Calorimetry is a method used to measure changes in enthalpy, or heat, that occur

Question

Calorimetry is a method used to measure changes in enthalpy, or heat, that occur during chemical processes. Two common calorimeters are constant-pressure calorimeters and constant-volume (or "bomb") calorimeters. Bomb calorimeters are used to measure combustion and other gas-producing reactions, in which the reaction is observed in a strong, sealed vessel. A simple constant-pressure calorimeter can be made from a foam coffee cup and a thermometer, in which energy changes in a reaction are observed via the change in temperature of the solution in the cup. The idea behind calorimeters is that if they are sufficiently insulated from the outside environment, any energy gained or lost in the chemical reaction will be directly observable as a temperature and/or pressure change in the calorimeter. In the following experiment, a coffee-cup calorimeter containing 100 mL of H_2O is used. The initial temperature of the calorimeter is 23.0 degree C. If 9.90 g of CaCl_2 is added to the calorimeter, what will be the final temperature of the solution in the calorimeter? The heat of solution Delta H_soln of CaCl_2 is - 82.8 kJ/mol.

Explanation / Answer

numberof moles of CaCl2 ,using molar mass
=9.90 g of CaCl2 / 110.98 g/mol = 0.0892 moles of CaCl2

find the available heat:
0.0892 moles of CaCl2 * -82.8 kJ/mol = -7.385 kJ can be released to the water
find dT water
dH = m C dT

100 g x 4.184 J/g deg x (Tf - 23) = 7385
7385 Joules = (109.9 g mixture) (4.184 J/g-C) (dT)
dT = 16.06 Celsius rise in temp

find final temp
23 C + 16.06C = 39.06 C========answer

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