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 23.24 g of BaBr2(s) are dissolved in 108.40 g of water, the temperature of the solution increases from 25.88 to 29.32 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.75 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of BaBr2(s) in kJ/mol.

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

Hdissolution =  kJ/mol

Explanation / Answer

mass of BaBr2 = 23.24 g

mass of water = 108.40 g

total mass of solution = 23.24 + 108.40 = 131.64 g

temperature rise = 29.32 - 25.88 = 3.44 oC

specific heat = 4.18 J / g oC

Q = m Cp dT + Cp dT

   = 131.64 x 4.184 x 3.44 + 1.75 x 3.44

   = 1900.71 J

moles of BaBr2 = 23.24 / 297.135 = 0.0782 mol

Hdissolution = - 1900.7 x 10^-3 / 0.0782

Hdissolution   = - 24.3 kJ/mol

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