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Compiled by Mr. Chris D A-level Chemistry 0193868950 1
Chemical Equilibria & Ionic Equlibria CIE AS level Questions
Question 1 (a) The table shows some standard enthalpies of formation.
(i) Use these standard enthalpies of formation to calculate a value for the standard enthalpy change of this synthesis.
(3 marks) (ii) State why the standard enthalpy of formation for hydrogen gas is zero. ........................................................................................................................................... ...........................................................................................................................................
(1 mark) (b) State and explain what happens to the yield of methanol when the total pressure is increased in this synthesis.
Effect on yield ..................................................................................................................... Explanation ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(3 marks)
Compiled by Mr. Chris D A-level Chemistry 0193868950 2
(c) The hydrogen required for this synthesis is formed from methane and steam in a reversible reaction. The equation for this reaction is shown below.
State and explain what happens to the yield of hydrogen in this reaction when the temperature is increased. Effect on yield ..................................................................................................................... Explanation ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(3 marks)
(d) Write an equation for the complete combustion of methanol liquid, with state symbol. ...........................................................................................................................................
(1 mark)
Compiled by Mr. Chris D A-level Chemistry 0193868950 3
Question 2 Brønsted–Lowry were instrumental in giving the modern definitions of acids and bases. (a) Define the term Brønsted–Lowry acid. ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(b) Three equilibria are shown below. For each reaction, indicate whether the substance immediately above the box is acting as a Brønsted–Lowry acid (A) or a Brønsted–Lowry base (B) by writing A or B in each of the six boxes.
(c ) Ammonia and ethylamine are examples of weak Brønsted–Lowry bases. (i) Write an equation for the reaction of ethylamine (CH3CH2NH2) with water to form a weakly alkaline solution. ........................................................................................................................................... ...........................................................................................................................................
(1 mark) (ii) In terms of this reaction, state why the solution formed is weakly alkaline. ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
Compiled by Mr. Chris D A-level Chemistry 0193868950 4
A student carried out an experiment to determine the enthalpy change when a sample of methanol was burned.
The student found that the temperature of 140 g of water increased by 7.5 oC when
0.011 mol of methanol was burned in air and the heat produced was used to warm the water.
Use the student’s results to calculate a value, in kJ mol–1
, for the enthalpy change when
one mole of methanol was burned.
(The specific heat capacity of water is 4.18 JK–1
g–1
).
........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
Compiled by Mr. Chris D A-level Chemistry 0193868950 5
Question 3 The manufacture of food grade phosphoric acid for use in cola drinks begins with the production of pure white phosphorus from the mineral fluoroapatite, Ca5F(PO4)3
3 (a) Complete the following equation for the manufacture of phosphorus.
(1 mark)
(b) As the phosphorus cools, it forms white phosphorus, P4
Give the oxidation state of phosphorus in each of the following.
P4 .............................................................
H3PO4 ......................................................
(c) Fertiliser grade phosphoric acid is manufactured from sulfuric acid and calcium phosphate. Use the following precise relative atomic mass data to calculate the relative molecular mass of each below:
(i) sulfuric acid (H2SO4) and
(ii) phosphoric acid (H3PO4) .
Compiled by Mr. Chris D A-level Chemistry 0193868950 6
(d) Concentrated phosphoric acid is used as a catalyst in the hydration of propene to form the alcohol CH3CH(OH)CH3 as the main organic product. The industrial name for this alcohol is isopropyl alcohol. (i) State the meaning of the term catalyst. ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(1 mark) (ii) State the meaning of the term hydration. ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(iii) Give two other subtances than can replace concentrated phosphoric acid in the hydration reaction. .....................................................................................
.....................................................................................
(e) Give the meaning of the term dynamic equilibrium. ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
Compiled by Mr. Chris D A-level Chemistry 0193868950 7
Question 4 (a) A mixture of 1.50 mol of hydrogen and 1.20 mol of gaseous iodine was sealed in a container of volume V dm3. The mixture was left to reach equilibrium as shown by the following equation.
