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.
