1. Balancing Chemical Reactions

2. Balancing
Chemical reactions obey the law of conservation of matter which states that matter is neither created nor destroyed, it only changes forms (becomes rearranged)
the number of atoms on the reactant side must equal the number of atoms on the product side
Reactants  products

3. Reactants Products Mg + 2HCl  MgCl2 + H2 Mass before = mass after (Law of conservation of matter) If the masses before and after are not equal, one of the compounds is in excess and the other is the limiting reagent.

4. Rules for Balancing Equations
1. Determine the correct formula for reactants and products
don’t forget diatomic molecules
H2 N2 O2 F2 Cl2 Br2 I2
and polyatomic molecules
(more than two) P4 S8
2. Count the # of atoms of each element in the reactants and products
3. Balance each atom using coefficients.
Never balance an equation by changing the subscripts
4. Check each atom to be sure that the equation is balanced.

5. Helpful hints:
All elements have a charge of zero when uncombined. (no ionic charge)
Leave H and O until the end
If stuck, go back to the beginning and put a 2 in front of the most complex chemical

6. Examples
1. Aluminum reacts with lead(II) hydroxide to produce aluminum hydroxide and lead.

7. Examples
1. Aluminum reacts with lead(II) hydroxide to produce aluminum hydroxide and lead. Al + Pb(OH)2  Al(OH)3 + Pb

8. Examples
1. Aluminum reacts with lead(II) hydroxide to produce aluminum hydroxide and lead. 2Al + 3Pb(OH)2  2Al(OH)3 + 3Pb

9. Examples
2. Sulfur plus oxygen yield sulfur dioxide

10. Examples
2. Sulfur plus oxygen yield sulfur dioxide S8 + O2  SO2

11. Examples
2. Sulfur plus oxygen yield sulfur dioxide S8 + 8O2  8SO2

12. Examples
3. Sodium sulfate and calcium borate react to make sodium borate and calcium sulfate.

13. Examples
3. Lithium sulfate and calcium borate react to make lithium borate and calcium sulfate.
Li2SO Ca3(BO3 )2  Li3BO3 + CaSO4

14. Examples
3. Lithium sulfate and calcium borate react to make lithium borate and calcium sulfate.
3Li2SO Ca3(BO3)2  2Li3BO CaSO4

15. Examples
4. Diphosphorus trioxide combines with water to create phosphorous acid.

16. Examples
4. Diphosphorus trioxide combines with water to create phosphorous acid.
P2O3 + H2O  H3PO3

17. Examples
4. Diphosphorus trioxide combines with water to create phosphorous acid.
P2O H2O  2H3PO3