Delta H denotes the change in enthalpy, or energy, between products and reactants in chemical reactions. The change in enthalpy across a chemical reaction is the standard enthalpy of formation of the products minus the standard enthalpy of formation of the reactants. The change in enthalpy, or delta H, is usually in the form of heat expressed in kilojoules per mole.
Enthalpy is also calculated when a substance changes states. For instance, there is more energy in steam at 212 degrees Fahrenheit than water at 212 degrees Fahrenheit because the pressure and volume of the substance has changed. The same is true in reverse, there is less energy in ice at 32 degrees Fahrenheit than in water at 32 degrees Fahrenheit. The enthalpy of vaporization is the amount of energy needed to change a liquid to a gas. The enthalpy of freezing is when a liquid becomes a solid. These enthalpies are expressed as delta H in equations.
Enthalpies of substances change when the temperature increases and decreases because molecules become more active in warmer temperatures and less active in colder temperatures. Particles are more energetic in warmer temperatures, therefore they have higher enthalpies.
Enthalpies with negative numbers denote reactions that give off heat, and enthalpies with positive numbers take in heat and feel colder. A kilojoule is a unit of energy, and a mole is the amount of a substance based upon its molecular or atomic weight.