Ch 2 · Brønsted–Lowry theory of acids and bases · AS & A Level Chemistry 9701

Topic 2 of 2 in Equilibria — AS & A Level Chemistry 9701 — 66 lessons.

Introduction

Acid–base chemistry is a form of equilibrium chemistry: acids and bases are defined by proton transfer, and the strength of an acid or alkali is described by how far it dissociates in water. This lesson covers the qualitative ideas needed to explain the pH scale, compare strong and weak acids, and interpret titration curves and indicator choice.

Key ideas — Common acids and alkalis, and the Brønsted–Lowry theory

Common acids include hydrochloric acid, HCl, sulfuric acid, H_2SO_4, nitric acid, HNO_3, and ethanoic acid, CH_3COOH. Common alkalis include sodium hydroxide, NaOH, potassium hydroxide, KOH, and aqueous ammonia, NH_3(aq). The Brønsted–Lowry theory defines an acid as a proton donor and a base as a proton acceptor; when an acid reacts with water it donates a proton to it:

🧑‍🏫 Teacher's explanation

Think of protons (H^+ ions) like hot potatoes in a game of musical chairs. An acid is a person holding a hot potato who is eager to pass it to someone else (a proton donor). A base is someone waiting to catch that potato (a proton acceptor).

📖 Definitions

🔍 Worked example — Easy

Identify the acid and base in the following reaction: NH_3(aq) + H_2O(l) ightleftharpoons NH_4^+(aq) + OH^-(aq)

🔍 Worked example — Standard

Calculate the concentration of H^+(aq) ions in a 0.10 ext mol dm^-3 solution of HCl(aq).

🔍 Worked example — Exam-style

A 0.050 ext mol dm^-3 solution of ethanoic acid, CH_3COOH, has a pH of 3.4. Calculate the concentration of the undissociated CH_3COOH molecules.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Strong and weak acids and bases

A strong acid or strong base dissociates (ionises) fully in aqueous solution, so essentially all of the molecules present release H^+ or OH^- ions. A weak acid or weak base only partially dissociates, setting up an equilibrium between the undissociated molecule and its ions:

🧑‍🏫 Teacher's explanation

Think of a crowd of people in a room. A 'strong acid' is like a group of very energetic people who all immediately start shouting at once; the room becomes incredibly loud (high concentration of H^+ ions) instantly. A 'weak acid' is like a group of shy people; only a few decide to speak up, while most stay silent (undissociated molecules). Even if you have the same number of people in the room, the 'shouting' level is much higher in the strong group.

📖 Definitions

🔍 Worked example — Easy

Calculate the [H^+] concentration in 0.10 mol dm^-3 HCl(aq). Assume HCl is a strong acid.

🔍 Worked example — Standard

A 0.10 mol dm^-3 solution of ethanoic acid (CH_3COOH) has a [H^+] concentration of 1.3 × 10^-3 mol dm^-3. Calculate the degree of dissociation (percentage of molecules that ionised).

🔍 Worked example — Exam-style

Compare the pH of 0.10 mol dm^-3 HCl and 0.10 mol dm^-3 CH_3COOH. Given [H^+] for HCl is 0.10 mol dm^-3 and [H^+] for CH_3COOH is 1.3 × 10^-3 mol dm^-3, calculate the ratio of [H^+] concentrations.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — The pH scale

The pH scale is a measure of hydrogen ion concentration: a lower pH corresponds to a higher [H^+]. Pure water, at pH 7, is neutral; acidic solutions have pH values below 7, and alkaline solutions have pH values above 7. Each fall of one pH unit corresponds to a ten-fold increase in [H^+], since pH is a logarithmic scale, defined as pH = -log_10[H^+].

🧑‍🏫 Teacher's explanation

Imagine you are measuring the intensity of an earthquake. A magnitude 7 earthquake is much more powerful than a magnitude 6, not just slightly stronger. The pH scale works exactly like this. It is a logarithmic scale, meaning every time the pH number goes down by 1, the concentration of hydrogen ions [H^+] actually increases by a factor of 10.

📖 Definitions

🔍 Worked example — Easy

Calculate the pH of a solution where the hydrogen ion concentration [H^+] is 1.0 × 10^-3 mol dm^-3.

🔍 Worked example — Standard

A solution has a pH of 4.0. Calculate the concentration of hydrogen ions [H^+] in mol dm^-3.

