Core practical 8 - calculate the enthalpy change for the thermal decomposition of potassium hydrogencarbonate

Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?

Heat energy must be supplied; hence, the temperature change measured is not solely due to the decomposition.

Show that the hydrochloric acid is in excess in both reactions

moles hydrochloric acid = 2 × 30/1000 = 0.06mol

mol K₂CO₃ = 3 /138.2 = 0.022mol

This reacts with 0.0434 moles of hydrochloric acid.

mol KHCO₃ = 3.5 /100.1 = 0.035mol

This reacts with 0.035 moles of hydrochloric acid.

Draw energy level diagrams for reactions 1, 2 and 3.

Reaction 1 – diagram shows exothermic reaction. Reaction 2 – diagram shows endothermic reaction. Reaction 3 – diagram shows endothermic reaction.

Explain why the reactions are conducted in a polystyrene cup rather than a glass beaker.

Polystyrene is a better insulator than glass. Therefore, less heat energy is lost to/gained from the surroundings, so temperature changes are more accurate.