Acids react in characteristic ways with metals, bases, oxides, and carbonates, and these reactions can be clearly represented using ionic equations showing the role of hydrogen ions.

Nature of Acid Reactions

Acids are substances that produce hydrogen ions, H⁺(aq) when dissolved in water. In aqueous reactions, it is these hydrogen ions that are chemically active, not the acid molecules themselves. Writing reactions in ionic form helps highlight this role and removes spectator ions that do not participate in the chemical change.

Many reactions of acids are examples of neutralisation, where acidic hydrogen ions react with basic species to form water, or redox reactions, particularly when acids react with metals.

Reactions of Acids with Metals

When an acid reacts with a reactive metal, a salt and hydrogen gas are produced. The metal atoms lose electrons and are oxidised, while hydrogen ions gain electrons and are reduced.

The general reaction is:

• Acid + Metal → Salt + Hydrogen

Only metals above hydrogen in the reactivity series will react readily with dilute acids. Common examples include magnesium, zinc, and iron reacting with dilute hydrochloric or sulfuric acid.

The key feature is the reduction of hydrogen ions to hydrogen gas.

When a reactive metal is added to a dilute acid, H₂(g) is released (seen as effervescence) and an aqueous salt is formed.

Magnesium reacts with hydrochloric acid, producing visible bubbles of hydrogen gas as the metal dissolves to form a magnesium salt in solution. Source

In ionic terms, the reaction can be simplified by removing spectator ions such as Cl⁻ or SO₄²⁻. This shows the essential chemistry clearly.

This ionic equation applies to all acid–metal reactions where hydrogen gas is evolved, regardless of the specific acid used.

Reactions of Acids with Bases and Alkalis

Bases are substances that neutralise acids. Alkalis are soluble bases that release hydroxide ions, OH⁻(aq), in solution. When acids react with bases or alkalis, the defining process is neutralisation.

The general reaction is:

• Acid + Base → Salt + Water

In aqueous solution, the key reacting species are hydrogen ions and hydroxide ions.

The ionic equation for all strong acid–strong alkali reactions is identical, regardless of the substances used.

In reactions with alkalis, the key chemical change is neutralisation, where H⁺(aq) reacts with OH⁻(aq) to form H₂O(l).

The images show bromothymol blue changing colour as alkali is added and hydrogen ions are neutralised by hydroxide ions. The specific example shown involves ascorbic acid, which goes beyond the syllabus focus but clearly illustrates neutralisation. Source

This equation demonstrates that the formation of water is the central chemical change, with all other ions acting as spectators.

Reactions of Acids with Metal Oxides

Metal oxides are basic and react with acids in neutralisation reactions to form a salt and water. These reactions are not redox processes; instead, they involve acid–base chemistry.

The oxide ion, O²⁻, is a strong base and reacts with hydrogen ions from the acid.

In molecular terms:

• Acid + Metal oxide → Salt + Water

When written ionically, the reaction focuses on hydrogen ions reacting with oxide ions.

This ionic equation shows that two hydrogen ions are required to neutralise each oxide ion, forming water. Solid metal oxides must first dissolve or react at the surface for the ions to participate.

Reactions of Acids with Metal Hydroxides

Metal hydroxides, whether soluble or insoluble, also react with acids via neutralisation. The reactive species is the hydroxide ion.

Examples include reactions of acids with sodium hydroxide, calcium hydroxide, or insoluble hydroxides such as copper(II) hydroxide.

The essential ionic process is identical to that of alkalis.

• Hydrogen ions react with hydroxide ions to form water.

• The remaining ions form the salt.

This reinforces that neutralisation reactions always involve H⁺ and OH⁻ ions, regardless of solubility.

Reactions of Acids with Carbonates

Carbonates and hydrogencarbonates react with acids to produce carbon dioxide, water, and a salt. These reactions are useful tests for carbonates due to the effervescence of CO₂.

The general reaction is:

• Acid + Carbonate → Salt + Water + Carbon dioxide

The chemistry involves hydrogen ions reacting with carbonate ions, CO₃²⁻.

Carbonates react with acids to produce an aqueous salt, water, and carbon dioxide gas, so you observe strong effervescence.

The diagram shows hydrogen ions reacting with carbonate ions to form carbon dioxide and water. The limewater test shown is additional detail beyond the core syllabus but helps confirm the identity of the gas produced. Source

This equation shows that two hydrogen ions are needed to fully react with each carbonate ion. The formation of a gas drives the reaction to completion.

Importance of Ionic Equations

Using ionic equations:

• Highlights the role of hydrogen ions in acid reactions

• Removes irrelevant spectator ions

• Allows comparison of reactions involving different acids

• Reinforces understanding of acids as H⁺ donors in aqueous solution

Across all these reactions, the defining chemical behaviour of acids is consistently shown by the involvement of hydrogen ions reacting with metals, bases, oxides, hydroxides, or carbonates.