SS1 Chemistry Lesson Note on Formulae and Equations

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FORMULAE AND EQUATIONS

CONTENTS:

1. Chemical formulae
2. Chemical equations
3. Empirical and Molecular formulae
4. Relative Molecular Mass and Percentage composition by mass of an element.

PERIOD 1: CHEMICAL FORMULAE

Chemical formula can be defined as a collection of two or more symbols to represent one molecule of the compound. For example, the formula of tetraoxosulphate(VI) acid is H₂SO₄. This formula shows that in a molecule of tetraoxosulphate(VI) acid, there are two atoms of hydrogen, one atom of sulphur and four atoms of oxygen.

The table below contain examples of compounds with their formulae

Compounds	Formulae
Oxygen molecule	O2
Hydrogen molecule	H2
Hydrogen chloride	HCl
Potassium chloride	KCl
Magnesium tetraoxosulphate(VI)	MgSO4
Hydrogen sulphide	H2S
Bromine Molecule	Br2
Ozone	O3

EVALUATION

Write the chemical formulae of the following:

(1) Sodium oxide, Calcium oxide, aluminium oxide,

(2) Carbon (IV) oxide, iron (II) oxide, copper(I) oxide

(3) Sodium tetraoxosulphate(VI), Calcium trioxonitrate(V), magnesium trioxocarbonate(IV).

PERIOD 2: CHEMICAL EQUATIONS

Chemical reactions are represented in form of equations which show the reactants and products in any given chemical reaction. For example, the reaction of aqueous hydrogen chloride and aqueous sodium hydroxide is represented by the equation:

HCl_(aq) + NaOH_(aq)    →        NaCl_(aq) + H₂O_(l)

Balancing Chemical Equations

All chemical equations must be balanced in order to comply with the law of conservation of mass. For example, to balance the equation for the reaction involving the combustion of ammonia gas in air, the following steps should be followed:

Step 1: The reactants (LHS) are NH₃ and O₂, while the products (RHS) are NO and H₂O

NH_3(g)  + O_2(g)    →  NO_(g) + H₂O_(g)

Step 2: Place a 4 in front of NH₃ and a 6 in front of H₂O. There are now 12 hydrogen atoms on both sides of the equation.

4NH_3(g)  + O_2(g)    →  NO_(g) + 6H₂O_(g)

Step 3: Place a 4 in front of NO to balance with the 4 nitrogen atoms of the LHS of the equation.

4NH_3(g)  + O_2(g)  →   4NO_(g) + 6H₂O_(g)

Step 4: Place a 5 in front of O₂. An “atom count” shows that the equation is balanced.

4NH_3(g)  + 5O_2(g)  →  4NO_(g) + 6H₂O_(g)

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