The standard enthalpy change of reaction (denoted ΔH° or ΔH^o)is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ... A chemical reaction is a process that results in the interconversion of chemical substances [1]. The substance or substances initially involved in a chemical reaction are called reactants. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...

For a generic chemical reaction

n_A A + n_B B + ... → n_P P + n_Q Q ...

the standard enthalpy change of reaction ΔH_r is related with the standard enthalpy change of formation ΔH_f of the reactants and products by the following equation: The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 atmosphere...

In this equation, the n_i corresponds to the stoichiometric coefficents.

Reactions with standard values

A common standard enthalpy change is the standard enthalpy change of formation, which has been determined for a vast number of substances. The enthalpy change of any reaction under any conditions can be computed, given the standard enthalpy change of formation of all of the reactants and products. Other reactions with standard enthalpy change values include combustion (standard enthalpy change of combustion), neutralisation (standard enthalpy change of neutralisation), and solution (standard enthalpy change of solution). The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 atmosphere... The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 atmosphere... Standard enthalpy of Combustion is the Enthalpy change when one mole of a substance is totally combusted in oxygen, and is measured at 298K and 1 atmospheric pressure. ... The standard enthalpy change of neutralization is the change in enthalpy that occurs when an acid and base undergo a neutralization reaction to form one mole of water under standard conditions(298k and 1atm), i. ...