Thermochemical equations

Such chemical equations show the mole ratio of reactants and products, their states and the enthalpy change(energy change)


2C8H18(g) + 25O2(g) => 16CO2(g)+ 18H2O(g) ΔH = -10108 kj/mol

The above thermochemical equation shows the reaction of two mole of octane with 25 mole of oxygen gas. 10108 kj of energy is released.


C8H18(g) + 12.5O2(g) => 8CO2(g)+ 9H2O(g) ΔH = -5054 kj/mol

It is reasonable to assume that if we react one mole of octane with 12.5 moles of oxygen half the amount of energy will be released as when one 2 mole of octane reacted. This is shown in the equation above.


If a reaction occurs in reverse the enthalpy change has the same magnitude but the opposite sign. Photosynthesis is the reverse reaction of respiration.


C6H12O6(aq) + 6O2(g) => 6CO2(g)+ 6H2O(l) ΔH = -2803kj/mol


The reaction above represents respiration while the reaction below is that of photosynthesis.

6CO2(g)+ 6H2O(l) => C6H12O6(aq) + 6O2(g) ΔH = +2803kj/mol