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7.1.4 Proton donors and acceptors; strong and weak acids

Proton donors and acceptors

Definition

Proton donor

A substance that gives a hydrogen ion, H+\text{H}^{+}H+, to another substance in a reaction.

Definition

Proton acceptor

A substance that takes a hydrogen ion, H+\text{H}^{+}H+, from another substance in a reaction.

  1. A hydrogen atom is made of one proton and one electron, so a hydrogen ion, H+\text{H}^{+}H+, is simply a proton.

  2. This allows acids and bases to be described by what happens to protons during a reaction.

  3. An acid is a proton donor, because it gives H+\text{H}^{+}H+ ions to another substance.

  4. A base is a proton acceptor, because it takes H+\text{H}^{+}H+ ions from another substance.

  5. Every reaction between an acid and a base is therefore a proton transfer from the acid to the base.

  6. When hydrogen chloride dissolves in water, it donates its protons, which is why the solution contains H+\text{H}^{+}H+ ions.

  7. In neutralisation, the hydroxide ion is the base, because it accepts a proton to form water.

    H+(aq)+OH−(aq)→H2O(l) \text{H}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightarrow \text{H}_2\text{O(l)} H+(aq)+OH−(aq)→H2​O(l)

  8. The oxide ion in a metal oxide also accepts protons, which is why metal oxides act as bases.

    O2−+2H+→H2O \text{O}^{2-} + 2\text{H}^{+} \rightarrow \text{H}_2\text{O} O2−+2H+→H2​O

  9. Ammonia is a base because it accepts a proton to form the ammonium ion.

    NH3+H+→NH4+ \text{NH}_3 + \text{H}^{+} \rightarrow \text{NH}_4^{+} NH3​+H+→NH4+​
Example

Ammonia and hydrogen chloride

  • Hydrogen chloride gas and ammonia gas react to form white solid ammonium chloride.
HCl(g)+NH3(g)→NH4Cl(s) \text{HCl(g)} + \text{NH}_3\text{(g)} \rightarrow \text{NH}_4\text{Cl(s)} HCl(g)+NH3​(g)→NH4​Cl(s)
  • Hydrogen chloride loses a proton and becomes a chloride ion, Cl−\text{Cl}^{-}Cl−, so it is the proton donor and acts as the acid.
  • Ammonia gains that proton and becomes an ammonium ion, NH4+\text{NH}_4^{+}NH4+​, so it is the proton acceptor and acts as the base.
  • No water is involved, which shows that the proton transfer idea describes acid and base behaviour even outside aqueous solution.

Strong and weak acids

Definition

Dissociation

The splitting of a compound into separate ions when it is in aqueous solution.

Definition

Strong acid

An acid that fully dissociates into ions when it is in aqueous solution.

Definition

Weak acid

An acid that only partly dissociates into ions when it is in aqueous solution.

  1. When an acid dissolves in water, its molecules split up, or dissociate, to release H+\text{H}^{+}H+ ions.
  2. The strength of an acid describes how completely it dissociates in aqueous solution.
  3. In a strong acid, every molecule dissociates, so the solution contains ions and no undissociated acid molecules.
  4. In a weak acid, only a small fraction of the molecules dissociate, so most of the acid stays as whole molecules.
  5. Hydrochloric acid, sulfuric acid and nitric acid are strong acids, while ethanoic acid is a weak acid.
  6. Hydrochloric acid

    1. Hydrochloric acid is a strong acid, as its dissociation equation shows.

      HCl(aq)→H+(aq)+Cl−(aq) \text{HCl(aq)} \rightarrow \text{H}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} HCl(aq)→H+(aq)+Cl−(aq)

    2. The single arrow shows that the dissociation goes to completion, leaving no HCl\text{HCl}HCl molecules.

  7. Ethanoic acid

    1. Ethanoic acid is a weak acid, as its dissociation equation shows.

      CH3COOH(aq)⇌H+(aq)+CH3COO−(aq) \text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^{+}\text{(aq)} + \text{CH}_3\text{COO}^{-}\text{(aq)} CH3​COOH(aq)⇌H+(aq)+CH3​COO−(aq)

    2. The reversible arrow shows that the ions join back together as fast as molecules split, so most of the acid stays as CH3COOH\text{CH}_3\text{COOH}CH3​COOH molecules.

    3. Only the hydrogen atom in the −COOH-\text{COOH}−COOH group is released as H+\text{H}^{+}H+, while the three hydrogen atoms in the CH3\text{CH}_3CH3​ group stay attached.

