International A LevelChemistryEdexcelExam Questions4: Rates, Equilibria and Further Organic Chemistry4.4 Chemical Equilibria

Chemical Equilibria (Edexcel International A Level Chemistry)

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4.4 Chemical Equilibria

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4.4 Chemical Equilibria

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1a
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Ethanol may be manufactured by the hydration of ethene.

C2H4 (g) + H2O (g) ⇌ C2H5OH (g)     ∆H to the power of ⦵= −45.3 kJ mol−1

In a laboratory investigation of this reaction, 1.00 mol of ethene was mixed with 1.00 mol of steam at 150 °C. At equilibrium, when the total pressure of the system was 50.0 atm, 0.450 mol of ethanol had formed.

i)
Give the expression for the equilibrium constant, Kp, for the reaction.

(1)

ii)

Calculate the equilibrium constant, Kp, for the hydration of ethene at 150 °C.

Include units with your answer.

(5)

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1b3 marks

The manufacture of ethanol is carried out at 230 °C and 70 atm; the overall conversion into ethanol is 95%.

Comment on these conditions in relation to their effect on the equilibrium and the overall yield of ethanol.

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2a
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Esters are used in flavourings and perfumes.
They can be made by reactions involving alcohols.

A flask containing a mixture of 0.200 mol of ethanoic acid and 0.150 mol of ethanol was left at 25°C, in the presence of a catalyst, until equilibrium had been established.

CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)

The ethanoic acid present in the equilibrium mixture required 34.8 cm3 of a 2.50 mol dm–3 solution of sodium hydroxide for complete neutralisation.

i)
Calculate the value of the equilibrium constant, Kc, for this reaction at 25°C.

(4)

ii)
The enthalpy change for this reaction is small.
Explain, by reference to the type and number of bonds being broken and made, how this might have been predicted.

(2)

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2b6 marks

An ester which smells of raspberries can be formed by either of two different reactions.

Reaction 1

HCOOH + (CH3)2CHCH2OH → HCOOCH2CH(CH3)2 + H2O

Reaction 2

HCOCl + (CH3)2CHCH2OH → HCOOCH2CH(CH3)2 + HCl

i)

Name the carboxylic acid, alcohol and catalyst used in Reaction 1.

(2)

HCOOH .............................................                   catalyst .........................

(CH3)2CHCH2OH ..............................

ii)

Explain one advantage and one disadvantage of using Reaction 2 rather than

Reaction 1.

(4)

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1a
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This question is about sulfuric acid and its salts.

The manufacture of sulfuric acid involves the equilibrium

2SO2(g) + O2(g) ⇌ 2SO3(g)   ΔrH = −197 kJ mol−1

i)
A catalyst of vanadium(V) oxide is used in this reaction.

State the effect, if any, of the catalyst on the value of the equilibrium constant, Kp.
(1)
ii)
The temperature used for this reaction in industry is 700 K.

Explain, in terms of the equilibrium constant and the equilibrium position, the effect of an increase in temperature on the equilibrium yield of sulfur trioxide.
(2)
iii)
Write the expression for the equilibrium constant, Kp, for this equilibrium.
State symbols are not required.
(1)
iv)
A mixture of 2.00 mol of sulfur dioxide and 1.00 mol of oxygen is allowed to reach equilibrium at 5.00 atm pressure.
1.60 mol of sulfur trioxide is formed.

Calculate the value of Kp.

Include units and give your answer to an appropriate number of significant figures.
(4)

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1b
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Sulfur trioxide is used to produce sulfuric acid.

i)
Commercial concentrated sulfuric acid contains 98.5 % H2SO4 and 1.5 % water by mass.
The density of concentrated sulfuric acid is 1800 g dm−3.

Calculate the concentration of this sulfuric acid in mol dm−3.
(2)
ii)
The pH of a 0.10 mol dm−3 solution of sulfuric acid at 25 °C is 0.97.
Calculate the concentration of hydrogen ions, in mol dm−3, in this solution.
(1)
iii)
In an aqueous solution of sulfuric acid, the following equilibria exist.

straight H subscript 2 SO subscript 4 left parenthesis aq right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis space rightwards harpoon over leftwards harpoon space straight H subscript 3 straight O to the power of plus space left parenthesis aq right parenthesis space plus space HSO subscript 4 superscript minus space left parenthesis aq right parenthesis space space space space space space space space space space space space space space K subscript straight a space very space large space space HSO subscript 4 superscript minus space left parenthesis aq right parenthesis space plus space straight H subscript 2 straight O left parenthesis straight l right parenthesis space rightwards harpoon over leftwards harpoon space straight H subscript 3 straight O to the power of plus space left parenthesis aq right parenthesis space plus space SO subscript 4 superscript 2 minus end superscript left parenthesis aq right parenthesis space space space space space space space space space space space space space space space space K subscript straight a space equals space 0.012 space mol space dm to the power of negative 3 end exponent

Explain, in terms of these equilibria, why the concentration of hydrogen ions in a 0.10 mol dm−3 solution of sulfuric acid is not 0.20 mol dm−3.

No calculation is required.

(2)

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1c
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A buffer solution is made from HSO4– and SO42– ions.

i)
Write two ionic equations involving HSO4– and SO42– ions to show how this solution acts as a buffer.
State symbols are not required.
(2)
ii)
A buffer solution is formed by mixing 25.0 cm3 of a solution that is 0.150 mol dm-3 with respect to SO42– ions with 75.0 cm3 of a solution that is 0.100 mol dm-3 with respect to HSO4– ions.
 
Calculate the pH of this buffer solution.
 
[Ka for HSO4– ions = 0.012 mol dm-3]
(5)

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2a7 marks

The reversible reaction between hydrogen chloride and oxygen produces water vapour and chlorine.

4HCl (g) + O2 (g) ⇌ 2H2O (g) + 2Cl2 (g)   ∆H = –114 kJ mol–1

Explain what effect, if any, each of the following changes has on the yield of chlorine at equilibrium and on the equilibrium constant, Kp.

i)
An increase in the total pressure
(3)
ii)
An increase in the temperature
(2)
iii)
The use of a catalyst
(2)

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2b
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0.850 mol of hydrogen chloride was mixed with 0.600 mol of oxygen and allowed to reach equilibrium in a closed flask.
At equilibrium the total pressure was 1.50 atm and there was 0.250 mol of chlorine in the flask.

4HCl(g) + O2(g) ⇌ 2H2O(g) + 2Cl2(g)

i)
Complete the table.
(3)
Substance Initial amount
/ mol		 Equilibrium amount
/ mol		 Mole fraction
at equilibrium
HCl 0.850    
O2 0.600    
H2O 0    
Cl2 0 0.250 0.189
Total moles at equilibrium =  


ii)
Write the expression for the equilibrium constant, Kp .
(1)

iii)
Use your answers to (b)(i) and (b)(ii) to calculate the value for Kp. Give your answer to an appropriate number of significant figures, and include units.
(3)

iv)
Use your answer to (b)(iii) to calculate a value for the total entropy change of the reaction, ΔStotal .
(2)

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2c3 marks

Draw a sketch of entropy against temperature for water to illustrate the entropy changes as temperature increases, including when water changes state.

A scale is not required for the vertical axis.

q20c-paper-4-2021-june-edexcel-ial-chemistry

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