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E3.6 · Explain Brønsted–Lowry acids and bases

Learn to explain brønsted–lowry acids and bases through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

Explaining acid–base reactions as proton transfer

An acidic solution can change the colour of an acid–base indicator. That visible change is evidence that the solution has different chemical properties from a neutral solution; it does not, by itself, tell us which particles transferred what. The Brønsted–Lowry model explains acid–base reactions by tracking a proton, written as H+\mathrm{H^+}, as it moves from one particle to another. In this lesson, you will use that transfer to identify acids and bases, then name the related particles.

What you will learn

  • Define a Brønsted–Lowry acid and a Brønsted–Lowry base.
  • Describe acid–base reactions as transfers of protons between particles.
  • Identify the acid, base, conjugate acid, and conjugate base in a reaction.
  • Explain why water can act as either an acid or a base.

1. Start with particles and proton transfer

You have already met acids and bases in earlier chemistry. For this lesson, the key particle is the hydrogen ion, written as H+\mathrm{H^+}. A proton is the positively charged particle at the centre of a hydrogen atom. In a Brønsted–Lowry acid–base reaction, a proton moves from one particle to another.
A Brønsted–Lowry acid is a particle that donates a proton. A Brønsted–Lowry base is a particle that accepts a proton. These definitions describe what a particle does in a reaction. To decide whether a substance is acting as an acid or a base, examine the proton transfer in the reaction rather than relying only on its name or formula.
Water provides a useful connection between what you can observe and the particle model. Water can take part in reactions with acids and bases. At the particle level, its role depends on whether it gives away a proton or accepts one. The same substance can therefore act as an acid in one reaction and a base in another.
H+\mathrm{H^+}
  • Acid: proton donor.
  • Base: proton acceptor.
  • Identify the transfer before assigning acid and base roles.

2. Track the proton in an equation

Chemical equations show the particles before and after a reaction. Compare the formulas on both sides to see which particle loses a hydrogen ion and which gains one. The particle that loses the proton is the acid. The particle that gains it is the base.
For example, when hydrogen chloride reacts with water, hydrogen chloride transfers a proton to a water molecule. The products are a hydronium ion, H3O+\mathrm{H_3O^+}, and a chloride ion, Cl−\mathrm{Cl^-}. Hydronium is a water molecule that has accepted a proton. The atoms and net charge are conserved: both sides contain one hydrogen chloride unit and one water molecule’s worth of atoms, and the total charge is zero.
The transferred proton is not shown floating freely in this equation. It is shown as part of the hydronium ion after water accepts it. This makes the change in particles clear while keeping the equation balanced.
HCl(aq)+H2O(l)→H3O+(aq)+Cl−(aq)\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)}
  • A proton transfer changes the particles’ formulas.
  • Hydronium forms when water accepts a proton.
  • Check that atoms and total charge are conserved.

3. Name the conjugate acid–base pairs

When an acid donates a proton, the particle left behind is its conjugate base. A conjugate base is the particle that can accept a proton to form the original acid. When a base accepts a proton, the product is its conjugate acid. A conjugate acid is the particle that can donate a proton to form the original base.
In the hydrogen chloride reaction, hydrogen chloride donates a proton and becomes chloride. So hydrogen chloride and chloride form one conjugate acid–base pair. Water accepts a proton and becomes hydronium. So water and hydronium form the other pair. Each pair differs by one proton.
Water can also donate a proton. For example, ammonia accepts a proton from water. Water becomes hydroxide, OH−\mathrm{OH^-}, and ammonia becomes ammonium, NH4+\mathrm{NH_4^+}. In this reaction, water is the acid and ammonia is the base. The conjugate pairs are water and hydroxide, and ammonia and ammonium.
These examples show why it is better to describe a substance’s role in a particular reaction than to assume that it always has the same role. Water accepts a proton in one reaction and donates one in another.
NH3(aq)+H2O(l)→NH4+(aq)+OH−(aq)\mathrm{NH_3(aq) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)}
  • Acid after donating a proton: its conjugate base.
  • Base after accepting a proton: its conjugate acid.
  • Members of a conjugate pair differ by one proton.