Strength and concentration

  1. Concentration is the amount of acid dissolved in each dm3\text{dm}^3dm3 of solution.
  2. A concentrated acid has a large amount of acid in each dm3\text{dm}^3dm3, while a dilute acid has a small amount.
  3. Strength and concentration are separate ideas, so an acid can be dilute and strong, or concentrated and weak.
  4. When a strong acid and a weak acid have the same concentration, the strong acid releases far more H+\text{H}^{+}H+ ions.
  5. The strong acid therefore has the lower pH, so 0.1 mol/dm30.1\ \text{mol/dm}^30.1 mol/dm3 hydrochloric acid is about pH 1 while 0.1 mol/dm30.1\ \text{mol/dm}^30.1 mol/dm3 ethanoic acid is about pH 3.
  6. The strong acid also reacts faster with magnesium or a carbonate, because more H+\text{H}^{+}H+ ions are available to collide with the solid each second.
  7. Equal volumes of the two acids at the same concentration still neutralise the same amount of alkali, because more ethanoic acid molecules dissociate as their H+\text{H}^{+}H+ ions are used up.
Practical

Comparing hydrochloric acid and ethanoic acid

  • Aim: to compare a strong acid and a weak acid of the same concentration using their pH values and their reactions with magnesium.
  • Apparatus: 1.0 mol/dm31.0\ \text{mol/dm}^31.0 mol/dm3 hydrochloric acid, 1.0 mol/dm31.0\ \text{mol/dm}^31.0 mol/dm3 ethanoic acid, two boiling tubes in a rack, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, universal indicator paper, glass rod, white tile, two 3 cm3\ \text{cm}3 cm strips of magnesium ribbon cut from the same roll, stop-watch and eye protection.
  • Method:
    • Measure 10 cm310\ \text{cm}^310 cm3 of each acid into a separate boiling tube.
    • Touch a drop of each acid onto universal indicator paper on the white tile with a clean glass rod, and record each pH.
    • Drop one strip of magnesium into each tube at the same moment and start the stop-watch.
    • Compare the rate of bubbling, and record the time taken for each strip to disappear completely.
    • Keep the concentration, volume, temperature and length of magnesium the same, so that the strength of the acid is the only variable that changes.
  • Results: hydrochloric acid turns the paper red at about pH 0 to 1 while ethanoic acid turns it orange at about pH 2 to 3, and the magnesium fizzes vigorously and disappears much sooner in the hydrochloric acid.
  • Conclusion: both acids contain the same amount of acid, but hydrochloric acid is fully dissociated, so it has a higher concentration of H+\text{H}^{+}H+ ions at any moment.
  • Safety: wear eye protection, treat both acids as irritants, and keep the tubes away from flames because the hydrogen produced is flammable.
Common Mistake
  • Do not use "strong" to mean "concentrated", or "weak" to mean "dilute", because strength is about dissociation and concentration is about the amount dissolved.
  • Do not say that a weak acid does not dissociate, because it dissociates partially.
  • Do not write a single arrow for the dissociation of ethanoic acid, because partial dissociation needs the reversible arrow.
Exam technique

Explaining acid strength

  • Define strength with the word dissociates together with completely or partially, and include "in aqueous solution".
  • When comparing the pH of a strong and a weak acid, state that the two acids have the same concentration.
  • Link each difference in pH or rate of reaction back to the concentration of H+\text{H}^{+}H+ ions.
  • Identify the proton donor and proton acceptor in a reaction by following which particle loses H+\text{H}^{+}H+ and which gains it.
Self review
  • What is an acid in terms of protons?
  • What is a base in terms of protons?
  • What is the difference between a strong acid and a weak acid?
  • Write the equation that shows ethanoic acid is a weak acid.
  • Why does 0.1 mol/dm30.1\ \text{mol/dm}^30.1 mol/dm3 hydrochloric acid have a lower pH than 0.1 mol/dm30.1\ \text{mol/dm}^30.1 mol/dm3 ethanoic acid?
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7.1.4 Proton donors and acceptors; strong and weak acids Revision Guide

  1. IGCSE
  2. /Chemistry
  3. /7.1.4 Proton donors and acceptors; strong and weak acids

Revision notes for CIE IGCSE Chemistry 7.1.4 Proton donors and acceptors; strong and weak acids: explanations and worked examples.

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