🔍 Worked example — Exam-style

Solution A has a [H^+] of 0.05 mol dm^-3. Solution B has a [H^+] of 0.0005 mol dm^-3. Calculate the difference in pH between the two solutions.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Calculate the pH of this solution. [1] (b) Calculate the concentration of hydroxide ions [OH^-] in this solution at 298 K. [2] [3 marks]

Key ideas — Neutralisation

Neutralisation occurs when an acid reacts with a base or alkali, and is best represented by the ionic equation:

🧑‍🏫 Teacher's explanation

Think of neutralisation as a 'reconciliation' between two opposites. In chemistry, acids are essentially 'proton donors'—they have extra hydrogen ions (H^+) looking for a place to go. Bases (specifically alkalis) are 'proton acceptors'—they have hydroxide ions (OH^-) looking to grab a proton. When they meet, they don't just sit there; they react to form the most stable, neutral substance possible: water (H_2O).

📖 Definitions

🔍 Worked example — Easy

Write the net ionic equation for the reaction between HCl(aq) and NaOH(aq).

🔍 Worked example — Exam-style

A student titrates 20.0 cm^3 of 0.50 mol dm^-3 H_2SO_4 with 0.20 mol dm^-3 KOH(aq). Calculate the volume of KOH needed to reach the equivalence point.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — pH titration curves

As alkali is added to acid, pH rises slowly at first, then rises very steeply near the equivalence point, before levelling off. A strong acid–strong base curve has a long, steep vertical section centred near pH 7. A weak acid–strong base curve starts at a higher initial pH, has a shorter vertical section centred above pH 7, and shows a gradual "buffer region" of slowly changing pH before the equivalence point. A strong acid–weak base curve has a shorter vertical section centred below pH 7. A weak acid–weak base curve has no sharp vertical section at all, only a gradual change in gradient.

🧑‍🏫 Teacher's explanation

Imagine you are adding a very strong, concentrated syrup (the alkali) to a large cup of bitter, acidic juice. At first, the juice barely changes because there is so much acid present. But as you get closer to the point where the acid is neutralized, every single drop of syrup causes a massive, sudden change in the taste (the pH). This is the 'vertical section' of a titration curve.

📖 Definitions

🔍 Worked example — Easy

A student titrates 25.0 cm^3 of 0.100 mol dm^-3 HCl with 0.100 mol dm^-3 NaOH. Calculate the pH at the equivalence point.

🔍 Worked example — Standard

A 50.0 cm^3 sample of 0.10 mol dm^-3 ethanoic acid (CH_3COOH) is titrated with 0.10 mol dm^-3 NaOH. Calculate the pH at the equivalence point (assume K_a of CH_3COOH = 1.8 × 10^-5).

🔍 Worked example — Exam-style

A titration curve for a weak acid and a strong base shows a vertical section between pH 7.5 and pH 10.5. Which indicator is most suitable: Methyl Orange (range pH 3.1–4.4) or Phenolphthalein (range pH 8.3–10.0)?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Selecting a suitable indicator

An indicator is suitable for a titration only if its full colour-change range lies entirely within the vertical (steep) section of the pH curve, so that it changes colour sharply over the one or two drops of alkali added at the equivalence point. Phenolphthalein (colourless to pink, range pH 8.2–10.0) is suitable for strong acid–strong base and weak acid–strong base titrations, where the vertical section includes this range, but unsuitable where a weak base is used. Methyl orange (red to yellow, range pH 3.1–4.4) is suitable for strong acid–strong base and strong acid–weak base titrations, but unsuitable where a weak acid is used. For a weak acid–weak base titration, no indicator gives a sharp colour change, and a pH meter must be used instead.

🧑‍🏫 Teacher's explanation

Imagine you are trying to catch a ball that is flying through a very narrow window. If the ball's path (the pH change) is very steep and fast, you need a sensor (the indicator) that reacts instantly at exactly that moment. If the ball's path is a slow, gentle curve, your sensor might trigger too early or too late because the pH is changing too slowly to catch it.

📖 Definitions

🔍 Worked example — Easy

A student titrates 0.10 mol dm^-3 HCl with 0.10 mol dm^-3 NaOH. The pH at the equivalence point is 7.0. Which indicator is most suitable: Methyl orange (range pH 3.1–4.4) or Phenolphthalein (range pH 8.2–10.0)?

🔍 Worked example — Standard

A titration involves 0.10 mol dm^-3 CH_3 extCOOH (weak acid) and 0.10 mol dm^-3 NaOH (strong base). The pH at the equivalence point is 8.8. Select the best indicator: Methyl orange (pH 3.1–4.4) or Phenolphthalein (pH 8.2–10.0).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Question. A solution of hydrochloric acid, a strong monoprotic acid, has a concentration of 0.0100\ mathrmmol\ dm^-3. (a) Calculate the pH of this solution. [2] (b) A second acidic solution has the same concentration but a pH of 3.4. State, with a reason, whether this second acid is strong or weak. [2]

Ch 2 · Brønsted–Lowry theory of acids and bases · AS & A Level Chemistry 9701

All chapters in Equilibria