text: Hess's Law states that the sum of the energy changes of all thermochemical equations included in an overall reaction is equal to the overall energy change. Since ΔH is a state function and is not dependent on how reactants become products as a result, steps (in the form of several thermochemical equations) can be used to find the ΔH of the overall reaction. For instance:
  :Reaction 1: C + O → CO
  This reaction is the result of two steps (a reaction sequence):
  :C + ½O → CO
  :ΔH = −110.5 kJ
  :CO + ½O → CO
  :ΔH = −283.0 kJ
  Adding these two reactions together results in Reaction 1, which allows ΔH to be found, so whether or not agents in the reaction sequence are equal to each other is verified. The reaction sequences are then added together. In the following example, CO is not in Reaction 1 and equals another reaction.
  :C + ½O + ½O → CO
  and
  :C + O → CO, Reaction (1)
  To solve for ΔH, the ΔHs of the two equations in the reaction sequence are added together:
  :(−110.5 kJ) + (−283.0 kJ) = (−393.5 kJ) = ΔH of Reaction (1) 
  Another example involving thermochemical equations is that when methane gas is combusted, heat is released, making the reaction exothermic. In the process, 890.4 kJ of heat is released, so the heat is written as a product of the reaction.