3.4 Chemical Equation

FORM 4

3.4
Chemical
Equation

Qualitative Aspect of Chemical Equations

• It is an equation that describe a chemical reaction.

• It is written in chemical formula.

• Description of chemical equation: Product
Reactants

2H2 (g) + O2 (g) 2H2O (  )

Meaning : Hydrogen Reacts Oxygen To produce Water
gas with gas liquid

Symbol Description

+ Reacts with

To produce

(s) Solid state

(l) Liquid state

(g) Gas state

(aq) Aqueous state

❖ Stoichiometry is a study of quantitative
composition of substances.

❖ Chemical equations give information of
quantitative aspects.

Quantitative Aspects of ❖ The coefficient in chemical formula indicates
Chemical Equations the number of moles or the number of
molecules of each reactants and products.

❖ Example: 2H2O (l)

2H2 (g) + O2 (g) 2 molecules or
2 mol of water
2 molecules or 2 1 molecule or
mol of hydrogen 1 mol of

gas oxygen gas

Writing Chemical ❖ According to the law of conservation of mass, matter
Equations cannot be created nor destroyed.

❖ This means that no atom are created or destroyed in
chemical reaction. Therefore:

• A chemical reaction must be balanced.
• Number of atoms of each element on the left and

on the right side of equation must be the same.

❖ Example : A balanced equation.

Left-side Right-side
2H2 (g) + O2 (g)
2H2O ( )

4 Hydrogen 2 oxygen 4 Hydrogen 2 oxygen
atom atom
atom atom

Steps to Write A Chemical Equation

Write the chemical Calculate the number
of atom of each
Write the chemical formula for each element
equation in word
reactants and

products

Balance the

Put the state symbols unbalanced number of
in the equation
A chemical equation atoms by adjusting the

coefficient of chemical

formula

Example – Writing A Calcium carbonate reacts with hydrochloric acid to produce calcium
Chemical Equation chloride, water and carbon dioxide gas.

Step 1 : Write the equation in words
Calcium carbonate + Hydrochloric acid Calcium chloride + Water + Carbon dioxide

Step 2 : Write the chemical formula for each reactants and products

CaCO3 + HCl CaCl2 + H2O + CO2

Step 3 : Calculate the chemical formula for each reactants and products

Element Numbers of atom on Numbers of atom on right
left side side

Ca 1 1

Solution C 1 1
O 3 3

H1 2

Cl 1 2

Step 4 : Balance the numbers of atoms by adjusting the coefficient in front

of the chemical formula.

CaCO3 + 2 HCl CaCl2 + H2O + CO2

Step 5 : Put the state symbols in the equation
CaCO3 (s) + 2HCl (l) CaCl2 (aq) + H2O (l) + CO2 (g)

Numerical Problems Involving Chemical Equations

❖ 1.2 g magnesium, Mg reacts with oxygen gas O2 to produce magnesium oxide, MgO.
Calculate the mass of magnesium produced.
[Relative atomic mass: Mg, 24]

Solution Step 1: Write the chemical equation

2Mg(s) + O2(g) 2MgO(s)

Step 2: Compare the mole ratio

2 mol of Mg reacts with 1 mol of O2 to produce 2 mol of MgO

Step 3: Calculate the number of moles of Mg

Number of moles = 1.2g = 0.05 mol
24

Step 4: Determine the number of moles of MgO

2 mol of Mg react with 1 mol of O2 produces 2 mol of MgO.
0.05 mol of Mg react with 0.05 of O2 produces 0.05 mol of MgO.

5
Step 5: Convert the number of moles into mass

Mass = Number of moles × Molar mass

= 0.05 mol × 40 g mol-1

=2g