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:

  1. 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 (

     

     

     

     

     

     

     

     

     

     

  1. 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

  1. Sodium hydroxide
  2. Calcium oxide
  3. Aluminium oxide
  4. Magnesium nitrate
  5. Potassium phosphate
  6. Ammonium carbonate
  7. Copper II chloride

 

Exercise

Write the formula of the following compounds

  1. Manganese IV oxide
  2. Calcium phosphate
  3. Copper II sulphate
  4. Magnesium sulphite
  5. Lead II nitrate
  6. Sodium phosphate
  7. Iron III phosphate
  8. Magnesium hydrogen carbonate
  9. aluminium sulphate
  10. zinc hydroxide
  11. ammonium sulphate
  12. 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.

  1. Word equations

    A word equation is written in words

    Examples

    1. Sodium chloride + magnesium nitrate → sodium nitrate + magnesium chloride
    2. Calcium phosphate + aluminium sulphate →
    3. Potassium fluoride + sodium carbonate →
    4. Lead ii chloride + potassium iodide →
    5. Ammonium hydroxide + copper II sulphate →
    6. Zinc chloride + sodium hydroxide →

       

  2. 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

    1. Count the atoms on each side of the equation
    2. 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.
    3. In balancing of equations put big numbers at the back of the formula which contains atoms that do not balance.
    4. 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

       

  • 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

  1. All acids are soluble
  2. All nitrates are soluble
  3. All sulphates are soluble EXCEPT BaSO4, CaSO4, PbSO4
  4. All chlorides, Iodides, Bromides are soluble except for silver, lead and mercury.
  5. All carbonates are INSOLUBLE except those formed by group 1 elements and ammonium carbonate, (NH4)2CO3.
  6. 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

     

    1. Write a balanced chemical equation
    2. Put state symbol into the equation
    3. Break down/separate the aqueous substances into ions. PLEASE DO NOT BREAK DOWN, SOLIDS, LIQUIDS AND GASES.
    4. Cancel out those ions which appear on both sides of the equation (spectator ions)
    5. What remains is the net ionic equation

       

      Examples

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