Summary: Chemical Reactions and Equations
Every day you see substances change. Ice melts, milk turns into curd, an iron gate slowly turns brown. Some of these are physical changes, where only the state or shape changes and the substance stays the same. Others are chemical changes, where old bonds break, new bonds form and a brand new substance with new properties appears. Such a change is called a chemical reaction.
Chemists write reactions in a short, smart language called a chemical equation. Reactants go on the left, products on the right, with an arrow between them. Symbols like (s), (l), (g) and (aq), an upward arrow for a gas and a downward arrow for a precipitate add detail. Because mass is never created or destroyed, every equation must be balanced so that each element has equal atoms on both sides.
Reactions are sorted into families. In combination, two or more reactants join to give one product. In decomposition, one reactant breaks into two or more products using heat, light or electricity. In displacement, a more reactive metal pushes a less reactive one out of its compound. In double displacement, two compounds swap ions and often form a precipitate. Reactions also either release heat (exothermic) or absorb it (endothermic).
Some reactions, like burning LPG, are over in a flash, while rusting takes months. The speed or rate of a reaction depends on the nature of the reactants, the size of their particles, their concentration, the temperature and the presence of a catalyst. This is why powdered tile fizzes faster in acid and why food stays fresh longer in a fridge.
Finally, you meet oxidation (gain of oxygen, loss of hydrogen or loss of electrons) and reduction (the opposite). The two always happen together, so we call them redox reactions. Redox explains real-life problems like the rusting of iron, the dulling of metals called corrosion, and the bad smell of old oil called rancidity, along with simple ways to prevent them.
Key points to remember
- In a physical change the composition of matter stays the same and the change is often reversible; in a chemical change the composition changes permanently.
- A chemical reaction is a process in which bonds in the reactants break and new bonds form to give products with new properties.
- In a chemical equation, reactants are written on the left, products on the right, and an arrow shows the direction of the reaction.
- Physical states are shown as (s), (l), (g) and (aq); an upward arrow (↑) shows a gas given off and a downward arrow (↓) shows a precipitate.
- A balanced equation has the same number of atoms of each element on both sides, in agreement with the law of conservation of mass.
- While balancing, only coefficients (numbers in front of formulae) are changed — never the subscripts of a formula.
- Combination: two or more reactants give a single product, e.g. 2Mg + O2 → 2MgO.
- Decomposition: one reactant gives two or more products, e.g. CaCO3 → CaO + CO2; decomposition by electricity is called electrolysis.
- Displacement: a more reactive element takes the place of a less reactive element in its compound, e.g. Zn displaces Cu from CuSO4.
- Double displacement: ions of two reactants are exchanged and usually a precipitate forms, e.g. AgNO3 + NaCl → AgCl↓ + NaNO3.
- Exothermic reactions give out heat (CaO + H2O); endothermic reactions absorb heat (decomposition of CaCO3).
- Rate of reaction depends on nature of reactants, particle size, concentration, temperature and catalyst (e.g. MnO2 for KClO3 and H2O2).
- Oxidation = gain of oxygen, loss of hydrogen or loss of electrons; reduction = loss of oxygen, gain of hydrogen or gain of electrons; together they form a redox reaction.
- Rust is hydrated iron(III) oxide, Fe2O3·xH2O; rusting needs both air and water and is faster in salt water. Rancidity is air oxidation of oils and fats.
