A chemical reaction occurs when two compounds collide in a certain orientation and with a certain amount of force. Because particles are constantly in motion, random collisions do occur, and some of them will be in the proper orientation. So, as long as the right energy is met will occur.
I guess you will get better answer from the other scientists who are more specialised in this area 🙂
This is to do with electrons. Atoms are made from protons and neutron on the inside and electrons on the outside.
Electrons have different energies and they group together around the atom in sets depending on their energy. The sets have a certain number of places that can be filled. Having a full set of electrons makes an atom more stable.
Most atoms have incomplete sets of electrons and they like to ‘share’ electrons with other atoms so that they get a full set. This ‘sharing’ is a chemical bond that keeps the atoms together.
Atoms like to react with atoms when the numbers of electrons in each one adds up to a complete set.
Chemical reactions are about making and breaking bonds – reorganising molecules. Bonds are of two kinds – ionic bonds where a positively charged species is attracted to a negatively charged species (a bit like magnets) and covalent bonds where two atoms are attached more as if they were tied together. In both cases energy is needed to break these initial bonds to start a reaction – even if more energy is returned when any new bonds are formed to give new substances. This is called the “reaction activation energy barrier” and explains why we don’t all just burst into flames (among other things)