Use standard enthalpies of formation to calculate the value of delata H degree for the rxn:NH3(g) + 3F2(g) = NF3(g) + HF(g)
To calculate the standard enthalpy change (ΔH°) for a reaction using standard enthalpies of formation, you need to apply the Hess's Law principle. Hess's Law states that the ΔH° for a chemical reaction is the sum of the ΔH° of the products minus the sum of the ΔH° of the reactants.
1. Write the balanced chemical equation for the reaction:
NH3(g) + 3F2(g) → NF3(g) + HF(g)
2. Determine the standard enthalpy of formation (ΔH°f) for each compound involved in the reaction. The standard enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its constituent elements in their standard states at a given temperature and pressure.
ΔH°f(NH3) = -45.9 kJ/mol (given)
ΔH°f(F2) = 0 kJ/mol (element in its standard state)
ΔH°f(NF3) = -100.6 kJ/mol (given)
ΔH°f(HF) = -273.3 kJ/mol (given)
3. Calculate the ΔH° for the reaction using Hess's Law:
ΔH° = ΣΔH°f(products) - ΣΔH°f(reactants)
ΔH° = [ΔH°f(NF3) + ΔH°f(HF)] - [ΔH°f(NH3) + 3ΔH°f(F2)]
= [-100.6 kJ/mol + (-273.3 kJ/mol)] - [-45.9 kJ/mol + 3(0 kJ/mol)]
= -373.9 kJ/mol + 45.9 kJ/mol
= -328 kJ/mol
Therefore, the value of ΔH° for the given reaction is -328 kJ/mol. This negative value indicates that the reaction is exothermic, meaning it releases energy into the surroundings.