4.1 Energy changes and conservation of matter  (Page 2/3)

We will use two of our earlier examples of chemical reactions to demonstrate this:

1. The decomposition of hydrogen peroxide into water and oxygen

2H ₂ O ₂ $\to$ 2H ₂ O $+$ O ₂

Left hand side of the equation

Total atomic mass = (4 $\times$ 1) + (4 $\times$ 16) = 68 u

Number of atoms of each element = (4 $\times$ H) + (4 $\times$ O)

Right hand side of the equation

Total atomic mass = (4 $\times$ 1) + (2 $\times$ 16) + (2 $\times$ 16) = 68 u

Number of atoms of each element = (4 $\times$ H) + (4 $\times$ O)

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

2. The synthesis of magnesium and oxygen to form magnesium oxide 2Mg $+$ O ₂ $\to$ 2MgO

Left hand side of the equation

Total atomic mass = (2 $\times$ 24,3) + (2 $\times$ 16) = 80,6 u

Number of atoms of each element = (2 $\times$ Mg) + (2 $\times$ O)

Right hand side of the equation

Total atomic mass = (2 $\times$ 24,3) + (2 $\times$ 16) = 80,6 u

Number of atoms of each element = (2 $\times$ Mg) + (2 $\times$ O)

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

Demonstration : the conservation of atoms in chemical reactions

Materials:

1. Coloured marbles or small balls to represent atoms. Each colour will represent a different element.
2. Prestik

Method:

1. Choose a reaction from any that have been used in this chapter or any other balanced chemical reaction that you can think of. To help to explain this activity, we will use the decomposition reaction of calcium carbonate to produce carbon dioxide and calcium oxide. CaCO ₃ $\to$ CO ₂ + CaO
2. Stick marbles together to represent the reactants and put these on one side of your table. In this example you may for example join one red marble (calcium), one green marble (carbon) and three yellow marbles (oxygen) together to form the molecule calcium carbonate (CaCO ₃ ).
3. Leaving your reactants on the table, use marbles to make the product molecules and place these on the other side of the table.
4. Now count the number of atoms on each side of the table. What do you notice?
5. Observe whether there is any difference between the molecules in the reactants and the molecules in the products.

Discussion

You should have noticed that the number of atoms in the reactants is the same as the number of atoms in the product. The number of atoms is conserved during the reaction. However, you will also see that the molecules in the reactants and products is not the same. The arrangement of atoms is not conserved during the reaction.

Law of constant composition

In any given chemical compound, the elements always combine in the same proportion with each other. This is the law of constant proportion .

The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.