A-Level Chemistry OCR Notes

2.1.2 Compounds, formulae, and equations

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Ionic Compounds

An ion is an atom or molecule with a net charge formed through the gain or loss of electrons
A cation is formed from the loss of electrons
An anion is formed from the gain of electrons
Ionic compounds are composed of oppositely charged ions. The overall charge is zero as the ionic charges balance.
e.g. write the formula of sodium carbonate

We need 2 x Na+ to balance the -2 charge of CO3^2-
The formula is Na2CO3

Balanced Equations

When a chemical reaction occurs, no atoms are created or destroyed. The atoms in the reactants rearrange to form the products.
In a balanced equation, there is the same number of atoms of each element in both the reactants and products.
State symbols are written after every species to indicate the physical state
- Solid (s)
- Liquid (l)
- Gaseous (g)
- Aqueous (aq) - dissolved in water
Ionic equations can be written for any reaction involving ions in solution, where only the reacting ions and the products they form are included.
The net ionic equation shows only the ions directly involved in the reaction (removing spectator ions). e.g.

NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Na+ (aq) + Cl- (aq) + Ag+ (aq) + NO3- (aq)→ AgCl (s) + Na+ (aq) + NO3- (aq)

Net ionic equation:
Cl- (aq) + Ag+ (aq) → AgCl (s)

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