Definition of Heat
Heat is defined as the energy transferred from one system to another by thermal interaction. It is measured in joules (J). Heat is transferred by conduction, convection, and/or radiation.
When a high temperature body is brought into contact with a low temperature body, heat flows from the high temperature body to the low temperature body spontaneously until their temperatures are equivalent. The high temperature body loses thermal energy, and the low temperature body acquires this same amount of thermal energy. The system is then said to be at thermal equilibrium.
Definition of Work
Work is the transfer of energy by any process other than heat. Like heat, the unit measurement for work is joules (J). There are many forms of work, including but not limited to mechanical, electrical, and gravitational work. For our purposes, we are concerned with P-V work, which is the work done in an enclosed chemical system. In this type of system, work is defined as the change in the volume (V) in liters within the system multiplied by a pressure (P). Assuming the system is at constant pressure, this equates to the following:
Most often, we are interested in the work done by expanding gases. Assuming the gases are ideal, we can apply the ideal gas law to the above equation to get the following:
Relationship Between Heat and Work
Heat and work are related. Work can be completely converted into heat, but heat can only be partially converted into work. Scientists and engineers have been able to exploit the principles of thermochemistry to develop technologies ranging from hot/cold packs to gasoline powered combustion engines.
For a closed system, the change in internal energy (∆U) is related to heat (Q) and work (W) as follows:
This means that the total energy within a system is affected by the sum of two possible energy transfers: heat and work.