4.2.1a Reactions of acids with metals
Acid plus metal gives a salt and hydrogen
Definition
Acid
A substance that produces hydrogen ions when it dissolves in water.
Definition
Salt
An ionic compound formed when the hydrogen ions in an acid are replaced by metal ions or ammonium ions.
- A reactive metal reacts with a dilute acid to form a salt and hydrogen gas.
- metal+acid→salt+hydrogen\text{metal} + \text{acid} \rightarrow \text{salt} + \text{hydrogen}metal+acid→salt+hydrogen
- The acid decides the salt: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.
- Magnesium and hydrochloric acid: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g)Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)
- Zinc and sulfuric acid: Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)
Example
- Adding magnesium ribbon to dilute sulfuric acid gives fast fizzing as bubbles of hydrogen stream off the metal.
- The magnesium slowly disappears and a colourless solution of magnesium sulfate is left behind.
The gas is hydrogen: the lighted-splint test
Definition
Hydrogen
A gaseous element that burns rapidly with a pop sound when tested with a burning splint.
- The bubbles given off are hydrogen, the least dense gas.
- Test for it by holding a lighted splint at the mouth of the tube.
- Hydrogen burns with a squeaky pop, which confirms the gas.
Exam technique
- Name the specific salt, not just "a salt": magnesium and sulfuric acid make magnesium sulfate.
- To test for hydrogen, write "lighted splint" and "squeaky pop", because both are needed for the mark.
Reactivity sets how fast the reaction goes
Definition
Metal
An element that forms positive ions in its chemical reactions.
- A more reactive metal reacts faster and fizzes more vigorously with the same acid.
- The rate falls in the order magnesium, zinc, iron, matching the reactivity series.
- Metals below hydrogen, such as copper, do not react with dilute acids.
- You can compare reactivity by timing how quickly the fizzing stops or how fast hydrogen is produced.
Self review
- What two products form when a metal reacts with a dilute acid?
- Which salt forms when zinc reacts with sulfuric acid?
- Describe the test for hydrogen and its result.
- Why does copper not react with dilute hydrochloric acid?
- Write the balanced equation for magnesium reacting with hydrochloric acid.
4.2.1b Redox in acid-metal reactions
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?
