Question

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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 0.85 g of CaCl2(s) are dissolved in 100.10 g of water, the temperature of the solution increases from 24.36 to 25.89 °C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.62 J/°C. Based on the student's observation, calculate the enthalpy of dissolution of CaCl (s) in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water. AH dissolution kJ/mol

Answer 1

Mass of CaCl2 = 0.85 g Number of moles of CaCl, = mass/mol.wt. = 0.85 g/(110.98 g/mol) = 0.007659 mol Mass of water = 100.10 g Mass of the solution = 100.10 + 0.85 = 100.95 g the specific heat of solution is same as that of water (4.184 J/(g.°C) Change in temperature = AT = 25.89 – 24.36 = 1.53 °C Heat associated with the calorimeter = C* AT = 1.62 J/°C * 1.53 °C = 2.4786 J Heat energy associated with this rise in temperature of solution = m*c* AT = 100.95 g * 4.184 d.g 1°C-1.x 1.53 °C = 646.2334 J At thermal equilibrium, qrxn + qwater + acal = 0 qrxn = -(qwater + acal) = -(646.2334 + 2.4786 ) = - 648.712 J Enthalpy of reaction = - 648.712 J Since the temperature increases by the dissolution of CaCl2, this is an exothermic reaction. So AH solution=-q = - 648.712 J For 1 mole of CaCl2 AH solution = -648.712 J/ 0.007659 mol =-84698.9 J/mol = -84.6989 kJ/mol = -84.70 kJ/mol AH solution = - 84.70 kJ/mol