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

Question

Calorimetry is a method used to measure enthalpy, or heat, changes 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, where 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; energy changes in a reaction are observed via a temperature change 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.

A total of 2.00 mol of a compound is allowed to react with water in a foam coffee cup and the reaction produces 151g of solution. The reaction caused the temperature of the solution to rise from 21.00 to 24.70 ?C. What is the enthalpy of this reaction? Assume that no heat is lost to the surroundings or to the coffee cup itself and that the specific heat of the solution is the same as that of pure water.

Explanation / Answer

The specific heat of the solution is same as that of the pure water.

So, Cp = 4.184 J / g oC

Mass of the solution (m) = 151 g

Initial temperature T1 = 21 oC

Final temperature T2 = 24.7 oC

Change in temperature Delta T = T2-T1

=24.7 - 21.0

= 3.7 oC

Heat generated Q = m x c x delta T

Q = 151 x 4.184 x 3.7

Q = 2337.6 J

Divide by 1000 to convert to kJ, we get,

Q = 2.34 kJ

Change in enthalpy Delta H = Q / number of moles

Delta H = 2.34 kJ / 2 mol

Delta H = 1.17 kJ / mol

The reaction releases the energy, so it is an exothermic reaction. And in exothermic reaction, delta H is always negative

So, our answer will be Delta H = -1.17 kJ/mol

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