Enthalpy data for the decomposition of potassium chlorate and its scaled variants are…

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Enthalpy data for the decomposition of potassium chlorate and its scaled variants are shown below. Reaction (1):

2\,\text{KClO}_{3}(s) \rightarrow 2\,\text{KCl}(s) + \frac{3}{2}\,\text{O}_{2}(g)

\Delta H_{1}=+89\,\text{kJ}

Assume standard conditions and constant external pressure. (a) The laboratory sometimes requires only

0.50

mol of

\text{O}_{2}(g)

from this decomposition. Write the balanced chemical equation that produces exactly

0.50

mol of

\text{O}_{2}(g)

and state the corresponding enthalpy change

\Delta H_{a}

. Express

\Delta H_{a}

numerically and include the sign. (b) For certain demonstrations, oxygen is instead produced by the endothermic decomposition of potassium chlorate carried out in reverse—chlorine dioxide reacts with potassium chloride to regenerate potassium chlorate. Write this reverse reaction so that it forms exactly

1.0

mol of

\text{KClO}_{3}(s)

, and state

\Delta H_{b}

. Explain in words why the magnitude of

\Delta H_{b}

is related to

\Delta H_{1}

the way it is. (c) Provide a short argument, based on conservation of energy, that justifies why the enthalpy change of a reaction is directly proportional to the amount (in moles) of each substance consumed or produced.

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