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 1.18 g of CaCl_2(s) are dissolved in 101.50 g of water, the temperature of the solution increases from 25.19 to 27.27 degree C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.79 J/degree C. Based on the student's observation, calculate the enthalpy of dissolution of CaCl_2(s) in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water. Delta H_dissolution = kJ/mol

Explanation / Answer

Heat absorbed by calorimeter = Heat capacity of calorimeter x Temperature rise

                                   = 1.79 J/C x ( 27.27 - 25.19)

                                      = 3.723 J

solution mass = CaCl2 mass + water mass = 1.18 + 101.5 = 102.68 g

Heat absorbed by solution = Spcecific heat of solution x temp rise x mass of solution

                            = 4.184 J/gC x (27.27-25.19) C x 102.68 g

                           = 893.6 J

Total heat absorbed = heat absorbed by calorimeter + Heat absorbed by solution

                        = 3.723 J + 893.6 J = 897.3 J

CaCl2 moles dissolved = mass of CaCl2 / Molar mass of CaCl2

            = 1.18 g / 110.98 g/mol

            = 0.01063

dH dissolution = Heat released in reaction / CaCl2 moles dissolved

            = - 897.3 J / 0.01063 mol   =- 84391 J/mol = - 84.4 KJ/mol

( -ve sign indicates heat is released. It is released by dissolving CaCl2 which si absorbed by solution to rise its temp_

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