Brønsted–Lowry - Chemistry

1. Fundamental Concepts

The Brønsted–Lowry theory defines acids and bases based on proton (H⁺) transfer:

Acid: proton donor (releases H⁺)

Base: proton acceptor (gains H⁺)

Acid-base reactions involve a proton transfer from acid to base, not limited to aqueous solutions.

Conjugate acid-base pair: two species differing by one H⁺.

Acid loses H⁺ → forms its conjugate base.

Base gains H⁺ → forms its conjugate acid.

Amphiprotic: can act as acid or base (e.g., H₂O, HCO₃⁻).

Equilibrium favors formation of weaker acids and bases from stronger ones.

Easy

Question:

In the reaction: \[ \text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- \]

Identify the Brønsted–Lowry acid.

Answer: HCl

Explanation: HCl donates a proton (H⁺), so it is a Brønsted–Lowry acid.

Medium

Question:

For the reaction: \[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]

Name the two conjugate acid-base pairs.

Answer:

1. \(\text{NH}_3\) and \(\text{NH}_4^+\)

2. \(\text{H}_2\text{O}\) and \(\text{OH}^-\)

Explanation:

\(\text{NH}_3\) gains H⁺ to become \(\text{NH}_4^+\)

\(\text{H}_2\text{O}\) loses H⁺ to become \(\text{OH}^-\)

Each pair differs by exactly one proton.

Hard

Question:

For the reaction: \[ \text{HCO}_3^- + \text{H}_2\text{PO}_4^- \rightleftharpoons \text{H}_2\text{CO}_3 + \text{HPO}_4^{2-} \]

Label: acid, base, conjugate acid, conjugate base.

Answer:

Acid: \(\text{H}_2\text{PO}_4^-\) (donates H⁺)

Base: \(\text{HCO}_3^-\) (accepts H⁺)

Conjugate acid: \(\text{H}_2\text{CO}_3\)

Conjugate base: \(\text{HPO}_4^{2-}\)

Explanation:

\(\text{H}_2\text{PO}_4^- \rightarrow \text{HPO}_4^{2-}\) (loses H⁺: acid → conjugate base)

\(\text{HCO}_3^- \rightarrow \text{H}_2\text{CO}_3\) (gains H⁺: base → conjugate acid)

Locate H⁺ transfer: the donor is the acid, the acceptor is the base.

Identify conjugate pairs: they differ by one H⁺.

Use charge:

Acid → conjugate base: charge becomes more negative (loses H⁺).

Base → conjugate acid: charge becomes more positive (gains H⁺).

Reaction direction: strong acid + strong base → weak acid + weak base.