Calorimetry exercise 5

In this experiment the enthalpy change for the following thermal decomposition is determined:

MgCO₃(s) ⇒ MgO(s) + CO₂(g)

by using Hess's Law. Two plastic cup calorimetry experiments were carried out. In both experiments 100 cm³ of 2M hydrochloric acid with a starting temperature of 19.6 ºC was used. In the first experiment 4.200 g of solid magnesium carbonate was added to the acid carefully. With stirring, the highest temperature reached was 25.2 ºC. In the second experiment 2.000 g of solid magnesium oxide was added to the acid. The highest temperature reached here was 36.6 ºC.

Construct a Hess's Law cycle and use it to calculate the enthalpy change for the thermal decomposition reaction. Heat loss is always a significant error when carrying out exothermic reactions. Suggest two reasons why heat loss may not be comparable between the two experiments.

Answer to exercise 5