At a given temperature, the equilibrium mixture contained 2.06 mol of hydrogen iodide. (i) Calculate the amounts, in moles, of hydrogen and of iodine in the equilibrium mixture. Moles of hydrogen ............................................................................................................. Moles of iodine ..................................................................................................................
(2 marks)
(ii) Write an expression for the equilibrium constant (Kc) for this equilibrium. ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(iii) Explain why Kc for this equilibrium has no units. ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(iv) A different mixture of hydrogen, iodine and hydrogen iodide was left to reach equilibrium at the same temperature in a container of the same volume. This second equilibrium mixture contained 0.38 mol of hydrogen, 0.19 mol of iodine and 1.94 mol of hydrogen iodide. Calculate a value for Kc for this equilibrium at this temperature.
........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
Compiled by Mr. Chris D A-level Chemistry 0193868950 8
(b) This question concerns changes made to the four equilibria shown in parts (b) (i) to (b) (iv) , given below. In each case, use the information in the table to help you choose from the letters A to E the best description of what happens as a result of the change described. Write your answer in the box. Each letter may be used once, more than once or not at all.
Compiled by Mr. Chris D A-level Chemistry 0193868950 9
Question 5 At high temperatures and in the presence of a catalyst, sulfur trioxide decomposes according to the following equation.
(a) In an experiment, 8.00mol of sulfur trioxide were placed in a container of volume
12.0 dm3 and allowed to come to equilibrium.
At temperature T1 there were 1.40 mol of oxygen in the equilibrium mixture. (i) Complete the table below to show the amount, in moles, of sulfur trioxide and of sulfur dioxide in the equilibrium mixture.
Sulfur trioxide
Sulfur trioxide
Oxygen
Initial mole
8.00
0
Equilibrium mole
1.40
(2 marks)
(ii) Write an expression for the equilibrium constant, Kc, for this equilibrium.
........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(iii) Deduce the units of Kc for this equilibrium.
........................................................................................................................................... ...........................................................................................................................................
(1 mark) (iv) Complete the table below for the concentration of each substance at equilibrium.
Then calculate a value of Kc for this equilibrium at temperature T1
Sulfur trioxide
Sulfur trioxide
Oxygen
Equilibrium concentration in moldm-3
8.00
0
(3 marks)
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(b) The experiment was repeated at the same temperature using the same amount of sulfur trioxide but in a larger container. State the effect, if any, of this change on: (i) the amount, in moles, of oxygen in the new equilibrium mixture ...........................................................................................................................................
(1 mark)
(ii) the value of Kc
...........................................................................................................................................
(1 mark) (c) The experiment was repeated in the original container but at temperature T2
The value of Kc was smaller than the value at temperature T1
State which is the higher temperature, T1 or T2
Explain your answer. Higher temperature ............................................................................................................ Explanation ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(3 marks)
Compiled by Mr. Chris D A-level Chemistry 0193868950 11
Question 6 Synthesis gas is a mixture of carbon monoxide and hydrogen. Methanol can be manufactured from the synthesis gas in a reversible reaction as shown by the following equation.
(a) A sample of synthesis gas containing 0.240 mol of carbon monoxide and 0.380 mol
of hydrogen was sealed together with a catalyst in a container of volume 1.50dm3.
When equilibrium was established, the equilibrium mixture contained 0.170 mol of carbon monoxide. The total pressure at equilibrium is 2.50atm (i) Calculate the amount, in moles, of methanol and the amount, in moles, of hydrogen in the equilibrium mixture.
Carbon monoxide
Hydrogen
Methanol
Initial mole
0.240
0.380
0
Equilibrium mole
0.170
(2 marks)
(ii) Determine the Partial pressure from each gas at equilibrium, in atm.
