Consider the following intermediate chemical equations.

what is the enthalpy of the overall chemical reaction

image below

250 kJ

Explanation: According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to Hess’s law, the chemical equation can be treated as algebraic expressions and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

  (1)

  (2)

Net chemical equation:

  (3)

Adding 1 and 2 we get,

  (4)

Now dividing equation (4) by 4, we get

  (4)