Coffee Cup Calorimetry: Heat of Solution

Question

1) 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 3.62 g of NH₄Cl(s) are dissolved in 108.60 g of water, the temperature of the solution drops from 23.12 to 20.75 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.61 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of NH₄Cl(s) in kJ/mol.

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

ΔH_dissolution =  kJ/mol

2) 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 3.81 g of CsClO4(s) are dissolved in 104.60 g of water, the temperature of the solution drops from 22.72 to 20.49 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.50 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of CsClO4(s) in kJ/mol.

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

ΔHdissolution = ___ kJ/mol

Answer 1

q = mC∆T + Ccal∆T

q = heat energy = ?

m = mass of water = 108.60 g

C = specific heat of water = 4.184 J/g/ºC

Ccal = calorimeter constant = 1.61 J/ºC

∆T = change in temperature = 23.12 - 20.75 = 2.37ºC

q = (108.60 g)(4.184 J/g/ºC)(2.37 ºC) + (1.61 J/ºC)(2.37ºC)

q = 1077 J + 4 J = 1081 J

moles NH4Cl = 3.62 g NH4Cl x 1 mol/53.5 g = 0.0677 moles

∆H_dissolution = 1081 J/0.0677 moles x 1 kJ/1000 J = 16.0 kJ/mole (3 significant figures)

(note: since the temperature decreased during dissolution, the process is endothermic and the sign of ∆H is positive)

#2 is done the same way as #1.