The standard enthalpy of formation is defined as the amount of energy released or absorbed when one mole of a substance is created under normal conditions from its pure elements. Energy is in the form of heat. Standard conditions are usually one atmosphere of pressure and 77 degrees Fahrenheit (298.15 Kelvins). The substance forms in its natural state of matter, either a liquid, solid or gas.
The standard enthalpy of formation is calculated by subtracting the sum of the standard enthalpies of the reactants from the sum of the standard enthalpies of the products in a chemical equation. Enthalpies are listed as joules or kilojoules of energy. One mole is 6.22 x 10^23 atoms of a particular substance. For example, one mole of carbon is 12 grams as indicated by carbon's atomic mass of 12.
The standard enthalpy of formation for pure elements in their normal state is zero. Some standard enthalpies of formation for certain substances include 33.18 kJ/mol for nitrogen dioxide, -296.8 kJ/mol for sulfur dioxide and -241.8 kJ/mol for water. A negative number in front of the standard enthalpy indicates an exothermic reaction, when energy is released into the surroundings and it feels warmer. A positive number means the reaction is endothermic as it absorbs energy from the surrounding environment and it feels colder.