CHEMICAL FORMULAE AND EQUATIONS
The formulae of compounds are written from the name of ions of atoms and the charge on the ions of atoms combined together.
There are two ways of writing the formulae of compounds:
- Prefix method
Prefixes are used to indicate the number of atoms present in a compound.
- Mono- = 1
- Di- = 2
- Tri- = 3
- Tetra- = 4
Examples
Carbon monoxide (CO)
Carbon dioxide (CO2)
Sulphur dioxide (SO2)
Sulphur trioxide (SO3)
Dinitrogen monoxide (
Dinitrogen pent oxide (
- Valency method
This is when valences of atoms, ions or radicals (compound ions) are used to determine the proportions of atoms in a compound.
Negative ions (anions)
Name of ion | formulae | Valency |
Fluoride | ||
Chloride | ||
Sulphide | ||
oxide | ||
Nitride | ||
Phosphide | ||
iodide | ||
bromide |
Positive ions (cations)
Name of ion | formulae | valency |
silver | 1 | |
Sodium | ||
Potassium | ||
Copper I | ||
Copper II | ||
Zinc | ||
Calcium | ||
Magnesium | ||
Lead II | ||
Lead IV | ||
Iron II | ||
Iron III | ||
Barium | ||
Aluminium | ||
Cobalt II | ||
Gold III |
Polyatomic ions/compound ions/radicals
Name of ion | Formulae | valency |
hydroxide | OH– | 1 |
nitrate | NO3– | 1 |
nitrite | NO2– | 1 |
Sulphate | SO42- | 2 |
sulphite | SO32- | 2 |
carbonate | CO32- | 2 |
phosphate | PO43- | 3 |
Hydrogen carbonate | HCO3– | 1 |
Phosphit5e | PO33- | 3 |
ammonium | NH4+ | 1 |
Examples
Write the formula of the following compounds
- Sodium hydroxide
- Calcium oxide
- Aluminium oxide
- Magnesium nitrate
- Potassium phosphate
- Ammonium carbonate
- Copper II chloride
Exercise
Write the formula of the following compounds
- Manganese IV oxide
- Calcium phosphate
- Copper II sulphate
- Magnesium sulphite
- Lead II nitrate
- Sodium phosphate
- Iron III phosphate
- Magnesium hydrogen carbonate
- aluminium sulphate
- zinc hydroxide
- ammonium sulphate
- lithium oxide
EQUATIONS
There are two parts to an equation, which are reactants and products. The two are separated by an arrow. Reactants are found on the left hand side and products on the right hand side
There are two ways to write equations of chemical reactions.
- Word equations
A word equation is written in words
Examples
- Sodium chloride + magnesium nitrate → sodium nitrate + magnesium chloride
- Calcium phosphate + aluminium sulphate →
- Potassium fluoride + sodium carbonate →
- Lead ii chloride + potassium iodide →
- Ammonium hydroxide + copper II sulphate →
- Zinc chloride + sodium hydroxide →
- Sodium chloride + magnesium nitrate → sodium nitrate + magnesium chloride
- Symbolic equations
These are equations where symbols or formulas of compounds are used.
Examples
Rewrite the following word equations as symbolic equations.
BALANCING OF EQUATIONS
Rules for balancing equations
- Count the atoms on each side of the equation
- If the number of each type of an atom on either side of the equation is not the same, we have to balance the equation.
- In balancing of equations put big numbers at the back of the formula which contains atoms that do not balance.
- Carry on until the atoms on each side of the equation are the same
NB: Only symbolic equations could be balanced.
Common acids
Hydrochloric acid HCl
Sulphuric acid H2SO4
Nitric acid HNO3
Carbonic acid H2CO3
Phosphoric acid H3PO4
Common di-atomic molecules
- Count the atoms on each side of the equation
- Hydrogen H2
- Oxygen O2
- Nitrogen N2
- Chlorine Cl2
- Fluorine F2
- Bromine Br2
- Iodine I2
Examples
Balance the following equations
Ionic equations
These are equations which show the ions which take part in a chemical reaction.
Ions which do not take part in a chemical reaction are called SPECTATOR IONS.
STATE symbol | Physical states (state symbol) meaning |
g | Gaseous substance |
l | Liquid |
s | Solid (insoluble substance) precipitates |
aq | Aqueous (substance that are soluble in water/dissolved in water) |
SOLUBILTY RULES
- All acids are soluble
- All nitrates are soluble
- All sulphates are soluble EXCEPT BaSO4, CaSO4, PbSO4
- All chlorides, Iodides, Bromides are soluble except for silver, lead and mercury.
- All carbonates are INSOLUBLE except those formed by group 1 elements and ammonium carbonate, (NH4)2CO3.
- All hydroxides and oxides are insoluble except those formed by group 1 elements and ammonium.
NB: All ammonium compounds are soluble
All compounds formed by group 1 are soluble
HOW TO WRITE AN IONIC EQUATION
- Write a balanced chemical equation
- Put state symbol into the equation
- Break down/separate the aqueous substances into ions. PLEASE DO NOT BREAK DOWN, SOLIDS, LIQUIDS AND GASES.
- Cancel out those ions which appear on both sides of the equation (spectator ions)
- What remains is the net ionic equation
Examples
- Write a balanced chemical equation