IGCSE Chemistry 0620/21/O/N/21 Q14


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14 Ammonia is made by reacting nitrogen with hydrogen.
The equation for the reaction is shown.

N2(g) + 3H2(g) 2NH3(g)

The forward reaction is exothermic.

Which changes in temperature and pressure decrease the yield of ammonia?

temperature    pressure
A    decrease        decrease
B    decrease         increase
C     increase        decrease
D    increase         increase

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Answer ( 1 )


    Because the reaction is exothermic, in order for the equilibrium to shift backwards, causing the yield to decrease and reactant to increase, the temperature has to be increased.

    The reactants involve 4 moles of gas whereas the the yield involves 2 moles of gas. The pressure before reaction is higher than that after reaction. Therefore, to decrease yield is to shift the equilibrium backwards. Hence, pressure needs to be decreased.

    Ans: C Temperature: Increase, Pressure: Decrease

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