Chem Think 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 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 Missing atoms or new atoms.
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 + O2H2O If we use only the atoms shown, we’d have 4 atoms of H and 3 atoms of O as reactants. This would
make 2 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 + 2 O2 ----> H2O Which is the same as:

# of atoms in Reactants	Element	# of atoms in Product
2	H	2
2	O	1

11. This idea is called the Law of Conversion 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 ---> CuO
14. In the unbalanced equation there are:
Reactants Products
Cu atoms : 1 Cu atoms: 1
O atom: 2 O atom: 1

15. To balance this equation, we have to add 1 Cu molecules to the products, because this reaction doesn’t make lone O atoms.

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

17. The balanced equation for this reaction is 2 Cu + 1 O2 ---> 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? (Use the table to keep track of the atoms on each side.)
1 CH4 + 2 O2 ---> 2 H2O + 1 CO2

# of atoms in Reactants	Element	# of atoms in Products
1	C	1
4	H	2
2	O	3

19. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 1 N2 + 3 H2 ---> 2 NH3

# of atoms in Reactants	Element	# of atoms in Products
2	N	1
2	H	3

20. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 2 KClO3 ---> 2 KCl + 3 O2

# of atoms in Reactants	Element	# of atoms in Products
1	K	1
1	Cl	1
3	O	2

21. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 4 Al + 3 O2 ---> 2 Al2O3

# of atoms in Reactants	Element	# of atoms in Products
1	Al	2
2	O	3

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