Enthalpy change when a reaction starts from a given pressure and temperature and returns back to the same pressure and temperature after the reaction is complete ## 1. Standard Enthalpy of reaction Enthalpy of reaction at a standard pressure and conditions (1 bar and 25C) It is related to [[Enthalpy of formation]] $\Delta H_{reaction} = \sum_i\Delta_{f}H_{products}^{\ominus}-\Delta_{f}H_{reactants}^{\ominus}$ $\displaylines{\Delta H_{reaction} = \sum_i n_i^{''}\left[{\Delta_{f}}_{STP}H^{\ominus} + \left(h_f(T)-h_{fSTP}^{\ominus}\right)\right]_{products}- \\ \sum_i n_i^{'}\left[{\Delta_{f}}_{STP}H^{\ominus} + \left(h_f(T)-h_{fSTP}^{\ominus}\right)\right]_{reactants}}$ In order for this reaction to produce high temperature (i.e. combustion) - The sensible enthalpy of the products $\left(h_f(T)-h_{fSTP}^{\ominus}\right)$ should be high - The standard enthalpy of the products needs to be a large negative value. i.e the combustion should produce reactants which are stable i.e. stoichiometric reaction - The sensible enthalpy of the reactants should be low (they react at low temps) - The standard enthalpy of the reactants should be high - especially the fuel