When a solid dissolves in water, heat may be evolved or absorbed. The heat of di

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 5.03 g of NH4Cl(s) are dissolved in 104.80 g of water, the temperature of the solution drops from 24.77 to 21.35 °C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.80 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.

Explanation / Answer

mass of water = 104.80 g

mass of salt = 5.03 g

mass of solution = 104.80 + 5.03 = 109.83 g

temperature drop = 21.35 - 24.77 = - 3.42

Q = m Cp dT + Cp dT

    = 109.83 x 4.18 x - 3.42 + 1.80 x - 3.42

   = - 1577.7 J

moles of NH4Cl = 5.03 / 53.49 = 0.0940

Enthalpy = - Q / n = 1577.7 x 10^-3 / 0.0940

enthalpy of dissolution = 16.78 kJ/mol

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