ChemThink; Chemical Reactions

1. Starting materials in a chemical reaction are called the 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 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.
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):
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.
Remember: reactions are not limited to 1 molecule each of reactants. We can use as many as we need to balance the chemical equation.
A balanced chemical reaction shows:
a) What atoms are present before (in the reactants) and after (in the products)
b) How many of each reactant and product is present before and after.
10. So to make H2O from oxygen gas and hydrogen gas, the balanced equation would be:
2 H2 + 1 O2= 2H2O
Which is the same as:

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

11. This idea is called the Law of Conservation 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 + 22O2= 2CuO

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 2 CuO molecule to the products, because this reaction doesn’t make lone Oxygen 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 add more Cu 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? (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.) 1N2 + 3H2 =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= 2KCl + 3 O2

# of atoms in Reactants	Element	# of atoms in Products
1	K	1
1	CI	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.)4Al + 3O2=2Al2O3

# 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(making) new 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 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.