1. Molecular rep of a rxn CH4 + O2  CO2 + H2O
Notice that there are 4 hydrogen atoms in the reactants and only 2 in the products.
Notice that there are only 2 oxygen atoms in the reactants and 3 in the products

2. Chemical Equation A representation of a balanced chemical reaction:
CH O2  CO H2O
reactants products

3. Molecular rep of a rxn
CH O2  CO H2O

4. Chemical Equation CH4 + 2O2  CO2 + 2H2O
The equation is balanced – there are the same number of each atom on each side of the arrow.
1 molecule of methane reacts with 2 molecules of oxygen
To produce:
1 molecule of carbon dioxide and 2 molecules of water

5. To Balance Equations
Start with the element that is most complex, usually
Make sure the same numbers of atoms of that element are on both sides of the equation
You CANNOT change the formula of a molecule in an equation
You must change the COEFFICIENT of the molecule to make there be the same numbers

6. Balance the Reaction 1 Ca + AlCl3  CaCl2 + Al
Chlorine is the most complex
There are 3 Cl’s in the reactants and 2 Cl’s in the products
The least common multiple between 3 and 2 is…6
In order to make 6 Cl’s on each side, put the coefficient 2 in front of the AlCl3 and the coefficient 3 in front of the CaCl2.

7. Balance the Reaction 1 Ca + 2AlCl3  3CaCl2 + Al
Now you have changed the number of Aluminum’s in the reactants to 2. There is only 1 Al in the products
In order to make there be 2 Al’s in the products, put the coefficient 2 in front of the Al
You have also changed the number of Calcium’s in the products to 3. There is only 1 Ca in the reactants
Put the coefficient 3 in front of the Ca.

8. Balance the Reaction 1 3Ca + 2AlCl3  3CaCl2 + 2Al
Check to see that there are equal numbers of each atom on each side of the reaction…
The reaction is Balanced!!

9. Balance the Reaction 2 C2H5OH + O2  CO2 + H2O
Hydrogen is the most complex
There are 6 H’s in the reactants and only 2 in the products
In order to make 6 H’s in the products, put the coefficient 3 in front of the H2O

10. Balance the Reaction 2 C2H5OH + O2  CO2 + 3H2O
Carbon is the next most complex
There are 2 C’s in the reactants and only 1 in the products
In order to make 2 C’s in the products, put the coefficient 2 in front of the CO2

11. Balance the Reaction 2 C2H5OH + O2  2CO2 + 3H2O
Now you have changed the numbers of Oxygen atoms in the products – there are 7 O’s
Changing the number of C2H5OH’s will force you to change the coefficients on the products
One of the O’s is used up in the C2H5OH, so there are 6 O’s that need to be accounted for by the O2’s.
Put the coefficient 3 in front of the O2 so there are 7 total O’s in the reactants.

12. Balanced Reaction 2! C2H5OH + 3O2  2CO2 + 3H2O
Check to make sure that there are the same numbers of each atom on each side!

13. Which of the following correctly balance the chemical equation given below?
CaO + C  CaC2 + CO2
1. CaO2 + 3C  CaC2 + CO2
2. 2CaO + 5C  2CaC2 + CO2
3. CaO + (2.5)C  CaC2 + (0.5)CO2
4. 4CaO + 10C  4CaC2 + 2CO2

14. Notice
The number of atoms of each element must be the same on both sides of a balanced equation.
Subscripts must not be changed to balance an equation.
A balanced equation tells us the ratio of the number of molecules that react and are produced in a chemical reaction.
Trial and error is a valid method to balance a chemical equation.

15. Balance the following Reactions
N H2  NH3
H2O  H O2
Mg + FeCl3  MgCl Fe
Br FeCl3  FeBr Cl2

16. Check your answers N2 + 3H2  2NH3 2H2O  2H2 + O2
3Mg + 2FeCl3  3MgCl2 + 2Fe
3Br2 + 2FeCl3  2FeBr3 + 3Cl2

17. Balance the following reactions
NaCl + MgBr2  MgCl2 + NaBr
Pb(NO3)2 + Na2SO4  NaNO3 + PbSO4
CH4 + O2  CO2 + H2O
C4H10 + O2  CO2 + H2O

18. Check Your Answers 2NaCl + MgBr2  MgCl2 + 2NaBr
Pb(NO3)2 + Na2SO4  2NaNO3 + PbSO4
CH4 + 2O2  CO2 + 2H2O
C4H /2O2  4CO2 + 5H2O or
2C4H O2  8CO2 + 10H2O

19. Balance the Reactions Al + O2  Al2O3 C3H8 + O2  CO2 + H2O
LiCl + CuSO4  CuCl2 + Li2SO4
NaCl + F2  NaF + Cl2

20. Check Your Answers 4Al + 3O2  2Al2O3 C3H8 + 5O2  3CO2 + 4H2O
2LiCl + CuSO4  CuCl2 + Li2SO4
2NaCl + F2  2NaF + Cl2