Question

In: Chemistry

Which of the following anions act as weak bases in solution? Check all that apply. Br−...

Which of the following anions act as weak bases in solution? Check all that apply.

Br−

NO2−

CN−

Cl−

Expert Solution

We have to consider the Arrhenius basicity here to compare the basicity. An Arrhenius base reacts with water to give OH^- ion in solution, thus making the solution basic.

A^-(aq) + H₂O(l) --> AH(aq) + OH^-(aq)

But some anions do not react with water, they are considered as neutral anion; these are the anions of strong mono-protic acids, e.g., Cl^-, Br^-, NO₃^- and ClO₄^-. The corresponding acids are strong acids and dissociate completely in solution.

HCl ---> H⁺ + Cl^-

Cl^-(aq) + H₂O (l) ----> No Reaction

Thus, Cl^-, Br^-, NO₃^- and ClO₄^- are not bases, they are neutral.

Anions that form from weak acids (incomplete dissociation) acts as acids (e.g., HSO₄^-, H₂PO₄^-) or bases.

Now we have to compare between NO₂^- and CN^-. These are conjugate bases of weak acids. We know, conjugate bases of weak acids are strong bases. Between HNO₂ and HCN, former is stronger than latter. It is clear from the pK_a value of the two acids.

pK_a of HNO₂ = 3.15

pK_a of HCN = 9.31

So, HCN is weaker acid than HNO₂.

Thus, NO₂^- is weaker base than CN^-.

Answer: NO₂^- acts as a weak base in solution. Though CN^- also is a weak base, it is stronger than NO₂^-.

queen_honey_blossom answered 2 years ago

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