IMAT 2018 Q49 [Ionic Equation]

Consider this ionic equation in aqueous solution:

CH_3O^– + H_2O → CH_3OH + OH^–

A. Water is acting as a Brønsted–Lowry base.

B. The CH_3O^– ion is a Lewis acid.

C. The final mixture of solutions would have a pH below 7.

D. CH_3OH is the conjugate acid of CH_3O^–.

E. The hydroxide ion is the conjugate acid of the water.

What is Bronsted-Lowry’s acid-base theory (1923)?

They describe acids and bases in terms of how they react (ex. water can act as an acid or base depending)

Bronsted-Lowry Acid: Proton Donor (Donates H+)
Bronsted-Lowry Base: Proton Acceptor (Receives H+)

What are Lewis acids and bases?

Lewis acid - accepts pair of electrons
Lewis base - donates pair of electrons

(A Bronsted-Lowry acid or base is a Lewis acid or base but a Lewis acid or base is not always a Bronsted-Lowry acid or base.)

According to Brønsted–Lowry:
CH_3O^– : Protein acceptor = Brønsted–Lowry Base
H_2O : Proton donor = Brønsted–Lowry Acid
CH_3OH : conjugate acid (when base accepts proton)
OH^– : conjugated base (when acid loses hydrogen)

According to Lewis:
CH_3O^– : Electron donor = Lewis Base
H_2O : Electron acceptor = Lewis Acid
CH_3OH : conjugate acid
OH^– : conjugated base

The final solution will be basic as we have OH^- ions present (which are a strong base, and we have CH_3OH which is a weak acid.

A. Water is acting as a Brønsted–Lowry base.

Incorrect, water is acting as a Brønsted–Lowry acid because it is donating a proton to CH_3O^– .

B. The CH_3O^– ion is a Lewis acid.

Incorrect, CH_3O^– is an Electron donor and therefore a Lewis Base

C. The final mixture of solutions would have a pH below 7.

Incorrect. The final ions in solution are a weak acid CH_3OH and highly basic OH^- ions, resulting in a basic solution.

D. CH_3OH is the conjugate acid of CH_3O^–.

CH_3O^– is a base, and it will accept a proton to form CH_3OH. A conjugate acid is formed when the base accepts a proton, therefore CH_3OH is the conjugate acid of CH_3O^-.