ChemThink: Chemical Reaction

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 chemical bonds.

5. Chemical reaction 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 missing atoms or new atoms.

8. Chemical reactions only rearrange the bonds in 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 balenced equation would be 2 H2 + 2 O2 = 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. The balanced equation for this reaction is 2 C2 + O2 = 2 CO2.

14. In the unbalanced equation, there are 1 Cu atom and 2 O atoms as the reactants, and 1 Cu atom and 1 O atom as the products.

15. To balance this equation, we have to add 3 molecules to the products, because this reaction doesn't make lone Cu atoms.

16. When we added a molecule of CuO, now the number of oxygen atoms is balanced but the number of Cu atoms don't match. Now we have to had more Cu atoms to the reactants. 

17. The balanced reaction for this equation is 2 Cu + O2 = 2 CuO. 

18. The balanced equation for this reaction is 1 CH4 + 2 O2 = 2 H2O + 1 CO2.

19. The balanced equation for this reaction is 1 N2 + 3 H2 = 2 NH3.

20. The balanced equation for this reaction is 2 KCIO3 = 2 KCI + 3 O2.

21. The balanced equation for this reaction is 4 Al + 3 O2 = 2 Al2O3.

Summary

1. Chemical reactions always involve breaking bonds, making bonds, or both.

2. The Law of Conservation of Mass says that the same atoms must be the same before the reaction and after.

3. To balance a chemical equation, you change the coefficients in front of each substance until there are the same number of each type of atom in both reactants and products.