Question

Which of the following substances has the highest proton affinity?
A. $N{{H}_{3}}$
B. $P{{H}_{3}}$
C. ${{H}_{2}}O$
D. ${{H}_{2}}S$

Answer 1

Hint: The proton affinity which is represented by ${{P}_{A}}$ or ${{E}_{PA}}$ can be defined as an anion which is a neutral atom or molecule and it is the negative of the enthalpy change in the reaction between the above species and proton in the gas phase.

Complete step by step answer:
Reactions in which proton affinity occurs are always exothermic in the gas phase an exothermic term refers to the reaction in which energy is released when the reaction takes place in the forward reaction and enthalpy is negative in this case while the proton affinity is positive. This is the same sign convention as is used for electron affinity. The property related to the proton affinity is actually a gas-phase basicity which is the negative of the Gibbs energy.
Those compounds which have a high tendency to donate an electron pair always show high electron affinity. Those chemical species which have the ability to donate an electron pair to a given Lewis acid in order to form adduct are known as Lewis bases. The most common example of Lewis bases are ammonia, alkyl amines, and other conventional amines. Generally Lewis bases are anionic in nature and their base strength generally depends on the ${{P}_{ka}}$ of the corresponding parent acid.

Hence out of the option given in the question ammonia shows highest proton affinity, option A is true.

Note: Lewis bases are electron-rich species that we can say that they have the ability to donate electron-pairs and these are classified in the category of nucleophiles while Lewis acids are kept in the category of electrophiles.