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 2.74 g of NH4ClO4(s) are dissolved in 112.40 g of water, the temperature of the solution drops from 22.28 to 20.50 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.86 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of NH4ClO4(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 absorbed(q) = m*s*DT + c*DT

m = mass of solution = 112.4+2.74 = 115.14 g

s = specific heat of solution = 4.184 j/g.c

DT = 22.28 - 20.5 = 1.78

C = heat capacity of the calorimeter = 1.86 j/c

q = 115.14*4.184*1.78 + 1.86*1.78

= 860.82 joule

= 0.86 kj

No of mol of NH4ClO4(s) dissolved = 2.74/117.5 = 0.0233 mol

Hdissolution = q/n = 0.86/0.0233 = 36.91 kj/mol

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