2.3.4Inorganic Chemistry
Carbon Dioxide from Thermal Decomposition — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 10 of 52
Part 1 of 4Thermal decomposition
Definition
- Definition Thermal decomposition is a reaction in which one substance is broken down into two or more simpler substances by heating.
- It differs from a reaction with oxygen because nothing else needs to be added.
Metal carbonates
- Metal carbonates Many metal carbonates split up when heated to give a metal oxide and carbon dioxide gas.
- The general word equation ismetal carbonate → metal oxide + carbon dioxide.
Reactivity link
- Reactivity link Carbonates of less reactive metals, such as copper, break down easily on gentle heating.
- Carbonates of very reactive metals,
- such as sodium and potassium, are far more stable and need much more intense heat,
- or do not decompose in a school lab.
Part 2 of 4Heating copper(II) carbonate
1
Place a spatula of green copper(II) carbonate powder in a dry test tube, and fit a delivery tube leading into a second tube of limewater.
2
Heat the carbonate strongly with a Bunsen burner, keeping the heating tube clamped at a slight angle.
3
Watch the solid: it slowly darkens from green to black as copper(II) oxide forms.
4
Observe the limewater: the carbon dioxide bubbles through and turns the limewater cloudy (milky).
5
Before stopping the heating, remove the delivery tube from the limewater so that the liquid cannot suck back into the hot tube.
Part 3 of 4Equation and observations
| Item | Detail |
|---|---|
| Word equation | copper(II) carbonate → copper(II) oxide + carbon dioxide |
| Symbol equation | CuCO3(s) → CuO(s) + CO2(g) |
| Colour change of solid | Green to black |
| Test for the gas | Limewater (calcium hydroxide solution) turns from clear to milky |
| Chemical in the milkiness | Insoluble calcium carbonate: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) |
Part 4 of 4Why it matters
CO2 from decomposition
Cement and lime
- Cement and lime Heating calcium carbonate (limestone) to make quicklime and cement releases large volumes of carbon dioxide.
Climate
- Climate This carbon dioxide adds to the greenhouse effect, making cement production a notable source of atmospheric CO2.
Mass change
- Mass change The solid left behind is lighter than the original carbonate because the carbon dioxide gas escapes.
Key terms
| Term | Meaning |
|---|---|
| Thermal decomposition | A reaction in which heat breaks a compound down into simpler substances. |
| Carbonate | A compound containing the CO32− ion. |
| Limewater | A solution of calcium hydroxide used to test for carbon dioxide. |
| Metal oxide | A compound formed between a metal and oxygen. |
| Reactivity series | A list of metals in order of how readily they react. |
Avoid these mistakes
1
Saying that the limewater 'turns white' because of the gas itself. It turns milky because insoluble calcium carbonate forms.
2
Leaving the delivery tube in the limewater when the heating stops, which causes suck-back.
3
Describing the colour change the wrong way round. Green copper(II) carbonate becomes black copper(II) oxide.
4
Assuming all carbonates decompose equally easily. The more reactive the metal, the more stable its carbonate.
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