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 2.81 g of FeBr3(s) are dissolved in 119.20 g of water, the temperature of the solution increases from 25.78 to 28.59 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.70 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of FeBr3(s) in kJ/mol.

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

Hdissolution =  kJ/mol

Explanation / Answer

Heat gained = Q = (m Cp dT)water + (Cp dT)calorimeter

                Q = 119.2 x 4.18 x (28.59 °C - 25.78 ) + 1.70 x (28.59 °C - 25.78 )

                     = 1400 + 4.78

                      = 1404.87 J

mass of FeBr3 = 2.81 g

molar mass of FeBr3 = 295.56 g/mol

moles = mass / molar mass

           = 2.81 / 295.56

           = 0.00951

for 0.00951 mole ------------- 1404.87 J heat

for 1 mole           ---------------- ??

Hdissolution = 1404.87 / 0.00951

                     = 147766.3 J/mol

                      = 147.76 kJ/mol

Hdissolution   = 147.76 kJ/mol

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