Question

On moving from left to right in a period what is the order of acidic character of hydrides?
A. $N{H}_{3} < {H}_{2}O < HF$
B. $ HF < {H}_{2}O < N{H}_{3}$
C. ${H}_{2}O < HF < N{H}_{3}$
D. ${H}_{2}O < N{H}_{3} < HF$

Answer 1

Hint: As we move from left to right in a period, the acidic character of an element increases. Consecutively, the acidic character of the hydrides also increases as we go from left to right in a period.

Complete step by step answer:
- Acidic character is the ability to or a tendency to lose a proton.
- As we move from left to right in a period, we know that the atomic size of the elements decreases. - And due to this decrease in size, the electronegativity, which is the capability of an element to donate electrons, also increases.
- And as a result, the charge separation that is the amount of positive or negative charge, also increases.
- Due to the increase in the electronegative character of the elements, the acidic character of the elements also increases as we move from left to right in a period because the molecule gets more polar and hydrogen gets a larger partial positive charge.
- And as the acidic character increases, the ionization of the hydrides also increases, and thus we can say that as we move along a period, the acidic character of the hydrides also increases.
- Now, looking at the facts discussed above, we can say that, $HF$ has the strongest acidic character and ${H}_{2}O$ has less acidic character than $HF$ and $N{H}_{3}$ has the least acidic character among the three.
Therefore, the increasing order of acidic character of hydrides is: $N{H}_{3} < {H}_{2}O < HF$. Hence, option (a) is the correct answer.

Note: Covalent hydrides have low melting as well as boiling points. Since the compounds which are bonded to hydrogen are more electronegative in nature than hydrogen itself, therefore, they tend to or are more likely to form hydrogen cation rather than hydrogen anion that the ionic hydrides form.