Chemical Reactions and Equations — Quick Revision
A chemical equation must be balanced because matter cannot be created or destroyed — the number of atoms of each element must be the same on both sides. In a combination reaction two or more substances form a single product, while a decomposition reaction breaks one substance into two or more. In a displacement reaction a more reactive element pushes out a less reactive one from its compound; in a double displacement reaction two compounds exchange ions, often forming a precipitate. Oxidation is gain of oxygen or loss of hydrogen and reduction is the opposite; when both happen together the reaction is called a redox reaction. Corrosion (such as the rusting of iron) and rancidity (oxidation of fats and oils) are everyday effects of oxidation.
Key Points
- •Balance equations — atoms of each element equal on both sides
- •Combination: A + B → AB
- •Decomposition: AB → A + B
- •Displacement: A + BC → AC + B
- •Double displacement: AB + CD → AD + CB
- •Oxidation: gain of oxygen / loss of hydrogen
- •Reduction: loss of oxygen / gain of hydrogen
- •Corrosion = rusting of metals; rancidity = oxidation of fats/oils