1. Balancing chemical equations

2. Main Ideas Chemical Reactions are represented by Chemical Equations.
Chemical Equations are balanced to show the same number of atoms of each element on each side.
The Law of Conservation of Mass says that atoms won’t be created or destroyed in a chemical reaction. That is why you have to balance chemical equations!

3. Chemical Reactions are Everywhere
Cooking
Respiration

4. CHEMICAL REACTIONS
Zn + I2
Reactants
Zn I2
Product

5. Parts of a Reaction Equation
Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow).
A + sign separates molecules on the same side
The arrow is read as “yields”
Example
C + O2  CO2
This reads “carbon plus oxygen react to yield carbon dioxide”

6. Subscripts vs. Coefficients
The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

7. Chemical Equations – Counting Atom
Counting Atoms
3H2O
coefficient subscript
How do I know how many atoms are in a molecule?
Easy!! There are rules to follow:

8. Chemical Equations – Counting Atoms
Subscripts
Multiply the atom directly in front of them (no subscript = 1)
CH4 has 1 atom of C and 4 atoms of H
C6H14 has 6 atoms of C and 14 atoms of H
Multiply all atoms in preceding parenthesis
Ca(OH)2 subscript multiplies both O and H.
1 Ca, 2 O and 2 H

9. WARNING!
Don’t ever play around with subscripts (those little numbers that tell you how many atoms are in a molecule) e.g.
C6H22O11

10. Chemical Equations – Counting Atoms
Coefficients - Multiply all atoms in the compound they precede, including those in parenthesis.
2CH4 has 2 C and 8 H
2C6H12O6 has 12 C, 24 H and 12 O
3Ca(OH)2has 3 Ca, 6 O and 6 H
Note: Coefficients do not carry past + signs or the reaction arrow ( ) in a chemical equation. They are for a single molecule or compound.

11. Chemical Equations – Counting Atoms
MnO2 _________________
2 MnO2 _________________
4 MnO2 _________________
C2H6 _________________
5 C2H6 _________________
Ca(HCO3)2 __________________
2 Ca(HCO3)2 __________________
C2H2OH _______________
3 C2H2OH _______________
Mrs. Johannesson

12. Chemical Equations – Counting Atoms
MnO Mn = O = 2
2 MnO2 Mn = O= 4 (2x2)
4 MnO2 Mn = O= 8 (4x2)
C2H6 C = H = 6
5 C2H6 C = 10 (5)x H = 30 (5x6)
Ca(HCO3) Ca = H =2 C = 2 O = 6
2 Ca(HCO3)2 Ca = H =4 C = 4 O = 12
C2H2OH C = H = 3 O = 1
3 C2H2OH C = H = 9 O = 3
Mrs. Johannesson

13. Chemical Equations – Counting Atoms
Mass is neither created nor destroyed in a chemical reaction
total mass stays the same
atoms can only rearrange
2 H2 + O2 2H2O
4 H
2 O
4 H
2 O
Mrs. Johannesson

14. Balancing Equations Na + Cl2 NaCl Example Reactants Products Na Na Cl

15. Balancing Equations NaCl Na + Cl2 Example Reactants Products Total 31 Na 1 Cl 2 Cl 1
Total 3
Total 2

16. Balancing Equations 2NaCl Na + Cl2 Add coefficients only Example
to balance
Example
2NaCl
Na Cl2
Reactants
Products Na 1 Na Cl 2 Cl
Total 3
Total

17. WE SAY THAT THE EQUATION IS BALANCED!!
Balancing Equations
2Na Cl2
2NaCl
Reactants
Products Na Na Cl Cl 2
Total
Total
NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS
WE SAY THAT THE EQUATION IS BALANCED!!

18. Chemical Equations – Balancing Practice
Step 1: Determine the reactants and products.
Step 2: Complete an atom inventory of both sides of the chemical equation.
C3H O → CO H2O
C = C = 1
H = H = 2
O = O = 3 (2+1)
Total left = Total right = 6
Is this equation balanced?? NO.

19. Chemical Equations - Balancing Practice
Step 3: Use coefficients to balance the equation. NEVER change the subscript in order to balance the equation!!
C3H O → CO H2O
C =3 C = 3
H = H = 2
O = O = 7 (6+1)
Is this equation balanced?? No.

20. Chemical Equations - Balancing Practice C3H8 + 5O2 → 3CO2 + 4H2O
C = C = 3
H = H = 8
O = O = 10 (6 + 4)
Total = Total = 21
Is this equation balanced?? Yes.
Hint: Whenever possible, save the “O” until last. The oxygen atoms will often take care of themselves.

21. Balance Equations in the simplest form
Check to see that the coefficients used are the smallest integers that give the balanced equation.
The balanced equation is:
4H2(g) + 2O2(g)  4H2O(l)
or could be:
2H2(g) + O2(g)  2H2O(l)
preferred

22. Chemical Equations – Balancing Practice
Na O2 → Na2O
CuCl2 + H2S → CuS HCl
Al2O → Al O2
4. NaCl + F2 → NaF Cl2

23. Conservation of Mass Lab
Mass Reactants = Mass of Products
State Law of Conservation of Mass in your own words