CAIE IGCSE Chemistry 6.3.3 Reversible Reaction in a Closed System Questions

Describe dynamic equilibrium as equal opposing reaction rates with constant concentrations in a closed system, not as a reaction that has stopped.

Exam points

state that a closed system allows nothing to enter or leave
describe equilibrium as equal forward and reverse reaction rates
distinguish constant concentrations from reactions having stopped completely
describe equilibrium characteristics in reversible ammonia and aqueous reactions

[Maximum number: 2]

Hydrogen iodide thermally decomposes into iodine and hydrogen. The reaction is reversible.

\underset{\text { colourless gas }}{2 \mathrm{HI}(\mathrm{~g})} \rightarrow \underset{\substack{\mathrm{I}_{2}(\mathrm{~g}) \\ \text { purple gas }}}{\rightleftharpoons}+\underset{\substack{\mathrm{H}_{2}(\mathrm{~g}) \\ \text { colourless gas }}}{ }

Fig. 4.1 shows a gas syringe containing a mixture of hydrogen iodide, iodine and hydrogen gases. The gas syringe is sealed and the mixture is heated to $300^{\circ} \mathrm{C}$ . The mixture of gases reaches equilibrium and is purple.

Fig. 4.1

State what is meant by the term equilibrium.

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CAIE IGCSE Chemistry 6.3.3 Reversible Reaction in a Closed System Questions question 1