2.1.4(a)-(c) - Acids, bases, alkalis and neutralisation

Common acids, bases and alkalis

An acid is a substance that releases H⁺ ions when it is in aqueous solution. Aqueous solution means the substance is dissolved in water, shown by the state symbol (aq).

The common acid formulae you must recognise are:

The ion left after the acid releases H⁺ helps you name the salt. Hydrochloric acid forms chlorides, sulfuric acid forms sulfates, nitric acid forms nitrates, and ethanoic acid forms ethanoates.

An alkali is a water-soluble base that releases OH⁻ ions in aqueous solution. Sodium hydroxide and potassium hydroxide contain OH⁻ ions in their formulae, so their aqueous dissociation is direct:

The common alkalis are:

Ammonia is the unusual one in this list because its formula does not contain OH. In water, some ammonia molecules react with water to produce ammonium ions and hydroxide ions:

So the formula to remember is NH₃, not NH₄OH.

Worked example: connecting formulae to ions

Hydrochloric acid has formula HCl. In aqueous solution, it releases H⁺ and leaves Cl⁻. Sodium hydroxide has formula NaOH. In aqueous solution, it gives Na⁺ and OH⁻. If the acid and alkali react, H⁺ and OH⁻ form water, while Na⁺ and Cl⁻ remain dissolved as the spectator ions of aqueous sodium chloride.

Strong and weak acids

The strength of an acid is about how far its molecules or formula units dissociate in water. It is not about how concentrated the solution is.

Hydrochloric acid is treated as a strong acid:

The single arrow shows that, for this qualitative model, the dissociation goes essentially to completion.

Ethanoic acid is a weak acid:

The reversible arrow shows that dissociation is incomplete. At any moment, there is a mixture of CH₃COOH molecules, H⁺ ions and CH₃COO⁻ ions.

Worked example: choosing the correct language

A student says, "Ethanoic acid is weak because there is not much acid in the bottle." That is not the right reason. A weak acid could be concentrated or dilute. Ethanoic acid is weak because only a small proportion of CH₃COOH particles dissociate into H⁺ and CH₃COO⁻ in water.

Another student says, "Hydrochloric acid is strong because it dissociates completely in water." That is the correct idea for this boundary.

Neutralisation by H⁺ and OH⁻

Neutralisation is an acid-base reaction. In the acid-alkali case, H⁺ ions from an acid react with OH⁻ ions from an alkali to form water. Metal oxides, carbonates and aqueous ammonia also neutralise acids, using the full equations taught below.

This ionic equation describes hydrogen ions reacting with hydroxide ions. For a weak acid, the net equation normally keeps the undissociated acid as a molecule; for ammonia, it shows NH₃ gaining H⁺. Do not cancel away a species that is itself reacting. The other ions are still important because they form the salt, but they do not change during the H⁺ and OH⁻ step.

Worked example: hydrochloric acid and sodium hydroxide

Hydrochloric acid provides H⁺ and Cl⁻. Sodium hydroxide provides Na⁺ and OH⁻.

The neutralisation step is:

The remaining Na⁺ and Cl⁻ ions stay in aqueous solution; the full equation names their salt as sodium chloride:

Worked example: sulfuric acid and potassium hydroxide

Sulfuric acid has formula H₂SO₄, so one formula unit has two acidic H atoms. Potassium hydroxide has one OH in each formula unit. You need two KOH for one H₂SO₄:

The salt is potassium sulfate because K⁺ combines with SO₄²⁻.

Aqueous ammonia also forms salts with acids. For example:

This is still an acid-base reaction, but the useful full equation shows ammonium chloride formation rather than pretending ammonia has the formula NH₄OH.

Acids with bases, carbonates and metal oxides

A base neutralises an acid. In this lesson, the important bases are alkalis, metal oxides and carbonates.

For an acid reacting with a hydroxide alkali:

For an acid reacting with a metal oxide:

The oxide ion, O²⁻, combines with H⁺ ions to form water. For example, copper(II) oxide reacts with sulfuric acid to make copper(II) sulfate and water:

For an acid reacting with a carbonate:

The carbonate ion needs two H⁺ ions overall to form water and carbon dioxide. For example:

Worked example: predicting products and balancing

Write the equation for magnesium oxide reacting with nitric acid.

1. Identify the base: magnesium oxide, MgO.
2. Identify the acid: nitric acid, HNO₃.
3. Name the salt from the metal ion and acid anion: magnesium nitrate, Mg(NO₃)₂.
4. Add water because metal oxide + acid gives salt + water.
5. Balance the equation:

The coefficient 2 is needed because magnesium nitrate contains two nitrate ions.

Salt formulae and complete neutralisation

Writing a salt formula combines the charge balance of the ions with the number of acidic hydrogens or basic groups available for neutralisation.

1. Write the formula of the acid and the base.
2. Decide the salt name from the positive ion in the base and the negative ion from the acid.
3. Write the salt formula using ion charges.
4. Add water for alkalis and metal oxides; add water and carbon dioxide for carbonates.
5. Balance atoms and charges by changing coefficients, not formulae.

Worked example: barium hydroxide and hydrochloric acid

Barium hydroxide is Ba(OH)₂ and hydrochloric acid is HCl. The salt uses Ba²⁺ and Cl⁻, so the salt formula is BaCl₂. Because this is acid + alkali, the other product is water.

The 2 in front of HCl is needed because BaCl₂ contains two chloride ions. The 2 in front of water is needed because Ba(OH)₂ contains two OH groups.

Why a weak acid can be completely neutralised

Ethanoic acid initially supplies fewer H⁺ ions than the same concentration of a strong monoprotic acid, but those initial ions are not the whole acid supply. When OH⁻ removes H⁺, more CH₃COOH dissociates. With sufficient sodium hydroxide, the acid can therefore be consumed in a 1:1 amount ratio:

The net equation is $\ce{CH3COOH(aq) + OH^-(aq) -> CH3COO^-(aq) + H2O(l)}$. It keeps the weak acid molecule and shows sodium ions are spectators. Weak describes partial dissociation in water, not an inability to neutralise a base.

Common traps

Do not use "strong" when you mean "concentrated". A dilute strong acid is still strong if it fully dissociates. A concentrated weak acid is still weak if it only partially dissociates.

Do not change formulae to balance equations. For example, potassium sulfate is K₂SO₄, not KSO₄, because two K⁺ ions are needed for one SO₄²⁻ ion.

Do not forget carbon dioxide in carbonate reactions. Carbonates with acids form salt, water and CO₂.

Do not write ammonia as NH₄OH. The formula required here is NH₃, and acids convert it into ammonium salts such as NH₄Cl.

In equation questions, a good final check is to read the products aloud: "salt plus water" for acid with alkali or metal oxide, and "salt plus water plus carbon dioxide" for acid with carbonate. Then check the salt formula and the balancing numbers.

Quick Recap

• Acids release H⁺ in water; alkalis are soluble bases that produce OH⁻. Ammonia is NH₃ and produces OH⁻ by reacting with water.
• Strong and weak describe extent of dissociation; concentrated and dilute describe amount per volume.
• Neutralisation consumes the acidic species: H⁺ + OH⁻ → H₂O is the strong acid–hydroxide case.
• Use ion charges to write the salt, then balance coefficients. Hydroxides and metal oxides give water; carbonates also give CO₂.