Monday, 4 August 2014

(O level Chem) Questions on reversible reactions


The knowledge on reversible reactions that includes Le Chatelier's Principle may not be in the current syllabus but I still write out the solutions for two Cambridge questions. The first question is on the Haber process while the second question is on steam reforming.


Le Chatelier's principle: When a reversible reaction at equilibrium is disturbed by a change in concentration, temperature, volume or pressure, the reaction shifts to minimize the effect of the disturbance.  


  1. Ammonia is manufactured by the Haber Process. 
N2(g) + 3H2(g) = 2NH3(g)
200 atmospheres
450 ºC
catalyst: iron

The forward reaction is exothermic.

(a) State one use of ammonia. [1]
(b) Describe how the reactants are obtained. [2]
(c) The percentage of ammonia in the equilibrium mixture varies with temperature and pressure.
  1. Will increasing temperature increase or decrease the percentage of ammonia at equilibrium? Explain your answer. [2]
  2. Will increasing pressure increase or decrease the percentage of ammonia at equilibrium? Explain your answer. [2]
(d) Catalysts do not alter the position of equilibrium. Explain why a catalyst is used in this process. [2]
[Solution]
(a) Ammonia is used to manufacture ammonium fertilizers. [1]
(b) Nitrogen is obtained from the fractional distillation of liquid air. [1]
Hydrogen is industrially obtained from the reaction between methane and steam. [1]
(c) (i) The forward reaction is exothermic and so the backward reaction is endothermic.
When temperature increases, the equilibrium shifts to the left so as to absorb the heat supplied. [1]
The percentage of ammonia decreases. [1]
(ii) There are 1 + 3 = 4 moles of gaseous reactants to 2 moles of gaseous product.
When the pressure increases, the equilibrium shifts to the right so as to result in lesser moles of gas. [1]
The percentage of ammonia increases. [1]
(d) The catalyst increases the rate of attainment of equilibrium. [1]


  1. The hydrogen used in the Haber process is obtained by an industrial process called steam reforming. It's a reversible reaction between methane and steam.

CH4(g) + H2O(g) = CO(g) + 3H2(g) ∆H = +210 kJ/mol

The reaction is carried out in the presence of nickel catalyst. The conditions used are 30 atmospheres pressure and a temperature of 750 ºC.

(a) If the temperature of the reaction mixture is increased to 900 ºC, explain what happens to the position of equilibrium. [2]
(b) If the pressure of the reaction mixture is increased to 50 atmospheres, explain in terms of collisions between reacting particles what happens to the speed of the forward reaction. [2]
(c) Explain the advantages of using a catalyst in this manufacture of hydrogen. [2]
(d) In the reaction, 210 kJ of heat energy is used to form 3.0 moles of hydrogen. Calculate how much heat energy is needed to make 1000 kg of hydrogen.
(e) Describe how hydrogen is used to manufacture margarine. [2]
[Solution]
(a) The forward reaction is endothermic while the backward reaction is exothermic.
When temperature increases, the equilibrium shifts to the right [1] so that heat is absorbed [1].
(b) When the pressure is increased, the number of molecules per unit volume increases. [1]
The frequency of collision increases and therefore the rate of reaction increases. [1]
(c) Using a catalyst, the rate of attainment of equilibrium increases. [1]
The heated catalyst supplies the heat for more hydrogen formed at equilibrium. [1]
(d) Mass of 3.0 moles of hydrogen H2 = 3.0 mol x (1 + 1) g/mol = 6.0 g
To form 6.0 g of H2, 210 kJ of heat is used.
To form 1000 kg of H2, (210 kJ x 1000 000 g)/6.0 g [1] = 35000 000 kJ [1]
(e) Margarine is formed when liquid vegetable oils that are unsaturated react with hydrogen [1] at about 60 ºC in the presence of nickel catalyst [1].


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