0.6 Physical and chemical change  (Page 5/7)

$2\mathrm{Mg}+{\mathrm{O}}_2\to 2\mathrm{MgO}$

Energy is needed to break the $\mathrm{O}-\mathrm{O}$ bonds in the oxygen molecule so that new $\mathrm{Mg}-\mathrm{O}$ bonds can be formed, and energy is released when the product ( $\mathrm{MgO}$ ) forms.

Despite all the energy changes that seem to take place during reactions, it is important to remember that energy cannot be created or destroyed. Energy that enters a system will have come from the surrounding environment and energy that leaves a system will again become part of that environment. This is known as the conservation of energy principle.

Conservation of energy principle

Energy cannot be created or destroyed. It can only be changed from one form to another.

Chemical reactions may produce some very visible and often violent changes. An explosion, for example, is a sudden increase in volume and release of energy when high temperatures are generated and gases are released. For example, $\mathrm{NH}{}_4{\mathrm{NO}}_3$ can be heated to generate nitrous oxide. Under these conditions, it is highly sensitive and can detonate easily in an explosive exothermic reaction.

Conservation of atoms and mass in reactions

The total mass of all the substances taking part in a chemical reaction is conserved during a chemical reaction. This is known as the law of conservation of mass . The total number of atoms of each element also remains the same during a reaction, although these may be arranged differently in the products.

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

1. The decomposition of hydrogen peroxide into water and oxygen

$$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2\mathrm{H}{}_2\mathrm{O}+{\mathrm{O}}_2$$

Left hand side of the equation

$\mathrm{Total\; atomic\; mass}=(4\times 1)+(4\times 16)=68\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(4\times \mathrm{H})+(4\times \mathrm{O})$

Right hand side of the equation

$\mathrm{Total\; atomic\; mass}=(4\times 1)+(4\times 16)=68\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(4\times \mathrm{H})+(4\times \mathrm{O})$

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

Left hand side of the equation

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

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(2\times \mathrm{Mg})+(2\times \mathrm{O})$

Right hand side of the equation

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

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(2\times \mathrm{Mg})+(2\times \mathrm{O})$

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

Activity : 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. ${\mathrm{CaCO}}_3\to {\mathrm{CO}}_2+\mathrm{CaO}$
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 ( ${\mathrm{CaCO}}_3$ ).
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.