Reversible reactions in a sealed container
4 marks · NormalA student heats ammonium chloride in a sealed test tube. The following reversible reaction occurs:
Answer the following questions about this experiment.
State what is meant by a sealed container in this experiment.
Describe what happens to NH3 and HCl in the sealed container after heating.
Explain why both the forward and reverse reactions occur in this setup.
Explain how equilibrium is reached during the reaction.
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(a) A closed container where substances cannot enter or leave
(b) NH3 and HCl gases form and remain in the tube, able to recombine to form NH4Cl
(c) Both gases are kept in, so forwards (NH4Cl → NH3 + HCl) and reverse (NH3 + HCl → NH4Cl) reactions occur
(d) When the rates of the forward and reverse reactions become equal, so the amounts of NH4Cl, NH3 and HCl no longer change
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Marking points
B1 (a) · 1 mark · Describes a sealed container as one where no substances enter or leave
B2 (b) · 1 mark · Correctly describes gases stay in the container and can recombine
M1 (c) · 1 mark · Explains both reactions can occur because all substances are kept in the container
M2 (d) · 1 mark · Explains equilibrium is when forward and reverse rates are equal and amounts stay constant
Worked solution
a) A sealed container is a closed system where no gases or solids can escape or be added.
b) NH3 and HCl are formed by heating NH4Cl, but cannot escape and so can recombine to form NH4Cl again.
c) As all reactants and products are kept in the system, both the forward reaction (decomposition) and the reverse reaction (recombination) can proceed.
d) Equilibrium is reached when the decomposition and recombination occur at the same rate, so the amounts of all substances stay constant.