The five basic types of chemical reactions are combination, decomposition, single-replacement, double-replacement, and combustion.

To calculate an energy change for a reaction:

1. add together the bond energies for all the bonds in the reactants – this is the ‘energy in’
2. add together the bond energies for all the bonds in the products – this is the ‘energy out’
3. energy change = energy in – energy out.

Chemical reactions involve breaking chemical bonds between reactant molecules (particles) and forming new bonds between atoms in product particles (molecules). The number of atoms before and after the chemical change is the same but the number of molecules will change.

How can I tell if a chemical reaction is occurring? A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change.

We can tell if a chemical reaction has taken place when one or more of the following things happen:

1. There has been a colour change inside the reaction flask.
2. A gas has formed. Usually we know a gas has formed when we can see bubbles.
3. A solid has formed.

Chemical reactions occur when chemical bonds between atoms are formed or broken. The substances that go into a chemical reaction are called the reactants, and the substances produced at the end of the reaction are known as the products.

Three things must happen for a reaction to occur.

• Molecules must collide.
• Molecules must collide with enough energy to begin to break the old bonds so new bonds can form. ( Remember activation energy)
• Molecules must collide with the correct orientation.