State and illustrate Hesse's law.

Explanation

Hess's law : Whether a chemical reaction takes place in a single step or in several steps, the total change n enthalpy remains the same.
Consider the formation of `CO_(2)` from C.
1 Method (Single step) : `C(S) + O_(2)(g) rarr underset("Heat liberated = xkJ")(CO_(2)(g) : Delta H = -xkJ)`
II Method (Two steps) `C(s) + (1)/(2)O_(2)(g) rarr CO(g), Delta H = -x_(1)kJ`
`CO(g) + (1)/(2)O_(2)(g) rarr CO_(2)(g), Delta H = -x_(2)kJ`.
Total heat liberated `= (x_(1) + x_(2))kJ`
According to Hess's law, `x = x_(1) + x_(2)`.