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 same atoms at the end that we had at the start.

7. In every reaction there can never be any missing atoms or new atoms when the reaction is over.

8. Chemical reactions only rearrange the bonds in the atoms that are already there.

9. Let’s represent a reaction on paper. For example, hydrogen gas (H2) reacts with oxygen gas (O2) to form water (H2O):

H2 + O2 → H2O

If we only use atoms shown, we’d have 2 atoms of H and 1 atom 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 + 1 O2 → 2 H2O

Which is the same as:

# of atoms in Reactants	Element	# of atoms in Products
4	H	4
2	O	2

11. This idea is called the Law of Conservation of Mass.

12. There must be the same atoms and the same number of mass before the reaction (in the reactants) and after the reaction (in the products).

13. What is the balanced equation for this reaction? 2 Cu + 1 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 CuO molecules to the products, because this reaction doesn’t make lone Oxygen atoms.

16. When we added a molecule of CuO, now the number of CuO atoms is balanced but the number of Oxygen atoms don’t match. Now we have to add more Cuo atoms to the reactants.

17. The balanced equation for this reaction is

2 Cu + 1 O2 → 2 CuO

This is the same thing as saying: Reactants         →    Products
# Cu atoms 2 # Cu atoms 2
# O atoms 2 # O atoms 2

18. What is the balanced equation for this reaction?

1 CH4 + 2 O2  → 2 H2O + 1 CO2

# of atoms in Reactants	Element	# of atoms in Products
1	C	1
4	H	4
4	O	4

19. What is the balanced equation for this equation?

1 N2 + 3 H2 → 2 NH3

# of atoms in Reactants	Element	# of atoms in Products
2	N	2
6	H	6

20. What is the equation for this reaction?

2 KClO3  → 2 KCl + 3 O2

# of atoms in Reactants	Element	# of atoms in Products
2	Cl	2
6	O	6
2	K	2

21. What is the equation for this reaction?

4 Al + 3 O2 → 2 Al2O3

# of atoms in Reactants	Element	# of atoms in Products
4	Al	4
6	O	6

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 present before and after the reaction.
3) To balance the 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.