Standard enthalpy of reaction  The standard enthalpy of reaction (denoted Δ"H"r⦵) is the enthalpy change that occurs in a system when matter is transformed by a given chemical reaction, when all reactants and products are in their standard states. For a generic chemical reaction  the standard enthalpy of reaction Δ"H"r⦵ is related to the standard enthalpy of formation Δ"H"f⦵ of the reactants and products by the following equation neglecting the heat of mixing of reagents and products or assuming ideal solutions involved:  In this equation, "vB" is the stoichiometric number of entity B. The standard enthalpy of formation, which has been determined for a vast number of substances, is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states. The enthalpy change of any reaction under any conditions can be computed from the above equation, given the standard enthalpy of formation of the reactants and products. The enthalpy change is defined as the amount of heat absorbed or evolved in the transformation of the reactants at a given temperature and pressure into the products at the same temperature and pressure. The heat of a reaction depends upon the conditions under which the reactions are carried out. There are two general conditions under which thermochemical measurements are made. The magnitudes of the heat effects in these two conditions are different. In the first case the volume of the system is kept constant during the course of the measurement by carrying out the reaction in a closed and rigid container and as there is no change in the volume and so no work is also involved. From the first law of thermodynamics we have a relation, formula_4  That is, the heat of a reaction at constant volume is equal to the change in the internal energy (Δ E) of the reacting system. The thermal change that occurs in a chemical reaction is only due to the difference in the sum of internal energy of the products and the sum of the internal energy of reactants. This also signifies that the amount of heat absorbed at constant volume could be identified with the change in the thermodynamic quantity internal energy. At constant pressure on the other hand, the system is either kept open to the atmosphere or confined within a container on which a constant external pressure is exerted and under these conditions the volume of the system changes. The thermal change at a constant pressure not only involves the change in the internal energy of the system but also the work performed either in expansion or contraction of the system.