Equilibrium Question 834
Question: Which one of the following arrangements represents the correct order of the proton affinity of the given species:
Options:
A) $ {I^{-}}<{F^{-}}<H{S^{-}}<NH_2^{-} $
B) $ H{S^{-}}<NH_2^{-}<{F^{-}}<{I^{-}} $
C) $ {F^{-}}<{I^{-}}<NH_2^{-}<H{S^{-}} $
D) $ NH_2^{-}<H{S^{-}}<{I^{-}}<{F^{-}} $
Show Answer
Answer:
Correct Answer: A
Solution:
The species with the greatest proton affinity will be the strongest base, and its conjugate acid will be the weakest acid. The weakest acid will have the smallest value of $ K_{a} $ . Since HI is a stronger acid than HF which is a stronger acid than $ H_2S $ , a partial order of proton affinity is $ {I^{-}}<{F^{-}}<H{S^{-}} $ Since $ NH_3 $ is a very weak acid, $ NH_2^{-} $ must be a very strong base. Therefore the correct order of proton affinity is $ {I^{-}}<{F^{-}}<H{S^{-}}<NH_2^{-} $
