Thermochemistry

Energy is a state function. There are a number of different forms of energy, which is the ability to do work. Work is done anytime a force is applied to make an object move. There is energy of motion, called kinetic energy and energy of position or stored energy, called potential energy. Potential and kinetic energy are interconvertible; that is, one form can change to the other. Different types of energy include thermal energy, electrical energy, radiant energy, chemical energy, mechanical energy and nuclear energy. One type of energy can be converted to another. However, energy can neither be created nor destroyed. It is always conserved. For example, passing electrical energy through a tungsten filament converts it to light energy. All the electrical energy is not converted to light however. Some of it is converted to thermal energy, which is why a light bulb becomes hot after some time.

In most chemical reactions, chemical energy is converted to some other, more useful form of energy. For example, in a flashlight, chemical energy from the batteries is converted to electrical energy. In a car, chemical energy from the combustion of the fuel is converted into mechanical energy. Thermochemistry concerns itself with the relation between chemical reactions and thermal energy. Thermal energy is the energy of motion of particles such as atoms, molecules or ions. Thermal energy depends on the quantity of a substance present and is thus known as an extensive property. The thermal energy provided by a drop of water is much less than that provided by a pot full of water. Temperature, however, is a property that is not dependent on the quantity of substance. The temperature of a drop of boiling water is the same as that of a pot of boiling water. Heat is the transfer of thermal energy that occurs between two objects when they are at different temperatures. If the two objects are at the same temperature, no thermal energy is transferred and no heat is felt. That is how we can tell if an object is hot by touching it. When heat is released from the system in a chemical reaction, the reaction is said to be exothermic. When heat is absorbed by the system, the reaction is said to be endothermic. In an endothermic reaction, the surroundings provide the heat for the reaction while in an exothermic reaction, the surroundings are heated by the reaction. For this reason it is accepted that exothermic quantities are negative quantities, since the system is losing energy and endothermic quantities are positive quantities since the system is gaining energy.