Consider the following thermochemical equations.

PCl5 (s)→PCl3 (g)+Cl2 (g) ΔH∘rxn= 87.9kJmol
2P (s)+3Cl2 (g)→2PCl3 (g) ΔH∘rxn= −574kJmol
Using this data, determine the heat of formation for PCl5.

The for the reaction is -749.8 kJ.

Explanation:

Hess’s law of constant heat summation states that the amount of heat absorbed or evolved in a given chemical equation remains the same whether the process occurs in one step or several steps.

According to this law, the chemical equation is treated as ordinary algebraic expressions and can be added or subtracted to yield the required equation. This means that the enthalpy change of the overall reaction is equal to the sum of the enthalpy changes of the intermediate reactions.

The chemical reaction for the formation of follows:

The intermediate balanced chemical reaction are:

(1)       ( × 2)

(2)     

The expression for enthalpy of the reaction follows:

Putting values in above equation, we get:

Hence, the for the reaction is -749.8 kJ.