CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2
Chemistry 9.1 More chemical reactions — Combustion, Oxidation and Thermal Decomposition
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Lesson summary
9.1 More chemical reactions — Combustion, Oxidation and Thermal Decomposition — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe what happens to atoms in chemical reactions, combustion, oxidation, thermal decomposition and the impacts of nitrogen oxides.Rea…
- Rearrangement
- Conservation
- Reactants and products
- First reaction
- Second reaction
- Atom counts
Part 1 of 7Atoms in chemical reactions
- Rearrangementin a chemical reaction atoms rearrange and join together differently.
- Conservationthe number of atoms of each element does not change, so the total mass of products equals the total mass of reactants, including any gas taken from the air.
- Reactants and productsyou cannot get the reactants back again from the products.
Part 2 of 7Nitrogen and oxygen in a car engine
1First reaction1 point
— at the high temperature of a car engine nitrogen and oxygen react to make nitrogen monoxide, NO, which goes into the air.
- One N2 molecule + one O2 molecule gives two NO molecules.
2Second reaction1 point
— nitrogen monoxide reacts with oxygen from the air to make nitrogen dioxide, NO2.
- Two NO molecules + one O2 molecule gives two NO2 molecules.
3Atom counts
— a nitrogen molecule has 2 atoms, NO has 2 atoms and NO2 has 3 atoms.
Part 3 of 7Impacts of nitrogen dioxide
- Acid rain — nitrogen dioxide dissolves in rain to make acid rain.
- Acid rain damages some buildings and harms ecosystems such as lakes and forests.
- It damages the waxy coating on the leaves of crops such as carrots, which slows their growth.
- Health — over time the gas damages the lungs, and it makes asthma worse.
- Sources — outside it is made in car engines, and inside it is made in gas stoves.
- Reducing exposure — walk or cycle instead of going by car, and open windows when a gas stove is burning.
Part 4 of 7Combustion
1Combustion1 point
= burning, a fast reaction of a substance with oxygen that releases heat and often light.
- Fuels containing carbon and hydrogen burn to make carbon dioxide and water: methane + oxygen → carbon dioxide + water.
2Incomplete combustion1 point
— when the oxygen supply is limited, carbon monoxide (a poisonous gas) and soot (carbon) form.
- A yellow, sooty flame shows incomplete combustion and a blue flame shows complete combustion.
3Fire triangle
— burning needs fuel, oxygen and heat, and removing any one puts the fire out.
Part 5 of 7Oxidation
1Oxidation1 point
= a reaction in which a substance gains oxygen.
- All combustion is oxidation, but oxidation can also be slow.
2Rusting
— iron rusts by slow oxidation and needs both oxygen and water.
3Metals1 point
— they burn in oxygen to form metal oxides: magnesium + oxygen → magnesium oxide, a white powder made with a bright white flame.
- The oxide has a greater mass than the metal because the mass of oxygen has been added.
Part 6 of 7Thermal decomposition
- Thermal decomposition = the breaking down of a compound into simpler substances by heating, which needs no second reactant.
- Metal carbonates — many decompose into a metal oxide and carbon dioxide: copper carbonate → copper oxide + carbon dioxide.
- Green copper carbonate turns black as copper oxide forms.
- Carbon dioxide turns limewater cloudy.
- The mass of solid decreases because the gas escapes into the air.
- Limestone — calcium carbonate decomposes in the same way to calcium oxide, which is used in cement and in the steel industry, and in agriculture to reduce soil acidity.
- Baking soda — heating sodium hydrogencarbonate releases carbon dioxide, which makes cakes rise.
Part 7 of 7Comparing the three reaction types
| Reaction | What happens | Typical evidence |
|---|---|---|
| Combustion | Substance burns in oxygen | Heat, light, new oxide |
| Oxidation | Substance gains oxygen | Colour change, mass gain |
| Thermal decomposition | One compound splits on heating | Colour change, gas, mass loss |
- Fire extinguishers — water removes heat, and foam and carbon dioxide extinguishers and fire blankets exclude oxygen.
- A pan of burning oil must not be treated with water because the oil floats and spreads.
- Rust prevention — painting, oiling, galvanising (coating with zinc) and sacrificial protection keep oxygen or water away from the iron.
- Powders — flour and metal dust can burn explosively because their large surface area lets oxygen reach many particles at once.
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