Acid-metal reactions are redox: the metal loses electrons
Definition
Hydrogen ion
A positively charged ion, H+\text{H}^{+}H+, produced by acids in aqueous solution.
Definition
Electron
A negatively charged subatomic particle that can be transferred between species during a redox reaction.
- When a metal reacts with an acid, the metal atoms lose electrons and form positive metal ions.
- Losing electrons is oxidation, so the metal is oxidised.
- The hydrogen ions from the acid gain those electrons and form hydrogen molecules.
- Gaining electrons is reduction, so the hydrogen ions are reduced.
- Because electrons are transferred, the whole reaction is a redox reaction (OIL RIG).
Key Idea
- Oxidation is loss of electrons; the metal atom becomes a positive ion.
- Reduction is gain of electrons; hydrogen ions become hydrogen gas.
Half equations show the electron transfer
Definition
Ion
An atom or group of atoms with an overall electrical charge because electrons have been lost or gained.
- Write a half equation for the metal losing electrons.
- Mg(s)→Mg2+(aq)+2e−\text{Mg}(s) \rightarrow \text{Mg}^{2+}(aq) + 2\text{e}^{-}Mg(s)→Mg2+(aq)+2e−
- Write a half equation for the hydrogen ions gaining electrons.
- 2H+(aq)+2e−→H2(g)2\text{H}^{+}(aq) + 2\text{e}^{-} \rightarrow \text{H}_2(g)2H+(aq)+2e−→H2(g)
- The electrons lost by the metal are the same electrons gained by the hydrogen ions, so they cancel.
Exam technique
- Balance the charges as well as the atoms; add electrons until both sides have the same total charge.
- Electrons on the right mean oxidation; electrons on the left mean reduction.
Naming what is oxidised and what is reduced
- In every acid-metal reaction the metal is oxidised and the hydrogen ions are reduced.
- The negative ion from the acid, such as chloride or sulfate, is a spectator ion and does not change.
- For zinc and hydrochloric acid the ionic equation is Zn(s)+2H+(aq)→Zn2+(aq)+H2(g)\text{Zn}(s) + 2\text{H}^{+}(aq) \rightarrow \text{Zn}^{2+}(aq) + \text{H}_2(g)Zn(s)+2H+(aq)→Zn2+(aq)+H2(g)
- Name the exact species: the zinc atoms are oxidised and the hydrogen ions are reduced.
Self review
- Define oxidation and reduction in terms of electrons.
- Write the half equation for magnesium atoms forming magnesium ions.
- Write the half equation for hydrogen ions forming hydrogen gas.
- In an acid-metal reaction, which species is oxidised and which is reduced?
- Why are the chloride ions in the reaction called spectator ions?
