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Heat of reaction

Heat of reaction is defined as the amount of heat evolved or absorbed when the reacting species, as represented by a balanced chemical equation, have completely reacted. Enthalpy or heat of reaction is given by
Standard states: In the calculation of heat of reaction, it is a convention to assume that the heat of formation of elements in their standard state is zero. A pressure of 1 atm and a constant temperature are taken as standard states.
State Standard states
Gas Ideal gas at the given temperature and 1 atm
Liquid Pure liquid at the given temperature and 1 atm
Solid Stable crystalline form at given temperature and 1 atm pressure

Factors affecting heat of reaction

  • Physical state of reactants and products
    H2(g) + ½ O2(g) H2O(g); ΔH = –57.8 kcal
    H2(g) + ½ O2(g) H2O(l); ΔH = –68.32 kcal
  • Allotropic forms of the element
    C(diamond) + O2(g) CO2(g); ΔH = –94.3 kcal
    C(amorphous) + O2(g) CO2(g); ΔH = –97.6 kcal
  • Temperature
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