Standard enthalpy change is the energy exchange that occurs when the surroundings are at standard conditions, which includes constant atmospheric pressure, a room temperature of 25°C, normal physical states for all substances involved, and a concentration of 1 mol per liter for solutions. The types of standard enthalpy changes are reaction, formation and combustion.
Enthalpy is defined as the total energy of the reacting materials, which is given by the sum of the internal energy of the system plus the product of the system's pressure and volume. Reaction enthalpy change occurs when the amount of reactants in the chemical equation react under standard conditions to create the products. Formation enthalpy change occurs when one mole of a compound is formed from its elements that are in their standard states and in standard conditions. Combustion enthalpy change occurs when one mole of an element reacts completely with oxygen under standard conditions.
Reactions are also classified as exothermic or endothermic depending on the direction of the standard enthalpy change. Exothermic reactions occur when the enthalpy change is negative, which means energy is lost to the environment and the chemical bonds are formed in the reactions. Endothermic reactions occurs when the enthalpy change is positive. In this reaction, energy is gained from the environment and the chemical bonds are broken.