4. A reliable way to classify a reaction

First, compare the reactant and product particles. Look for a product that has one more hydrogen ion than a reactant particle, and another that has one fewer. Next, identify the particle that gained the proton as the base and the particle that lost it as the acid. Finally, match each reactant with the product formed from it to name the conjugate pairs.
The equation must also make chemical sense. Check that each type of atom appears in the same total amount on both sides. Check that the total charge is the same on both sides. These checks help catch a missing ion or an incorrect formula, but they do not replace identifying the proton transfer.
A common error is to label water as always being an acid or always being a base. Instead, use the equation: if water gains a proton, it is the base; if it gives away a proton, it is the acid. Another error is to call the product of an acid’s donation the conjugate acid. It is the conjugate base because it has lost a proton.
  • Compare reactants and products to locate the proton transfer.
  • Use the direction of transfer to assign acid and base roles.
  • Confirm atom and charge balance.

Worked example

Classifying ammonia’s reaction with water

For the reaction of ammonia with water, identify the Brønsted–Lowry acid and base, both conjugate particles, and the two conjugate acid–base pairs.
  1. Write the reaction
    Ammonia accepts a proton from water. The equation shows ammonium and hydroxide as the products.
    NH3(aq)+H2O(l)→NH4+(aq)+OH−(aq)\mathrm{NH_3(aq) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)}
  2. Find the proton acceptor
    Ammonia changes into ammonium, which has gained one hydrogen ion. Therefore, ammonia accepts the proton and acts as the Brønsted–Lowry base. Its product, ammonium, is the conjugate acid.
    NH3→NH4+\mathrm{NH_3 \rightarrow NH_4^+}
  3. Find the proton donor
    Water changes into hydroxide, which has one fewer hydrogen atom. Therefore, water donates the proton and acts as the Brønsted–Lowry acid. Hydroxide is its conjugate base.
    H2O→OH−\mathrm{H_2O \rightarrow OH^-}
  4. Check the equation
    There are five hydrogen atoms and one nitrogen atom on each side, as well as one oxygen atom on each side. The total charge is zero on both sides because the product charges add to zero.
    (+1)+(−1)=0(+1)+(-1)=0
Answer: Ammonia is the base, and ammonium is its conjugate acid. Water is the acid, and hydroxide is its conjugate base. The conjugate pairs are NH3/NH4+\mathrm{NH_3/NH_4^+} and H2O/OH−\mathrm{H_2O/OH^-}.
Check: Each conjugate pair differs by one proton, and the reaction conserves atoms and net charge.

Common mistakes and how to avoid them

Calling a substance an acid or base without checking the reaction.
Correction: Identify which particle donates the proton and which accepts it in that reaction.
Calling the product of an acid’s proton donation its conjugate acid.
Correction: The acid loses a proton, so the resulting particle is its conjugate base.
Assuming water must always act as an acid or always act as a base.
Correction: Water’s role depends on whether it donates or accepts a proton in the reaction.

Lesson summary

  • A Brønsted–Lowry acid donates a proton.
  • A Brønsted–Lowry base accepts a proton.
  • An acid becomes its conjugate base after donating a proton.
  • A base becomes its conjugate acid after accepting a proton.
  • Water can act as either an acid or a base, depending on the proton transfer.

Check your understanding

Question 1

In HCl(aq)+H2O(l)→H3O+(aq)+Cl−(aq)\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)}, which particle is the Brønsted–Lowry base?
  1. HCl\mathrm{HCl}
  2. H2O\mathrm{H_2O}
  3. H3O+\mathrm{H_3O^+}
  4. Cl−\mathrm{Cl^-}
Show answer and explanation
H2O\mathrm{H_2O}
Water accepts a proton from hydrogen chloride, so water is the base.

Question 2

In the same reaction, what is the conjugate base of hydrogen chloride?
  1. H2O\mathrm{H_2O}
  2. H3O+\mathrm{H_3O^+}
  3. Cl−\mathrm{Cl^-}
  4. HCl\mathrm{HCl}
Show answer and explanation
Cl−\mathrm{Cl^-}
Hydrogen chloride donates a proton and becomes chloride, its conjugate base.

Question 3

In NH3(aq)+H2O(l)→NH4+(aq)+OH−(aq)\mathrm{NH_3(aq) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)}, what role does water have?
  1. It is the base because it accepts a proton.
  2. It is the acid because it donates a proton.
  3. It is the conjugate acid because it accepts a proton.
  4. It is the conjugate base because it donates a proton.
Show answer and explanation
It is the acid because it donates a proton.
Water loses a proton and forms hydroxide, so water acts as the acid.

Key terms

Proton
The positively charged particle at the centre of a hydrogen atom; in these reactions, it is represented as H+\mathrm{H^+}.
Brønsted–Lowry acid
A particle that donates a proton in a reaction.
Brønsted–Lowry base
A particle that accepts a proton in a reaction.
Conjugate acid
The particle formed when a base accepts a proton.
Conjugate base
The particle formed when an acid donates a proton.

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