Question

What is a thermochemical equation? How is this different than a chemical equation? Please provide detailed answer and examples of a thermochemical and chemical equations.

Answer 1

A thermochemical equation is a balanced stoichiometric chemical equation with associated enthalpy change ΔH mentioned along.
e.g.,
A + B -> C,

ΔH =123.45 kJ

is a thermochemical equation specifying that for each moles of A and B reacting and C being produced, 123.45 kJ energy is absorbed (since, ΔH is positive). The balanced reaction indicates amount of energy released or absorbed for the specified number of moles of reactants reacting.

A general chemical equation merely specifies reactants and products.

C (s) + O2 (g) -> CO(g), is a chemical equation. In addition to this information, if it is balanced and then specified that such is the enthalpy associated with the balanced equation, then it becomes a thermochemical equation.

C (s) + 1/2 O2 (g) -> CO (g) (balanced equation)
ΔH = −110.5 kJ (enthalpy information)

So, now this equation tells that one mole of carbon reacts with half a mole of oxygen mole to form one mole of carbon monoxide with release of 110.5 kJ of energy (notice the negative sign indicating release of energy).
Further such a reaction can be multiplied with some constant and enthalpy has to be multiplied with the same constant to indicate that the amount of energy release or absorbed will change by the same factor.

E.g. multiplying the aforementioned reaction equation with 2;

2C (s) + O2 (g) -> 2CO (g)
ΔH = −110.5*2 kJ = -221 kJ