Acids, Bases and Salt Preparations
Inorganic Chemistry
Acid + metal → salt + hydrogen
- Only a metal above hydrogen in the reactivity series reacts with a dilute acid (see topic 13 section 1)
- General reaction:
metal + acid → salt + hydrogen
- Test for the gas: a lit splint at the mouth of the tube gives the squeaky pop of hydrogen
- Examples (using a +2 metal):
- Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
- Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
- Acid → salt naming rule:
- Hydrochloric acid gives a chloride
- Sulfuric acid gives a sulfate
- Nitric acid gives a nitrate
Completing the equation for a reaction of an acid
Question: Complete the chemical equation for an acid reacting with a metal or a carbonate, by writing the products or by adding the state symbols (1–2 marks).
Set in 8 of the 23 papers. For the products, a metal gives the salt and hydrogen and a carbonate gives the salt, carbon dioxide and water. Multiples or fractions are allowed and state symbols are ignored even when wrong; for the carbonate, one mark is for the carbon dioxide and water, the other for the correct salt with the equation balanced.
For state symbols, the solid metal or carbonate is (s), the dilute acid and the salt formed are (aq), hydrogen or carbon dioxide is (g) and water is (l). Capitals are accepted, but all must be right for the single mark; a two-mark version splits the reactants from the products. In a precipitation both solutions are (aq) and only the precipitate is (s).
Acid + base → salt + water
- Reacting any acid with a base is a and always gives the same two products: a salt plus water
acid + base → salt + water
| Acid + base | Equation |
|---|---|
| HCl + NaOH | NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) |
| H2SO4 + CuO | CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) |
| HNO3 + ZnO | ZnO(s) + 2 HNO3(aq) → Zn(NO3)2(aq) + H2O(l) |
Acid + metal carbonate → salt + carbon dioxide + water
- Acids react with metal carbonates to give three products: a salt, carbon dioxide gas, and water
acid + metal carbonate → salt + carbon dioxide + water
- A distinguishing feature is the as carbon dioxide is released — passing the gas through limewater turns it milky, confirming CO2
- Examples:
- CuCO3(s) + 2 HCl(aq) → CuCl2(aq) + CO2(g) + H2O(l)
- ZnCO3(s) + H2SO4(aq) → ZnSO4(aq) + CO2(g) + H2O(l)
- Na2CO3(s) + 2 HNO3(aq) → 2 NaNO3(aq) + CO2(g) + H2O(l)
Observations when an acid reacts with a metal or carbonate
Question: State what you would observe when a metal or a carbonate is added to a dilute acid (1–2 marks).
Set in 3 of the 23 papers. The observation is effervescence (bubbles, fizzing); in the two-mark version the second mark is for the metal getting smaller or disappearing. "A gas is given off" and "hydrogen is produced" are ignored: they name a product rather than something you can see. For a metal the scheme also accepts that the metal dissolves, a colourless solution forms or the mixture gets hot; for a carbonate, that the solid disappears. "Crystals form" is ignored, since nothing crystallises while the acid is still reacting.
The follow-up test for the hydrogen is asked in 5 of the 23 papers, twice straight after an acid and metal equation: a lighted splint gives a squeaky pop. A glowing splint is rejected (that is the oxygen test), and "burns with a squeaky pop" on its own is not enough.