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.

Question

In the laboratory a general chemistry student finds that when 8.01 g of CsBr(s) are dissolved in 111.10 g of water, the temperature of the solution drops from 24.31 to 21.97 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.64 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of CsBr(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 drop in temperature is 2.34ºC = ∆T

Since the temperature decreased, this means that heat is lost from the surroundings and absorbed by the system, making this an endothermic reaction. Some heat goes into dissolution and some goes to the calorimeter.

Heat going to the system = q = mC∆T = (110.10 g)(4.184 J/g/ºC)(2.34ºC) = 1078 J (ignoring mass of CsBr)

Heat going to the calorimeter = q = Ccal x ∆T = 1.64 J/ºC x 2.34ºC = 3.8 J = 4 J

Total heat = 1078 J + 4 J = 1082 J

This is for dissolution 8.01 g of CsBr. Now, for the heat to dissolve 1 mole of CsBr.

Molar mass CsBr = 213g/mol

8.01 g CsBr x 1 mol/213 g = 0.0376 moles CsBr

Molar enthalpy of dissolution = 1082 J/0.0376 moles x 1 kJ/1000 J = 28.8 kJ/mole

Answer 2

You can use the formula Q=mcΔT where m is the mass of the water, C is the heat capacity, and ΔT is the change in temperature. Q is your heat.

Note, that the answer needs to be in kj/mole, so make sure to convert your units appropriately.