Correct Answer - Option 2 : 10.20 kcal/mol
Enthalpy of formation (Heat of formation):
- It is defined as the enthalpy change when one mole of a compound is formed from its elements.
- It is designated as ΔHf.
∆Hf = ΔHf (products) - ΔHf (reactants)
Where, ΔHf (products) = Enthalpy of formation of the product, ΔHf (reactants) = Enthalpy of formation of reactant
Standard enthalpy of formation:
- The standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation is called Standard Molar Enthalpy of Formation.
- Its symbol is ΔHfo.
- The enthalpy of formation of all free elements in their standard states is regarded as zero.
i.e., ΔHfo = ΔHfo (products) - ΔHfo (reactants)
Where, ΔHfo = Standard enthalpy of a formation, ΔHfo (products) = Standard enthalpies of formation of all products, ΔHfo (reactants) = Standard enthalpies of formation of all reactants
Given: H2 + I2 → 2HI ; ΔH = 20.40 kcal/mol
We know that, ∆Hf = ΔHf (products) - ΔHf (reactants)
⇒ ∆Hf = [2×∆Hf (HI) - ∆Hf (H2) - ∆Hf (I2)]
The enthalpy of formation of all free elements in their standard states is regarded as zero.
Therefore, ∆H(H2) = 0 and ∆H(I2) = 0
⇒ 20.40 kcal = 2×∆Hf (HI)
⇒ ∆Hf (HI) = 20.40/2
⇒ ∆Hf (HI) = 10.2 kcal/mol
Hence, the enthalpy of formation of HI is 10.2 kcal/mol.