water into steam at 373 K and 100 kPa, 
∆G = 0 kJ mol–1
Calculate the value of ∆S for the conversion of one mole of water into steam under these conditions. State the units.
(If you have been unable to complete part (d)(i) you should assume 
that ∆H = 45.0 kJ mol–1. This is not the correct answer.)
Calculation ..........................................................................................
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Units ....................................................................................................
(6)
(Total 10 marks)

 

4.        	In the second stage, carbon monoxide and hydrogen react according to the following 	equation:	CO(g) + 2H2(g)  CH3OH(g)
A 62.8 mol sample of carbon monoxide was added to 146 mol of hydrogen. When equilibrium was reached at a given temperature, the mixture contained 26.2 mol of methanol at a total pressure of 9.50 MPa. 
Write an expression for the equilibrium constant, Kp, for this reaction. Calculate a value for Kp at this temperature and give its units.
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 (8)
(Total 8 marks)
 
5.       This question concerns the weak acid, ethanoic acid, for which the acid dissociation constant, Ka, has a value of 1.74 × 10–5mol dm–3 at 25 °C.
		
In each of the calculations below, give your answer to 2 decimal places.
(a)	Write an expression for the term pH. Calculate the pH of a 0.150 mol dm–3 solution of ethanoic acid.
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(4)

(b)     A buffer solution is prepared by mixing a solution of ethanoic acid with a solution of sodium ethanoate.
(i)      Explain what is meant by the term buffer solution.
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(ii)     Write an equation for the reaction which occurs when a small amount of hydrochloric acid is added to this buffer solution.
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(3)
(c)     In a buffer solution, the concentration of ethanoic acid is 0.150 mol dm–3 and the concentration of sodium ethanoate is 0.100 mol dm–3.
(i)      Calculate the pH of this buffer solution.
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(ii)     A 10.0 cm3 portion of 1.00 mol dm–3 hydrochloric acid is added to 1000 cm3 of this buffer solution.
Calculate the number of moles of ethanoic acid and the number of moles of sodium ethanoate in the solution after addition of the hydrochloric acid. Hence, find the pH of this new solution.
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(8)
(Total 15 marks)

6.      (a)     At 50°C, the ionic product of water, Kw, has the value 5.48 × 10–14 mol2 dm–6.
(i)      Define the term Kw
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(ii)     Define the term pH
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(iii)     Calculate the pH of pure water at 50 °C. Explain why pure water at 50 °C is still neutral even though its pH is not 7.
Calculation ..........................................................................................
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Explanation ..........................................................................................
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(5)

(b)     At 25°C, Kw has the value 1.00 × 10–14 mol2 dm–6. Calculate the pH at 25 °C of
(i)      a 0.150 mol dm–3 solution of sodium hydroxide,
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(ii)     the solution formed when 35.0 cm3 of this solution of sodium hydroxide is mixed with 40.0 cm3 of a 0.120 mol dm–3 solution of hydrochloric acid.
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(8)
(c)     In a 0.150 mol dm–3 solution of a weak acid HX at 25 °C, 1.80% of the acid molecules are dissociated into ions.
(i)      Write an expression for Ka for the acid HX.
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(ii)     Calculate the value of Ka for the acid HX at this temperature and state its units.
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(5)
(Total 18 marks)

7.	Titration curves labelled A, B, C and D for combinations of different aqueous solutions of acids and bases are shown below.
All solutions have a concentration of 0.1 mol dm–3.
 
(a)     In this part of the question write the appropriate letter in each box.
From the curves A, B, C and D, choose the curve produced by the addition of
 
 	ammonia to 25 cm3 of hydrochloric acid		
 	sodium hydroxide to 25 cm3 of ethanoic acid		
 	nitric acid to 25 cm3 of potassium hydroxide		

(3)

(b)     A table of acid.base indicators is shown below.
The pH ranges over which the indicators change colour and their colours in acid and alkali are also shown.
 
 	Indicator                       pH range            Colour in acid          Colour in alkali	
 	Trapaeolin                     1.3 – 3.0                       red                            yellow
Bromocresol green        3.8 – 5.4                    yellow                           blue
Cresol purple                 7.6 – 9.2                    yellow                          purple
Alizarin yellow              10.1 – 12.0                  yellow                         orange	

(i)      Select from the table an indicator that could be used in the titration that produces curve B but not in the titration that produces curve A.
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(1)
(ii)     Give the colour change at the end point of the titration that produces curve D when cresol purple is used as the indicator.
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(1)
(Total 5 marks)