Year 9 · Unit 1 · Reference
Acids in this unit
In this unit you use two acids: hydrochloric acid and sulphuric acid. Both are solutions in water, so they have the state symbol (aq).
| Name | Formula |
|---|---|
| hydrochloric acid | HCl(aq) |
| sulphuric acid | H₂SO₄(aq) |
What we mean by an acid in this unit: a solution that is corrosive and reacts with reactive metals to give a salt and hydrogen, with metal oxides to give a salt and water, and with carbonates to give a salt, water and carbon dioxide.
Stay safe
Acids are corrosive. Wear goggles and a lab coat at all times when acid is out. Wash your hands at the end of the practical.
What acids do
| Acid reacts with | Products | Example from this unit |
|---|---|---|
| a reactive metal | a salt + hydrogen | magnesium + hydrochloric acid → magnesium chloride + hydrogen |
| a metal oxide | a salt + water | copper oxide + sulphuric acid → copper sulphate + water |
| a carbonate | a salt + water + carbon dioxide | sodium hydrogen carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide |
More metal examples from the unit:
- calcium + hydrochloric acid → calcium chloride + hydrogen
- iron + hydrochloric acid → iron chloride + hydrogen
- zinc + hydrochloric acid → zinc chloride + hydrogen
Comparing metals: the more vigorous the reaction with acid, the more reactive the metal.
Test for hydrogen: hold a lit splint at the mouth of the tube. Hydrogen burns with a squeaky pop.
The same reactions in formulae
Read the word equation first. Then open the formulae and find each substance in it.
Everything except hydrogen fades, so you can see where the acid's hydrogen ends up.
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Show it in formulae
Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)
The hydrogen leaves as hydrogen gas. That is the fizzing.
copper oxide + sulphuric acid → copper sulphate + water
Show it in formulae
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
The hydrogen ends up in water. No gas is made, so there is no fizzing.
sodium hydrogen carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide
Show it in formulae
NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
The hydrogen ends up in water. The fizzing is carbon dioxide.
- hydrogen
- chlorine
- sulphur
- oxygen
- carbon
- the metal
Balance them yourself
A formula cannot change. Only the big numbers in front can. Find out why magnesium needs 2 HCl.
- Count the atoms on each side. Every big number is 1 for now. Count each kind of atom in the reactants on the left and in the products on the right.
- Balance it with the big numbers. Make the two bars for each element equal. Change only the big numbers in front. The small numbers are locked.
- Mg + HCl → MgCl₂ + H₂: Balanced: Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)
- Zn + HCl → ZnCl₂ + H₂: Balanced: Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g)
- CuO + H₂SO₄ → CuSO₄ + H₂O: Balanced: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
- Same atoms before and after. Same atoms before and after, so the mass is the same.
Naming the salt
Look at the equations above:
- Hydrochloric acid makes chlorides: magnesium chloride, calcium chloride, sodium chloride.
- Sulphuric acid makes sulphates: copper sulphate.
Which salt? Pick a metal and an acid
The metal takes the place of the acid's hydrogen. The hydrogen leaves as a gas.
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)
Hydrochloric acid always gives a chloride. Each HCl has one chlorine and the salt needs two, so it takes 2 HCl.
magnesium + sulphuric acid → magnesium sulphate + hydrogen
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
Sulphuric acid always gives a sulphate. The S and the four O stay together and move from the acid to the salt.
zinc + hydrochloric acid → zinc chloride + hydrogen
Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g)
Hydrochloric acid always gives a chloride. Each HCl has one chlorine and the salt needs two, so it takes 2 HCl.
zinc + sulphuric acid → zinc sulphate + hydrogen
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
Sulphuric acid always gives a sulphate. The S and the four O stay together and move from the acid to the salt.
iron + hydrochloric acid → iron chloride + hydrogen
Fe(s) + 2 HCl(aq) → FeCl₂(aq) + H₂(g)
Hydrochloric acid always gives a chloride. Each HCl has one chlorine and the salt needs two, so it takes 2 HCl.
iron + sulphuric acid → iron sulphate + hydrogen
Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)
Sulphuric acid always gives a sulphate. The S and the four O stay together and move from the acid to the salt.
Why are the formulae different?
Sodium chloride is NaCl, but magnesium chloride is MgCl₂. The group number of the metal tells you why.
| Metal | Group | Outer electrons it gives away | Chlorine atoms it needs | Its chloride |
|---|---|---|---|---|
| sodium | 1 | 1 | 1 | NaCl(aq) |
| magnesium | 2 | 2 | 2 | MgCl₂(aq) |
| calcium | 2 | 2 | 2 | CaCl₂(aq) |
Each chlorine atom takes one electron. So a metal that gives away two electrons needs two chlorine atoms.
Zinc, iron and copper are not in group 1 or group 2. In the reactions of this unit each of them gives away two electrons, like magnesium.
The acids fit the same count. HCl has one hydrogen for a metal to replace. H₂SO₄ has two.
What is in each formula
Every substance on this page, with the atoms in one unit of it.
| Name | Formula | Atoms |
|---|---|---|
| hydrochloric acid | HCl(aq) | |
| sulphuric acid | H₂SO₄(aq) | |
| magnesium chloride | MgCl₂(aq) | |
| copper sulphate | CuSO₄(aq) | |
| sodium chloride | NaCl(aq) | |
| hydrogen | H₂(g) | |
| water | H₂O(l) | |
| carbon dioxide | CO₂(g) |
Acids and alkalis
Alkalis are the other side of this. Group 1 metals react with water to form hydroxides, which dissolve to give alkaline solutions. Universal indicator turns from green to purple in an alkaline solution.
Where you meet acids in Unit 1
- 1.6 Reactions of magnesium and calcium: magnesium in hydrochloric acid
- 1.6 Comparing metals in acid
- 1.7 Conservation of mass: sodium hydrogen carbonate and hydrochloric acid
- 1.7 Copper cycle: copper oxide and sulphuric acid
- 1.7 Physical or chemical change circus: hydrochloric acid with magnesium carbonate and with sodium hydrogen carbonate
- 1.7 Copper routes: copper oxide and sulphuric acid
- 1.8 Rates of reaction: hydrochloric acid with sodium thiosulphate or marble chips