Acids and Alkalis
A concise OCR A Level Chemistry A revision guide to the formulae of the common acids and alkalis, acids as sources of H⁺ ions and alkalis as sources of OH⁻ ions in water, and the difference between strong and weak acids in terms of dissociation.
- 2.1.4(a)-i
- 2.1.4(a)-ii
- 2.1.4(a)-iii
- 2.1.4(a)-iv
- 2.1.4(b)-i
- 2.1.4(b)-ii
What these spec points say
- 2.1.4(a)-i the formulae of the common acids (HCl, H2SO4, HNO3 and CH3COOH)
- 2.1.4(a)-ii the formulae of the common alkalis (NaOH, KOH and NH3)
- 2.1.4(a)-iii explanation that acids release H+ ions in aqueous solution
- 2.1.4(a)-iv explanation that alkalis release OH– ions in aqueous solution
- 2.1.4(b)-i qualitative explanation of strong acids in terms of relative dissociation
- 2.1.4(b)-ii qualitative explanation of weak acids in terms of relative dissociation
Strong and Weak Acids in Water
Choose a step to watch how a strong acid and a weak acid of the same concentration behave in water, and what happens when OH⁻ ions are added.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: pH, Dilution and Neutralisation
Before you start, check what you already know about acids, alkalis and the pH scale.
The Common Acids and Alkalis
OCR expects the formulae of four acids and three alkalis to be known without prompting.
| Acid | Formula | Alkali | Formula |
|---|---|---|---|
| Hydrochloric acid | HCl | Sodium hydroxide | NaOH |
| Sulfuric acid | H₂SO₄ | Potassium hydroxide | KOH |
| Nitric acid | HNO₃ | Ammonia | NH₃ |
| Ethanoic acid | CH₃COOH |
Ethanoic acid is written CH₃COOH rather than C₂H₄O₂ to show that only one of its four hydrogens is acidic: the one on the carboxyl group.
Key idea: Sulfuric acid is diprotic: one mole gives two moles of H⁺. The other three acids are monoprotic.
Quick Check: How Much Alkali Is Needed
Decide which of these four acids needs twice as much alkali per mole.
What Makes an Acid an Acid
An acid releases hydrogen ions, H⁺, when it dissolves in water. Hydrogen chloride gas is a covalent molecule, but in water it dissociates: HCl(aq) → H⁺(aq) + Cl⁻(aq). The H⁺ ions are what give acids their reactions with metals, carbonates and bases.
An alkali is a soluble base that releases hydroxide ions, OH⁻, in water. Sodium hydroxide is ionic and simply separates: NaOH(aq) → Na⁺(aq) + OH⁻(aq).
Ammonia is different: it reacts with water to make hydroxide ions, NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). That is why ammonia solution is alkaline even though the formula contains no OH.
Neutralisation is the reaction of these two ions: H⁺(aq) + OH⁻(aq) → H₂O(l), the ionic equation for every acid-alkali reaction.
Exam sentence: Acids release H⁺ ions in aqueous solution; alkalis release OH⁻ ions in aqueous solution; neutralisation forms water from H⁺ and OH⁻.
Acids release H⁺ ions and alkalis release OH⁻ ions in water, and every acid-alkali neutralisation is H⁺ and OH⁻ forming water.
Quick Check: A Base with No OH Group
Work out where the hydroxide ions in this solution come from.
Strong and Weak Acids
The difference is not concentration but the extent of dissociation.
- A strong acid dissociates completely in water: every HCl molecule becomes H⁺ and Cl⁻. Hydrochloric, sulfuric and nitric acids are strong.
- A weak acid dissociates only partially: in ethanoic acid a small fraction of the molecules give up H⁺ and most stay as CH₃COOH. The equilibrium CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) lies well to the left.
Particle diagrams of a strong acid, HCl, fully ionised into H⁺ and Cl⁻, and a weak acid, CH₃COOH, in which only a few molecules have dissociated into CH₃COO⁻ and H⁺
Same concentration, different dissociation: a strong acid is fully ionised, a weak acid only slightly.
Because a weak acid has far fewer H⁺ ions at the same concentration, it has a higher pH, conducts electricity less well and reacts more slowly with magnesium or a carbonate.
It still needs the same volume of alkali in a titration, because the equilibrium shifts right as H⁺ is removed until every molecule has reacted.
Key idea: Strong means fully dissociated; concentrated means a lot of acid per dm³. A dilute strong acid and a concentrated weak acid are both possible.
Quick Check: Two Acids, One Concentration
Use the two pH values to complete the comparison.
Quick Check: Strong, Weak, Concentrated or Dilute
In each round, pick the one statement that is accurate.
Common Exam Mistakes
- Writing that a weak acid is a dilute acid. Strength is about dissociation, concentration about moles per dm³.
- Saying ammonia contains OH⁻: it makes OH⁻ by reacting with water.
- Treating all four hydrogens in CH₃COOH as acidic; only the carboxyl hydrogen dissociates.
- Using a single arrow for the dissociation of a weak acid; it is an equilibrium.
Exam sentence: A strong acid is fully dissociated in aqueous solution; a weak acid is only partially dissociated, so an equilibrium exists between the acid molecules and their ions.
FAQs
These questions secure the acid and alkali ideas that are most often confused.
Is a weak acid the same as a dilute acid?
No. Weak means only partly dissociated. Dilute means few moles of acid per dm³. A concentrated solution of a weak acid is possible.
Why does ethanoic acid have only one acidic hydrogen?
Only the hydrogen on the carboxyl group, COOH, is released as H⁺. The three hydrogens on the CH₃ group stay bonded to carbon.
How does ammonia make a solution alkaline?
It reacts with water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions come from the water molecules, not from the ammonia.
Does a weak acid need less alkali than a strong acid?
No. At the same concentration and volume, both need the same volume of alkali. As OH⁻ removes H⁺, the weak acid equilibrium shifts right until all the acid has reacted.
Why is sulfuric acid called diprotic?
Each H₂SO₄ can release two H⁺ ions, so one mole needs two moles of sodium hydroxide. Hydrochloric, nitric and ethanoic acids each release one.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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