Question

The conjugate acid of \[{\text{NH}}_{\text{2}}^{\text{ - }}\] is
A.\[{\text{N}}{{\text{H}}_3}\]
B.\[{\text{N}}{{\text{H}}^{2 - }}\]
C.\[{\text{NH}}_4^ + \]
D.\[{\text{N}}_3^ + \]

Answer 1

Hint: If conjugate acid is present for any species then the given species will be base. A base is that species which gains protons or \[{{\text{H}}^{\text{ + }}}\].

Complete step by step answer:
An acid is a substance which loses \[{{\text{H}}^{\text{ + }}}\] ion or gains electrons. A base is a substance which either gains \[{{\text{H}}^{\text{ + }}}\] ions or lose electrons. In this question we are only concerned with loss and gain of \[{{\text{H}}^{\text{ + }}}\] ions.
Conjugate pairs are those pairs which differ by one \[{{\text{H}}^{\text{ + }}}\] ion. Since acid is a proton donor, whenever an acid loses its proton one anionic species and one \[{{\text{H}}^{\text{ + }}}\] will form. This anionic species will be the conjugate base of that acid. For example, let us consider any acid HA, it will dissociate as:
\[HA \to {H^ + } + {A^ - }\] the \[{A^ - }\] is termed as conjugate base.
Similarly for a base, if it takes \[{{\text{H}}^{\text{ + }}}\] the respective cationic species formed will be termed as conjugate acid.
In the above question it is clear that \[{\text{NH}}_{\text{2}}^{\text{ - }}\] is a base, because the respective species that is being formed is an acid. A base never donates \[{{\text{H}}^{\text{ + }}}\] . So option B and D are eliminated. Now since the conjugate pairs differ by one \[{{\text{H}}^{\text{ + }}}\] , option C has a difference of 2 hydrogen. Hence the only option left is option A, which is the correct option.

The correct option is A.

Note:
 The IUPAC name of \[{\text{NH}}_{\text{2}}^{\text{ - }}\] is Azanide. It is commonly called amide ion. Though there is also an amide functional group present in organic chemistry. When ammonia reacts with alkali metals \[{\text{NH}}_{\text{2}}^{\text{ - }}\] is formed. Alkali metals reacts with \[{\text{N}}{{\text{H}}_3}\] to form their respective amides such as sodium amide \[{\text{(NaN}}{{\text{H}}_2})\], potassium amide or lithium amide.