ChemThink: Chemical Reactions

1. Starting materials in a chemical reaction are called reactants.
2. The ending materials in a chemical reaction are called products.
3. The arrow indicates a chemical change has taken place.
4. All reactions have one thing in common: there is a rearrangement of the chemical bonds.
5. Chemical reactions always involve breaking old bonds, forming new bonds, or both.
6. In all reactions we still have all of the atoms at the end that we had at the start.
7. In every reaction there can never be any new atoms or missing atoms.
8. Chemical reactions only change the position and bonds of the atoms that are already
there.
9. If we use only the atoms shown, we’d have 2 atoms of H and 2 atoms of O as reactants. This would make 1 molecule of H2O, but we’d have 1 atom of O leftover. However, this reaction only makes H2O.
10. So to make H2O from oxygen gas and hydrogen gas, the balanced equation would be:

2 H2 + O2 ----> 2 H2O

11. This idea is called the Law of Conservation of Mass.
12. There must be the same mass and the same number of atoms before the reaction (in the reactants) and after the reaction (in the products).
13. What is the balanced equation for this reaction? 2 Cu + O2 ! 2 CuO
14. In the unbalanced equation there are: Reactants ----> Products

Cu atoms    1    Cu atoms 1

O atoms      2    O atoms 1

15. To balance this equation, we have to add two molecules to the products because this reaction doesn’t make lone atoms.
16. When we added a molecule of CuO, now the number of O atoms is balanced but the number of Cu atoms.
17. The balanced equation for this reaction is 2 Cu + O2 ----> 2 CuO
18. What is the balanced equation for this reaction? 1 CH4 + 2 O2 ---->  2 H2O + 1 CO2
19. What is the balanced equation for this reaction? 1 N2 + 3 H2 ----->  2 NH3
20. What is the balanced equation for this reaction? 2 KClO3 ----> 2 KCl + 3 O2
21. What is the balanced equation for this reaction? 4 Al + 3 O2 ----->  2 Al2O3

SUMMARY
1) Chemical reactions always involve breaking old bonds, forming new bonds, or both.
2) The Law of Conservation of Mass says that the same atoms must have the same mass and number of atoms before and after the bond.
3) To balance a chemical equation, you change the number in front of each substance until there are the same number of each type of atoms in both reactants and products.