Coffee Cup Calorimetry: Heat of Solution

Question

When a solid dissolves in water, heat may be evolved or absorbed. The heat of dissolution (dissolving) can be determined using a coffee cup calorimeter.

In the laboratory a general chemistry student finds that when 0.68 g of MgCl₂(s) are dissolved in 115.90 g of water, the temperature of the solution increases from 23.29 to 25.72 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.68 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of MgCl₂(s) in kJ/mol.

Assume the specific heat of the solution is equal to the specific heat of water.

ΔH_dissolution =  ____kJ/mol

Answer 1

The heat evolved form the dissolution of MgCl2 goes to raise the temperature of the calorimeter and the water.

As the heat capacity is 1.68 J/°C, and the temperature change is 2.43 °C,, the heat used to heat the calorimeter is 1.68 x 2.43 = 4.08 J.

The heat required to raise the temperature of the water is 4.18 J/°C-g x 115.90 g x 2.43 °C = 1177.2 J.

The total heat required for the above is 4.08 J + 1177.2 J = 1181.3 J.

To convert this to enthalpy in kj/mole, we need to calculate the number of moles of MgCl2 which is 0.67 g / 95.2 g/mole = 7.0 x 10^-3 moles .

The enthalpy of dissolution is 1181/7.0 x 10^-3 = 1.7 x 10^5 j or 1700 kj/mole.

Do check the math.