Categories of Chemical Reactions
Senior 2 · Chemistry
Categories of Chemical Reactions
Lesson 1 of 4
25 min
In a lime kiln, lumps of limestone are heated until they break down into quicklime, which builders and farmers use. In a fertiliser factory, nitrogen from the air is joined to hydrogen to make ammonia. In a kitchen in Rubavu, cooking gas burns with a blue flame. These are three different kinds of chemical reaction: decomposition, combination and combustion.
There are millions of reactions, but most fit a small number of patterns. Once you recognise the pattern, you can predict the products and write a balanced equation. By the end of this lesson you will be able to balance equations and to recognise combination, decomposition and combustion reactions.
In a chemical reaction, atoms are rearranged, but none are created or destroyed. This is the law of conservation of mass: the total mass of the products equals the total mass of the reactants. A balanced equation has the same number of atoms of each element on both sides.
Follow four steps:
Methane from Lake Kivu burns in oxygen to form carbon dioxide and water. Write the balanced equation.
Step 1 methane + oxygen → carbon dioxide + water
Step 2 CH4 + O2 → CO2 + H2O
Step 3 Count atoms. Carbon: 1 on each side, balanced. Hydrogen: 4 on the left, 2 on the right, so put 2 in front of H2O. Now oxygen: 2 on the left, but 2 + 2 = 4 on the right, so put 2 in front of O2.
| Atom | Left | Right |
|---|---|---|
| C | 1 | 1 |
| H | 4 | 2 × 2 = 4 |
| O | 2 × 2 = 4 | 2 + 2 = 4 |
Step 4 CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Try it: open Balancing Chemical Equations (version 2.0.2). In the Introduction screen, change the coefficients and watch the balance scales tip until both sides have the same atoms. Then try the Game screen.
In a combination reaction, two or more substances join to form a single product: A + B → AB.
2Mg(s) + O2(g) → 2MgO(s)
N2(g) + 3H2(g) → 2NH3(g) (the first step in making nitrogen fertilisers)
CaO(s) + H2O(l) → Ca(OH)2(s) (slaking quicklime to make slaked lime)
In a decomposition reaction, one substance breaks down into two or more simpler substances: AB → A + B. Many need heat (thermal decomposition), some need electricity or light, and some need a catalyst, a substance that speeds up a reaction without being used up.
CaCO3(s) → CaO(s) + CO2(g) (heating limestone in a lime kiln)
CuCO3(s) → CuO(s) + CO2(g) (green copper(II) carbonate turns black on heating)
2H2O2(aq) → 2H2O(l) + O2(g) (hydrogen peroxide, with manganese(IV) oxide as catalyst)
2H2O(l) → 2H2(g) + O2(g) (passing electricity through acidified water)
Combustion is a reaction in which a substance (a fuel) reacts rapidly with oxygen, giving out heat and usually light. Fuels made of carbon and hydrogen (hydrocarbons) such as methane, propane and butane give carbon dioxide and water when there is plenty of oxygen: this is complete combustion.
C(s) + O2(g) → CO2(g)
When the oxygen supply is limited, incomplete combustion produces carbon monoxide, a colourless, odourless and poisonous gas, or soot (carbon). This is why a charcoal stove must never be left burning in a closed room.
2C(s) + O2(g) → 2CO(g)
2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(l)
Propane, C3H8, is a component of the bottled gas used for cooking. Balance the equation for its complete combustion.
Solution. Balance C first: 3 carbons, so 3CO2. Then H: 8 hydrogens, so 4H2O. Oxygen is last because it appears in two products: 3 × 2 + 4 × 1 = 10 oxygen atoms on the right, so 5O2 on the left.
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l)
Tip
C, then H, then O last. When balancing a combustion equation, balance carbon, then hydrogen, and leave oxygen until the end, because it appears in more than one product. If you end up needing an odd number of oxygen atoms from O2, double every coefficient.
Never change a subscript. Turning H2O into H2O2 to balance oxygen changes water into hydrogen peroxide.
Balance each equation and name the type of reaction. Try each one before you read the solution.
H2 + O2 → H2O
Solution: 2H2(g) + O2(g) → 2H2O(l). Two substances join to form one product: combination (it is also the combustion of hydrogen).
KClO3 → KCl + O2
Solution: Oxygen: 3 on the left, 2 on the right; the lowest common multiple is 6, so 2KClO3 and 3O2. Then 2KCl balances K and Cl. 2KClO3(s) → 2KCl(s) + 3O2(g). One substance breaks down: decomposition.
Butane, C4H10, burns completely in oxygen.
Solution: C: 4CO2. H: 10 hydrogens, so 5H2O. O on the right: 8 + 5 = 13, an odd number, so double everything: 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l). Combustion.
Na + Cl2 → NaCl
Solution: 2Na(s) + Cl2(g) → 2NaCl(s). Combination.
Why is it dangerous to burn charcoal in a closed room?
Solution: With little oxygen, combustion is incomplete and produces carbon monoxide, 2C(s) + O2(g) → 2CO(g). Carbon monoxide is poisonous and cannot be seen or smelled.
Summary
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