The number of atoms of each element on the left hand side of the equation must be the same as the number of atoms of each element on the right hand side of the equation

Summary

Reactants are written on the left hand side of the chemical equation.

Products are written on right hand side of the chemical equation.

You can't change the formula of reactant or product molecules in the chemical equation.

You can only change the numbers of reactant or product molecules in the chemical equation.

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Example

When zinc metal reacts with hydrochloric acid, hydrogen gas and zinc chloride are produced.
Write a balanced chemical equation for this reaction.

Once you have written the formula for each reactant and product you cannot change them during the process of balancing the equation.

Write the unbalanced chemical equation by replacing the words in the word equation with the formulae above:

word equation:

zinc

+

hydrochlodic acid

→

hydrogen

+

zinc chloride

unbalanced chemical equation:

Zn

+

HCl

→

H_{2}

+

ZnCl_{2}

Balancing the equation:

Balance the Zn atoms:
Count the number of Zn atoms on the left hand side of the equation = 1

(ie 1 Zn atom in Zn)

Count the number of Zn atoms on the right hand side of the equation = 1

(ie, 1 Zn atom in ZnCl_{2})

number of Zn atoms on the left hand side = number of Zn atoms on the right hand side
Zn atoms are balanced.

Balance the H atoms in the chemical equation:

Zn + HCl → H_{2} + ZnCl_{2}

Count the number of H atoms on the left hand side of the equation = 1

(ie, 1 H atom in HCl)

Count the number of H atoms on the right hand side of the equation = 2

(ie, 2 H atoms in H_{2})

number of H atoms on the left hand side ≠ number of H atoms on right hand side
H atoms are NOT balanced.

In order to balance the number of H atoms, we need twice as many H atoms on the left hand side of the equation, but, we cannot change the formula of hydrochloric acid so we must multiply the number of molecules of hydrochloric acid by 2:

Zn + 2HCl → H_{2} + ZnCl_{2}

Check that the number of H atoms on each side of the equation is now the same:
Count the number of H atoms on the left hand side of the equation = 2

(ie, 2 H atoms in 2HCl)

Count the number of H atoms on the right hand side of the equation = 2

(ie, 2 H atoms in H_{2})

number of H atoms on the left hand side = number of H atoms on right hand side
H atoms are balanced.

Balance the Cl atoms using the new chemical equation :

Zn + 2HCl → H_{2} + ZnCl_{2}

Count the number of Cl atoms on the left hand side of the equation = 2

(ie, 2 Cl atoms in 2HCl)

Count the number of Cl atoms on the right hand side of the equation = 2

(ie, 2 Cl atoms in ZnCl_{2})

number of Cl atoms on the left hand side = number of Cl atoms on right hand side
Cl atoms are balanced.

The chemical equation is balanced when the numbers of atoms of each element on the left hand side of the equation is equal to the number of atoms of each element on the right hand side of the equation:

Zn

+

2HCl

→

H_{2}

+

ZnCl_{2}

number Zn atoms

1

=

1

Zn atoms are balanced

number H atoms

2

=

2

H atoms are balanced

number Cl atoms

2

=

2

Cl atoms are balanced

The number of atoms of each element on the left hand side of the equation is equal to the number of atoms of each element on the right hand side of the equation so the chemical equation is balanced.