Brønsted-Lowry Acids & Bases (College Board AP® Chemistry): Exam Questions

An alkaline solution is formed when sodium hydrogencarbonate (NaHCO₃) is dissolved in water.

Write an equation for the reaction and explain why the solution is alkaline.

Carbonic acid (H₂CO₃​) dissociates in two steps:

1. H₂CO₃ (aq) ⇌ H⁺ (aq) + HCO₃⁻ (aq)

2. HCO₃⁻ (aq) ⇌ H⁺ (aq) + CO₃²⁻ (aq)

Using this information, identify the conjugate base of HCO₃⁻. Justify your answer.

State whether the HCO₃^- ion is behaving as a Brønsted–Lowry acid or as a base and give a reason for your answer.

In aqueous solution, hydroxylamine (NH₂OH) reacts with hydrogen fluoride (HF):

NH₂OH (aq) + HF (aq) ⇌ NH₃OH⁺ (aq) + F⁻(aq)

Identify the Brønsted–Lowry acid and base in the forward reaction. Justify your answer in terms of proton transfer.

Identify the conjugate acid–base pairs and explain how they are related.

Predict which is the stronger conjugate base. Justify your answer.

CH₃COCH₂COCH₃ + NH₃ → CH₃COCH⁻COCH₃ + NH₄⁺

Identify the Brønsted–Lowry acid and base in the reaction above. Explain your answer.

Phosphine (PH₃) is considered a weaker base than ammonia (NH₃).

In a gas-phase experiment, PH₃ reacts with hydrogen bromide:

PH₃​ (g) + HBr (g) → PH₄⁺ ​(g) + Br⁻ (g)

Explain how this reaction supports the role of PH₃ as a Brønsted–Lowry base, and justify why PH₃ is less basic than NH₃ based on structure and electronegativity.

Methylamine (CH₃NH_2​) is a weak base that reacts with water as follows:

CH₃NH₂ (aq) + H₂O (l) ⇌ CH₃NH₃⁺ (aq) + OH⁻ (aq)

The conjugate acid, CH₃NH₃⁺, can also act as an acid in water:

CH₃NH₃⁺ (aq) + H₂O (l) ⇌ CH₃NH₂ (aq) + H₃O⁺ (aq)

Identify the conjugate acid-base pairs in the reaction between methylamine and water.

Using the equations given in part (a), explain why CH₃NH₃⁺​ is considered a weak acid, while OH⁻ is a strong base.

Compare the role and relative strength of water in the two reactions provided.

Explain how the stability of its conjugate pairs contributes to its behavior.

The reaction of acetic acid (CH₃COOH) with ammonia (NH₃​) in a non-aqueous solvent is shown below:

CH₃COOH + NH₃ ⇌ CH₃COO⁻ + NH₄⁺

Identify the Brønsted-Lowry acid and base in this reaction.

Predict how increasing the concentration of NH₃​ in the solvent could shift the position of equilibrium in this reaction. Justify your answer.

Explain how the solvent affects the strength of the acid and base in this reaction compared to the same reaction in water.

The following acids are arranged in order of increasing strength:

HF < HCl < HBr < HI

For each of the acids listed, write the formula of its conjugate base.

Explain why acid strength increases from HF to HI, referring to bond strength and atomic properties.

Based on the trend in acid strength, predict which conjugate base has the weakest tendency to accept a proton. Justify your answer.