Carbon monoxide
Hydrogen
Methanol
Partial Pressure at equilibrium, in atm
(2 marks)
(iii) Write the Kp expression ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(1 mark) (iv) Calculate Kp value for this reaction and state its unit , if any. ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(3 marks)
Compiled by Mr. Chris D A-level Chemistry 0193868950 12
(b) A different sample of synthesis gas was allowed to reach equilibrium in a similar
container of volume 1.50 dm3 at temperature T1
At equilibrium, the mixture contained 0.210 mol of carbon monoxide, 0.275 mol of hydrogen and 0.0820 mol of methanol. (i) Write an expression for the equilibrium constant Kc for this reaction. ........................................................................................................................................... ...........................................................................................................................................
(1 mark)
(ii) Calculate a value for Kc for the reaction at temperature T1 and state its units. Calculation ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Units ................................................................................................................................. ...........................................................................................................................................
(2 marks)
(iii) State the effect, if any, on the value of Kc of adding more hydrogen to the equilibrium mixture. ...........................................................................................................................................
(1 mark)
Question 7
Hydrogen is used in very large quantities as a fuel, as a reducing agent, and in the
production of ammonia. Hydrogen is manufactured by steam reforming of methane
from natural gas. Two reactions are involved, both being in equilibrium in closed
systems.
(a) Write the expression for the equilibrium constant, Kp, for reaction I. (1) ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
Compiled by Mr. Chris D A-level Chemistry 0193868950 13
(b) Reaction I occurs at a temperature of 1000 K and a pressure of 30 atm over a nickel
catalyst.
(i) State and explain the effect, if any, on the value of Kp of increasing the
pressure on the reaction. (1)
........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(ii) Explain, in terms of your answers to (a) and (b)(i), why an increase in the
pressure leads to a decrease in yield in reaction I. (2) ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(iii) Increasing the pressure on this heterogeneously-catalysed reaction I has very
little effect on the rate of the reaction. Suggest why this is so. (2) ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
(c) At a particular temperature and 30.0 atm pressure, a mixture of 2.00 mol of carbon
monoxide and 2.00mol of steam reacts to give an equilibrium mixture where 75 % of the
carbon monoxide had reacted.
(i) Calculate the moles of each gas at equilibrium. (4)
Carbon
monoxide
Steam Carbon dioxide Hydrogen
Initial mole
2.00
2.00
0
0 Equilibrium
mole
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(iii) Calculate the partial pressure of each gas at the given total pressure of
30.0 atm. (4)
Carbon monoxide Steam Carbon dioxide Hydrogen
Partial
pressure in
atm
(iv) Calculate the equilibrium constant, Kp for this reaction and state its unit, if
any. (4) Kp value:.............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................
Unit:………………………………………………………………………………………...
………………………………………………………………………………………………
8 Consider the equilibrium
Which of the following is true when the total pressure of the system is increased at
constant temperature?
Compiled by Mr. Chris D A-level Chemistry 0193868950 15
8. The dissociation of ethanoic acid in aqueous solution is represented by
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq)
Which of the following statements is true for this equilibrium?
9. Iron and steam at high temperature react in a closed vessel to give an equilibrium
mixture
Which of the following is the correct expression for Kp?
Compiled by Mr. Chris D A-level Chemistry 0193868950 16
The following information is required to answer questions 10 to 12.
The equation for the synthesis of methanol is as follows:
At equilibrium, when the temperature is 340 K, the total pressure is 20 atm. The moles of each component present at equilibrium are shown in the table below.
10. The mole fraction of hydrogen in the equilibrium mixture is ……….
A 0.23 B 0.46 C 0.49 D 0.92 11. The numerical value for the equilibrium partial pressure of the carbon monoxide, in atmospheres, is …….
A 3.0 B 4.6 C 5.0 D 9.2 12. Units for the equilibrium constant, Kp, for this reaction are ………….
A no units B atm C atm−1
D atm−2
