 Calorimetry

Objectives:

1.    Explain the relationship between the heat capacity and the specific heat of a substance.

2.    Explain how a calorimeter is used to determine the quantity of heat transferred in a chemical reaction.

Key Terms:

calorimetry        heat capacity        specific heat

Calorimetry is the study of heat flow and measurement.  Scientist that study the heat of reactions are actually measuring the changes in the environment surrounding the reaction.  To do this a device called a calorimeter is used.  A calorimeter is a device that separates the outside environment so that any loss or gain of energy in the products results in a change in temperature only in the calorimeter.  The rise (or drop) of temperature in the calorimeter is dependant on the amount of heat released and the heat capacity of the surroundings The heat capacity is the amount of heat energy needed to raise the temperature 1oC The heat capacity of an object depends on the mass and its composition The heat capacity of 1 gram of the substance is its specific heat

ex:    it takes 4.184J (1cal) to raise 1 gram of water 1oC The symbol q is used to symbolize the amount of heat lost or gained in the calorimeter The q of the reaction is equal to the opposite sign of the enthalpy of a reaction formula:    q = (massH2O) x (specific heat H2O) x (temperature change)

ex:    Examine the reaction of sodium hydroxide in 75g water

NaOH(s)    g    Na(aq) + OH(aq)

When the 0.050 moles of sodium hydroxide reacts in the calorimeter the temperature of the water raised from 19.8oC to 26.7oC.

qNaOH = (75g )x(4.184J)/goC)x(26.7oC-19.8oC) = +2170J

Enthalpy (remember that the sign must change for enthalpy)

DH = (1 mol NaOH / 0.050 mol NaOH) x -2.17kJ = -43kJ

Foods are also combusted in calorimeters to reveal the amount of energy (Calories) they contain.  Standard accepted values for food is as follows: Carbohydrates:    4Cal/g or 17kJ/g Fat:                         9Cal/g or 38kJ/g Protein                  4Cal/g or 17kJ/g