1. Constant volume calorimeter (i.e. bomb calorimeter): measures the quantity of energy released by a chemical reactions under constant volume (DE, V= constant). This type of calorimeter consists of a closed container in which the chemical reaction takes place. The heat that is released from the system (=chemical reaction) is transferred to an adjacent water reservoir (=surroundings). The change in water temperature is measured and the can be used to determine the energy content of food.
2. Constant pressure calorimerter (i.e. styrofoam or coffee cup calorimeter, soda can calorimeter): measures the quantity of heat released by chemical reactions under constant pressure (DH, P= constant). This type of calorimeter consists of an insulated vessel containing a solution (=surroundings) in which the reaction (=system) occurs. The temperature change of the solution is measured. Constant pressure calorimeters are easy to construct and we will use these in the laboratory and the discussions below.

Determination of reaction enthalpy using a constant pressure calorimeter:

When heat that is released by the chemical reaction (= system), then temperature of the solution (=surroundings) in which the reaction takes place will increase. Assuming that no heat is lost, then the heat released by the chemical reaction must be transferred to the solution and we can write

Heat is energy that causes a change in the thermal energy of a sample. When heat is added or removed from a sample it causes a change in temperature. Given a certain amount of heat that is transferred, then the amount by which the temperature changes is largely dependent on the heat capacity of the sample.

Heat capacity ( c) = quantity of heat required to raise the temperature of an object by 1 K (i.e. the heat capacity is dependent on the mass of the object).

Units: J/^oC or J/K

Specific heat capacity ( C)= heat needed to raise temperature of one gram of object by 1 K (i.e. the specific heat capacity is independent of the mass of the object and is therefore an intensive property).

Units: J/g*K or J/g*^oC

Specific heat of water = 1 cal/g ^oC or 4.184 J/g ^oC

The amount of heat q transferred: q_surr = m x C DT = - DH_system at constant pressure

Where m = mass of sample [g] , C = specific heat capacity [J/g^oC), and DT the change in temperature [^oC ], and DH_system is the reaction enthalpy [J].

Excercises.:

• Suppose you heat a cup of coffee: identify the system and its surroundings
• What quantity of heat must be added to heat 250 ml of water from 20^oC to 25^oC?
• What quantity of heat must be added to heat the water in a swimming pool from 20^oC to 25^oC (pool dimensions 25 m x 20 m x 1.5 m)?
• What quantity of heat must be added to heat a 250 g of aluminum from 20^oC to 25^oC

So far we just considered heat transfer from one object to another. Now lets look at a chemical reaction. A simple chemical reaction is an acid/base neutralization reaction. Such reactions are highly exothermic and can be conveniently carried out in a styrofoam calorimeter. Our experimental setup consists of

•                 system: acid + base
•                 surroundings : solution and calorimeter

q_surr = q_solution + q_calorimeter = m_solution x C_solution x DT + c_calorimeter x DT = -DH_reaction

(we generally assume that C_solution = C_water)

The heat capacity of the calorimeter c_calorimeter is determined through calibration. This can be achieved by adding a certain amount of hot water to a certain amount of cold water present in the calorimeter and measuring the change in temperature:

The heat capacity of the calorimeter is then

c_calorimeter = q/DT

Exercise: 50 ml of 0.1 M HCl are neutralized with 50 ml of 0.95 M NaOH. The temperature of the liquid rises from 21.5 to 24.7 ^oC. (heat capacity of the calorimeter c_calorimeter = 6.7 J/^oC)

• Calculate q_surr:
• Calculate the enthalpy change DH for the neutralization reaction:
• What is the neutralization enthalpy per mol of acid? (i.e. DH for the reaction H⁺ + OH^-)
• How do these results compare with the neutralization enthalpy per mol acid? (see appendix for values)