Calorimetry

During a chemical reaction, reactants are converted to products. If the reaction requires heat to make products, the reaction is said to be endothermic. Heat flowed into the system, causing the reactants to convert to products. If the reaction evolves heat, the reaction is said to be exothermic. Heat flow is denoted by the letter "q".

If a reaction is endothermic, q > 0.
If a reaction is exothermic, q < 0.

It is often important to know the magnitude and direction of the heat flow for a reaction. The process by which the heat flow is measured is termed calorimetry.

If you would like to measure the heat flow in a reaction, you must have a closed system called a calorimeter. We will use a coffee-cup calorimeter. It consists of two Styrofoam coffee cups. One cup is placed inside the other and covered with a lid. The reaction occurs in the innermost container and the cups provide enough insulation to contain the reaction.

The specific heat (C) of a substance is that amount of heat which is required to increase the temperature of one gram of a substance one degree Celsius. The equation which relates heat flow, temperature change and specific heat is:

where m is the mass of the substance, C is the specific heat, and DT is the change in temperature (T_final-T_initial). This reaction is useful in calculating the heat flow for a reaction if all other variables are known.

In Project 3, Exercise A, you will mass of meat to a high temperature and then place it in a calorimeter which contains water. From experience, you know that the metal will cool and the water will warm up. (Doesn't your hand warm up when you place it in warm water?) Because the calorimeter is a closed system, all the heat lost by the metal must be gained by the water. Therefore we can make the following assumption:

This equation implies that the heat lost by the metals is equal and opposite to the heat gained by the water. You should interpret the negative sign to mean "opposite of". If you substitute the solution for each q, this is the resulting equation:

This equation can be used to determine the specific heat of the metal in Exercise A. Don't forget that DT is T_final-T_initial.

In Project 3, Exercise C and D, you will be exploring Hess's Law. More information on Hess's Law can be found here.

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