Calorimetry answer 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.

The Hess's Law triangle is:

Using Hess's Law gives DH = DH₁ - DH₂

Experiment 1

MgCO₃ = 84

\ number of moles of MgCO₃ = 4.200/84 = 0.0500 mol

Temperature rise = 25.2 - 19.6 = 5.6 ºC

Heat energy (in J) = 4.18 × mass of solution (in g) × temperature change (in °C)

Heat evolved = 100 × 4.18 × 5.6 = 2341 J = 2.34 kJ

DH₁ = - 2.34/0.0500 = - 47 kJ mol^-1

Experiment 2

MgO = 40

\ number of moles of MgO = 2.000/40 = 0.0500 mol

The masses were obviously chosen to give the same number of moles

Temperature rise = 36.6 - 19.6 = 17.0 ºC

Heat energy (in J) = 4.18 × mass of solution (in g) × temperature change (in °C)

Heat evolved = 100 × 4.18 × 17.0 = 7106 J = 7.11 kJ

DH₂ = - 7.11/0.0500 = - 142 kJ mol^-1

DH = DH₁ - DH₂ = - 47 - - 142 = + 95 kJ mol^-1

The accepted value is + 101.6 kJ mol^-1. This reaction is quite endothermic. It does not take place at room temperature but requires heating.

Heat loss will be an error in both experiment 1 and experiment 2. We would expect the effect to be greater in experiment 2 where a higher temperature was reached. Another factor to consider is the production of a gas in experiment 1. The gas escapes from the plastic cup and will take away some of the reaction heat with it. This is not a problem in experiment 2.