LITo describe the reactions of acids with carbonates and with metal oxides.
SC: I can:
Try these 2questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.
When dilute HCl is dripped onto calcium carbonate chips, vigorous fizzing occurs. When the same acid is poured onto black copper(II) oxide, the solid dissolves but there is no fizzing. What is different about the two reactions?
A metal carbonate is a compound containing a metal cation and the carbonate anion (CO₃²⁻). Acids react with metal carbonates to form salts, carbon dioxide and water.
For example, calcium carbonate reacts with hydrochloric acid to produce calcium chloride, carbon dioxide and water.
Word form: hydrochloric acid + calcium carbonate → calcium chloride + carbon dioxide + water.
People burp when they take calcium carbonate to settle an upset stomach. The carbonate reacts with the stomach acids to produce carbon dioxide, which fills up the stomach and has to be released by burping.
To test if the gas released in a reaction is carbon dioxide, bubble the gas through limewater. Carbon dioxide turns limewater milky or cloudy.
A metal oxide is an ionic compound that contains a metal cation and oxide ions (O²⁻). Acid + metal oxide is a neutralisation reaction that gives a salt and water. Where do the atoms go?
For example, when hydrochloric acid (HCl) reacts with copper(II) oxide (CuO), the products are copper(II) chloride (CuCl₂) and water.
Word form: hydrochloric acid + copper(II) oxide → copper(II) chloride + water.
Figure 4.22 — Black copper(II) oxide dissolves in dilute hydrochloric acid to form a clear greenish copper(II) chloride solution. No gas is released — the oxide's O²⁻ leaves as part of the water.
This type of reaction is often used to illustrate how acids can neutralise basic compounds, as metal oxides are generally basic in nature.
Table 4.4 summarises all four acid reactions you have met. Use it to predict the products from whatever an acid is reacting with.
Table 4.4 — Reactions of acids:
| Reacts with acid to form → | Products |
|---|---|
| Acid + base | Salt + water |
| Acid + metal | Salt + hydrogen gas (H₂) |
| Acid + metal carbonate | Salt + water + carbon dioxide |
| Acid + metal oxide | Salt + water |
Lab safety with acids:
Figure 4.24 — Hazard pictograms that may appear on an acid bottle. The corrosive diamond means the acid eats skin and surfaces — wear gloves. The warning diamond means irritant — still treat with care.
Wrong — there is no carbon in a metal oxide. The oxygen from CuO ends up in water, not CO₂. Acid + metal oxide produces only salt + water (no gas).
Compare:
| Reaction | Fizz? | What ends up in the H₂O |
|---|---|---|
| 2HCl + CaCO₃ → CaCl₂ + CO₂ + H₂O | yes | half the C-O atoms (the rest go into CO₂) |
| 2HCl + CuO → CuCl₂ + H₂O | no | the oxide's O²⁻ |
Rule: CO₂ only comes from a carbonate (CO₃²⁻). No carbonate, no CO₂.
Wrong — fizzing means a gas is being produced, but two different reactions in §4.4 produce a gas:
You must run the gas test before claiming what it is. A copper coin in HCl does not fizz at all (Cu sits below H on the activity series); chalk in HCl fizzes vigorously — but the gas is CO₂, not H₂.
Rule: fizz alone doesn't identify the reactant — run the matching gas test.
Wrong — only two of the four acid reactions in Table 4.4 give just salt + water. Use the table to remember which:
| Acid + … | Products |
|---|---|
| base | salt + water |
| metal | salt + H₂(g) |
| metal carbonate | salt + water + CO₂(g) |
| metal oxide | salt + water |
Rule: the salt is always there; the second product changes — water (base/oxide), H₂ (metal), water + CO₂ (carbonate).
Hydrochloric acid is dripped onto calcium carbonate. Fill in the blanks:
Balanced symbol equation:
Use Table 4.4 to match each acid reaction on the left to the products on the right.
Describe in your own words how you would tell — from observations alone — whether an unknown white solid reacting with HCl was a metal, a metal carbonate, or a metal oxide.
Next class (Wed 13 May, P5): §4.5 — rate of reaction and collision theory. Why do some reactions happen in a flash while others take years?