In these reactions, a substance is made from its elements. Most composition reactions are exothermic.
Many, if not all composition reactions can be divided in steps.
The production of water from the elements Hydrogen and Oxygen needs the following first steps:
- H2 2H
- O2 2O
- Only if loose Hydrogen and oxygen atoms exist, a water molecule can be composed.
- 2H + O H2O
- In general (but not always!) exothermic processes will dominate at the composition of substances from the elements.
With some help of the table with bonding energies, you can calculate the energy effect of the composition of water:
- H2 + ½O2 H2O
- 1 mol H-H bonds are broken (cost how much energy?) sign +
- 1 mol O=O bonds are broken (cost how much energy?) sign +
- 2 mol O-H bonds are made (energy is delivered, how much? sign -