Using a Calorimeter to Measure Heat Energy in a Chemical Reaction
In the lab, chemists use a calorimeter to experimentally measure the heat associated with a chemical reaction. A calorimeter is an insulated chamber outfitted with a thermometer used to measure the temperature change of a chemical reaction taking place inside. The observed temperature change, ΔT, can then be used to calculate the amount of heat absorbed or released during the reaction.
Temperature change, ΔT, is the difference in temperature before and after a reaction. It is not a measure of the heat energy accompanying the reaction, but it can be used to calculate the heat content change. The calculation is done using a property of the reaction called specific heat capacity, often simply called specific heat. Specific heat capacity or specific heat is the amount of energy absorbed by 1 gram of a substance when its temperature is raised by 1°C. For example, the specific heat of water is 4.18 J/g•°C. This value indicates that increasing the temperature of 1 gram of water by 1°C requires 4.18 joules of heat energy.
The heat energy absorbed by the solution is determined using the following expression:
Q = C × ΔT × m
where C is the specific heat of the reaction mixture, m the mass of the reaction mixture, and ΔT the observed change in temperature.
Suppose that a reaction is carried out in an aqueous solution having a specific heat of 3.94 J/g•°C. The mass of the reaction solution is 100.0 g, and the temperature changes from a starting value of 25.2°C and decreases to 18.4°C at the end of the reaction. To calculate the energy change for the reaction:
1. First calculate the change in temperature, ΔT:
ΔT = 18.4°C – 25.2°C = –6.8°C
ΔT is negative because the temperature of the solution has decreased, indicating the solution has released energy.
2. Use the equation above to calculate the heat energy released by the solution:
Q = 3.94 J/g•°C × (–6.8°C) × 100.0 g = –2,680 J = –2.68 kJ
3. The reaction absorbs heat released by the solution. Thus, it is an endothermic reaction and ΔH = +2.68 kJ. A positive sign is added to the energy change to indicate that the reaction is endothermic.
However, if the reaction had resulted in an increase in temperature, then ΔT would be positive and Q would also be positive. In this case, the solution would have absorbed energy and the reaction would have produced energy. Therefore, a negative sign would be used to show that it is an exothermic reaction.
The temperature of a 10. g aluminum rod is raised from 15°C to 23°C. What is the specific heat of aluminum knowing that the energy absorbed as heat is 71.2 J?
C =
J/g•°C
