Reactions of acids and bases
Acids give hydrogen with metals and carbon dioxide with carbonates and bicarbonates. With bases or metal oxides, they give a salt and water.
Reactions of acids
| Reaction | General rule | Balanced example |
|---|---|---|
| Acid + metal | Salt + hydrogen | Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) |
| Acid + metal carbonate | Salt + water + carbon dioxide | Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) |
| Acid + metal hydrogen carbonate (bicarbonate) | Salt + water + carbon dioxide | NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g) |
| Acid + metal oxide | Salt + water | CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l); black copper oxide gives a bluish green solution |
| Acid + base (neutralisation) | Salt + water | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) |
Reactions of bases
| Reaction | General rule | Balanced example |
|---|---|---|
| Base + active metal (such as zinc) | Salt + hydrogen | Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g); the salt is sodium zincate |
| Base + non-metal oxide | Salt + water | Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) |
| Base + acid (neutralisation) | Salt + water | H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l) |
Activity: acids with metals and carbonates (lab)
- 1Put a few zinc granules in a test tube and pour dilute sulphuric acid slowly down the side.Bubbles of gas rise
- 2Bring a burning matchstick near the mouth of the tube.The gas burns with a pop sound: it is hydrogen
- 3Put about 0.5 g of sodium carbonate in a boiling tube fitted with a thistle funnel and a delivery tube. Dip the delivery tube into 2 mL of fresh lime water.
- 4Pour about 3 mL of dilute HCl down the thistle funnel.Brisk fizzing, and the lime water turns milky: the gas is carbon dioxide
- 5Keep passing the gas for some time.The milkiness disappears, because soluble calcium hydrogen carbonate forms
- 6★Repeat with sodium hydrogen carbonate.The same result: carbonates and bicarbonates both give CO₂
★ key step
The lime water test, in equations
- Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The white calcium carbonate makes the lime water milky.
- With excess CO₂: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq). Calcium hydrogen carbonate dissolves, so the milkiness goes.
- Egg shells, marble and limestone are calcium carbonate. They also fizz with acids: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Activity: neutralisation with phenolphthalein (also an online lab)
- 1Take 2 mL of dilute NaOH solution in a test tube and add one drop of phenolphthalein.The solution turns pink
- 2Add dilute HCl drop by drop, shaking all the time.The pink colour slowly fades
- 3★Stop when the colour just disappears.All the NaOH has reacted: the solution is now neutral
- 4Add one or two drops of NaOH again.The pink colour comes back, as the solution is basic once more
★ key step
Corrosive and caustic
The word caustic is used only for bases, never for acids.
Egg shells and the marble of old monuments fizz with acid because they are calcium carbonate. That is why acid rain slowly damages marble. Fresh whitewash (lime) hardens into a white coat as it takes up carbon dioxide from the air.
2 marks: “A metal X reacts with dilute sulphuric acid and gives a gas that burns with a pop sound. Name the gas, and write the equation if X is zinc.” Hydrogen. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
3 marks: “What happens when (i) dilute HCl is stirred with copper oxide, (ii) carbon dioxide is passed through lime water and (iii) zinc is heated with sodium hydroxide solution? Write balanced equations.” (i) The black solid dissolves to give a bluish green solution: CuO + 2HCl → CuCl₂ + H₂O. (ii) Lime water turns milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. (iii) Hydrogen is given off: Zn + 2NaOH → Na₂ZnO₂ + H₂.
2 marks: “What is neutralisation? Write the equation for sulphuric acid reacting with potassium hydroxide.” The reaction of an acid with a base to give a salt and water, in which both lose their properties. H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.
Metal gives H₂ (pop), carbonate gives CO₂ (milky), base or metal oxide gives salt + water.