Thermodynamics and Thermochemistry

Thermodynamics and Thermochemistry
The study of energy and its transformations is known as thermodynamics. Thermochemistry is the branch of thermodynamics that relates to chemical reactions and energy changes involving heat. Most thermochemical studies are conducted in a laboratory where the transfer of heat can be carefully monitored in a closed system. The measurement of heat transfer is called calorimetry. The device in which heat transfer is measured is called a calorimeter. A calorimeter can be a sophisticated device that isolates the reaction from its surroundings or it can be a simple device in which the reactants are in constant, close physical contact with the surroundings. In either case, it is important that temperature and heat transfer can be easily monitored. The reactants contained in a calorimeter are defined as the system. The solvent in which the reactants are dissolved, the calorimeter itself including the thermometer, and any air that is in contact with the solution are defined as the surroundings. When the reagents are mixed they react. Any heat generated in an exothermic reaction is transferred to the surroundings (the solvent, the calorimeter, and the air). An endothermic reaction must, conversely, draw heat from the surroundings. In order to determine the amount of heat that is absorbed by the calorimeter it is advisable that the calorimeter be calibrated (see part A below).
Heat transfer causes the temperature of a system and its surroundings to change due to absorption or release of heat. The change in temperature, ΔT, can be measured and used to calculate the heat absorbed or released by the reaction. Heat, Q, is then used to calculate the change in Enthalpy, ΔH. To obtain the temperature change it is necessary to measure the initial temperature of the system/surroundings and also to determine the final, maximum temperature after the reaction. The temperature at the time of mixing cannot be determined directly because the reaction doesn’t happen instantaneously, neither is the heat of the reaction instantly transferred to the surroundings (mainly the solvent) where it can be measured. There is also a delayed response in the thermometer. To compensate for these time delays, the temperature of the reactants is measured before mixing and then the temperature change is monitored at timed intervals after mixing. The actual value of the final temperature is then determined by constructing a time post reaction versus temperature graph. The temperature at the time of mixing, Tmax or Tfinal is determined by fitting a line through the data points and extrapolating back to the time of mixing