The study of the relationship between heat and work within chemical reactions is known as thermodynamics. Chemical thermodynamics helps to determine whether a reaction proceeds spontaneously as well as determining whether a reaction is reversible or irreversible. Chemical energy is the potential energy with a substance that can be used by making or breaking bonds, which can absorb or evolve heat. As heat and work are both forms of energy, they can be transformed into each other. Work and heat can both be described using the same unit of measure. Sometimes the calorie is the unit of measure, and refers to the amount of heat required to raise one (1) gram of water one (1) degree Celsius. Heat energy is measured in kilocalories, or 1000 calories. Typically, the SI units of Joules (J) and kilojoules (kJ) can be converted into calories. One calorie of heat is equivalent to 4.187 J. The term specific heat is also relevant, the heat required to raise one (1) gram of a material one (1) degree Celsius. Specific heat, given by the symbol “C.”

Chemically, when energy is converted to work, energy in the form of heat moves from one place to another, or energy is stored up in the constituent chemicals. Heat is defined as that energy that is transferred as a result of a temperature difference between a system and its surroundings. Mathematically, we can look at the change in energy of a system as being a function of both heat (q) and work (w). If q is positive, we say that the reaction is endothermic, that is, heat flows into the reaction from the outside surroundings. If q is negative, then the reaction is exothermic, that is, heat is given off to the external surroundings.

Enthalpy is a state property, the word enthalpy comes from the Greek “heat inside”. If a chemical system undergoes some kind of change, but has a fixed volume, the heat output is equal to the change in internal energy (q = ΔE). Define the enthalpy change, ΔH, of a system as being equal to its heat output at constant pressure. So in order to calculate the change in enthalpy, calculate the enthalpy of products minus the enthalpy of reactants.