In order to balance a chemical equation, it is important to understand the law of conservation of mass.

Definition 1: The law of conservation of mass

The mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. Matter can change form, but cannot be created or destroyed. For any chemical process in a closed system, the mass of the reactants must equal the mass of the products.

In a chemical equation then, the mass of the reactants must be equal to the mass of the products. In order to make sure that this is the case, the number of atoms of each element in the reactants must be equal to the number of atoms of those same elements in the products. Some examples are shown below:

Example 1:

Reactants

Atomic mass of reactants=55,8u+32,1u=87,9uAtomic mass of reactants=55,8u+32,1u=87,9u

Number of atoms of each element in the reactants: (1×Fe)(1×Fe) and (1×S)(1×S)

Products

Atomic mass of products=55,8u+32,1u=87,9uAtomic mass of products=55,8u+32,1u=87,9u

Number of atoms of each element in the products: (1×Fe)(1×Fe) and (1×S)(1×S)

Since the number of atoms of each element is the same in the reactants and in the products, we say that the equation is balanced.

Example 2:

Reactants

Atomic mass of reactants=(1+1)+(16+16)=34uAtomic mass of reactants=(1+1)+(16+16)=34u

Number of atoms of each element in the reactants: (2×H)(2×H) and (2×O)(2×O)

Product

Atomic mass of product=(1+1+16)=18uAtomic mass of product=(1+1+16)=18u

Number of atoms of each element in the product: (2×H)(2×H) and (1×O)(1×O)

Since the total atomic mass of the reactants and the products is not the same and since there are more oxygen atoms in the reactants than there are in the product, the equation is not balanced.

Example 3:

Reactants

Atomic mass of reactants=(23+6+1)+(1+35,4)=76,4uAtomic mass of reactants=(23+6+1)+(1+35,4)=76,4u

Number of atoms of each element in the reactants: (1×Na)+(1×O)+(2×H)+(1×Cl)(1×Na)+(1×O)+(2×H)+(1×Cl)

Products

Atomic mass of products=(23+35,4)+(1+1+16)=76,4uAtomic mass of products=(23+35,4)+(1+1+16)=76,4u

Number of atoms of each element in the products: (1×Na)+(1×O)+(2×H)+(1×Cl)(1×Na)+(1×O)+(2×H)+(1×Cl)

Since the number of atoms of each element is the same in the reactants and in the products, we say that the equation is balanced.

We now need to find a way to balance those equations that are not balanced so that the number of atoms of each element in the reactants is the same as that for the products. This can be done by changing the coefficients of the molecules until the atoms on each side of the arrow are balanced. You will see later that these coefficients tell us something about the mole ratio in which substances react. They also tell us about the volume relationship between gases in the reactants and products.

Remember that if you put a number in front of a molecule, that number applies to the whole molecule. For example, if you write 2H2O2H2O, this means that there are 2 molecules of water. In other words, there are 4 hydrogen atoms and 2 oxygen atoms. If we write 3HCl3HCl, this means that there are 3 molecules of HClHCl. In other words there are 3 hydrogen atoms and 3 chlorine atoms in total. In the first example, 2 is the coefficient and in the second example, 3 is the coefficient.

You will need: coloured balls (or marbles), prestik, a sheet of paper and coloured pens.

We will try to balance the following equation:

Take 1 ball of one colour. This represents a molecule of AlAl. Take two balls of another colour and stick them together. This represents a molecule of O2O2. Place these molecules on your left. Now take two balls of one colour and three balls of another colour to form a molecule of Al 2 O 3 Al 2 O 3 . Place these molecules on your right. On a piece of paper draw coloured circles to represent the balls. Draw a line down the center of the paper to represent the molecules on the left and on the right.

Count the number of balls on the left and the number on the right. Do you have the same number of each colour on both sides? If not the equation is not balanced. How many balls will you have to add to each side to make the number of balls the same? How would you add these balls?

You should find that you need 4 balls of one colour for AlAl and 3 pairs of balls of another colour (i.e. 6 balls in total) for O2O2 on the left side. On the right side you should find that you need 2 clusters of balls for Al 2 O 3 Al 2 O 3 . We say that the balanced equation is:

Repeat this process for the following reactions:

When balancing a chemical equation, there are a number of steps that need to be followed.

This simulation allows you to practice balancing simple equations.

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