Proton affinity is the enthalpy change due to acceptance of 1 mole of H + (g) by a species.
Determine the proton affinity of NH 3 (g) from the following data in kcal/mol in nearest possible integers.
LE of NH 4 F (s) = – 181 kcal/mol
Δ form of NH 4 F (s) = –112 kcal/mol
Δ form of NH 3 (g) = –280 kcal/mol
Δ atom of H 2 (g) = 104.2 kcal/mol
IE of H (g) = 311.9 kcal/mol
Δ atom H (F 2 ,g) = 37.8 kcal/mol
Δ eg H (F,g) = –79.6 kcal/mol
Text Solution
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46
Sol. 
N 2 (g) + 2H 2 (g) + 
F 2 (g) → NH 4 F (s) Δ H = –112
N 2 (g) + 
H 2 (g) → NH 3 (g) Δ H = –280
52.1 + 18.9 + 311.9 – 79.6 + PA – 181 = 168
or 122.3 + PA = 168
or PA = 45.7 kcal/mol ~ 46 kcal/mol.
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