Reversible reactions in a sealed container
4 marks · NormalA reversible reaction takes place in a sealed flask. At first, only reactants are present. The forward reaction starts quickly, but slows down over time.
Explain why the rate of the forward reaction decreases as the reaction proceeds.
Describe what is happening with the reverse reaction as the products begin to form.
State what is meant by equilibrium in this context.
Explain why equilibrium can only be reached in a sealed container.
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(a) Reactant concentration falls so forward rate decreases
(b) Reverse reaction rate increases as product concentration increases
(c) When the rates of forward and reverse reactions become equal
(d) A sealed container stops loss of reactants or products, so a constant system is possible
Common mistakes: Confusing the meaning of equilibrium; Not mentioning sealed container prevents loss of reactants/products
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Marking points
M1 (a) · 1 mark · States that forward rate decreases because reactant concentration decreases
M2 (b) · 1 mark · States that reverse rate increases as product concentration increases
B1 (c) · 1 mark · Defines equilibrium as rates of forward and reverse reactions becoming equal
B2 (d) · 1 mark · Explains why sealed container is needed for equilibrium (prevents loss/gain of reactants/products)
Worked solution
a) The forward reaction slows because the concentration of reactants decreases as they are used up, so fewer collisions occur each second.
b) As products form, their concentration increases, increasing the rate of the reverse reaction.
c) At equilibrium, the forward and reverse reactions happen at the same rate, so the amounts of substances remain constant.
d) Equilibrium can only be reached in a sealed container, because if substances could escape, their concentrations would not remain constant.