1. II. Balancing Equations
Ch. 7/8 – Chemical Reactions
II. Balancing Equations
C. Johannesson

2. Coefficient  subscript = # of atoms
A. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest possible ratio, if necessary.
5. Double check atom balance!!!
C. Johannesson

3. B. Helpful Tips Balance one element at a time.
Update ALL atom counts after adding a coefficient.
If an element appears more than once per side, balance it last.
Balance polyatomic ions as single units.
“1 SO4” instead of “1 S” and “4 O”
C. Johannesson

4. C. Balancing Example 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1
Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2
Al CuCl2  Cu AlCl3 Al Cu Cl 3 3 2
2 
 2
 6
3 
6 
 3
C. Johannesson

5. Mr. Rapps slides next
C. Johannesson

6. Compare the numbers of each kind of atom in the balanced equation with the numbers of each kind of atom in the sketched representation. Both the equation and the sketch have the same number of atoms in the reactants and in the products.

7. Practice Al(s) + O2(g) ---> Al2O3(s)
Inventory: 1Al and 2O’s on left and 2Al’s and 3O’s on right.
Start with Al
Need 2 on left to balance right
2Al(s) + O2(g) ---> Al2O3(s)
Al is balanced but O is not.
Double everything but O2

8. Practice 4Al(s) + O2(g) ---> 2Al2O3(s)
Al is still balanced, now we have 2O’s on left and 6O’s on right.
How can we balance that?
3x2 = 6 so we need 3 O2 molecules
4Al(s) + 3O2(g) ---> 2Al2O3(s)
Inventory: 4Al’s and 6O’s on left and 4Al’s and 6O’s on right
It’s balanced

9. Practice Balance: FeCl3 + NaOH  Fe(OH)3 + NaCl
Inventory: 1Fe, 3Cl’s, 1Na and 1OH on left and 1Fe, 3OH’s, 1Na and 1Cl on right
Balance Cl first  need 3 on right
FeCl3 + NaOH  Fe(OH)3 + 3NaCl
Na is now unbalanced  need 3 on left now
FeCl3 + 3NaOH  Fe(OH)3 + 3NaCl
Inventory again: 1Fe, 3Cl, 3Na, and 3OH on left and 1Fe, 3OH, 3Na, and 3Cl on right
It’s balanced!!!

10. Practice Ba(NO3)2 + Al2(SO4)3  BaSO4 + Al(NO3)3
Inventory: 1Ba, 2NO3’s, 2Al’s and 3SO4’s on left and 1Ba, 1SO4, 1Al, and 3NO3’s on right
Balance SO4’s first  need 3 on right
Ba(NO3)2 + Al2(SO4)3  3BaSO4 + Al(NO3)3
Balance Ba next  need 3 on left
3Ba(NO3)2 + Al2(SO4)3  3BaSO4 + Al(NO3)3
Balance NO3’s next  6 on left and 3 on right, use 2 as coefficient

11. Practice 3Ba(NO3)2 + Al2(SO4)3  3BaSO4 + 2Al(NO3)3
That gives 6NO3’s on both sides
Inventory: 3Ba’s, 6NO3’s, 2Al’s and 3SO4’s on left and 3Ba’s, 3SO4’s, 2Al’s, and 6NO3’s on right.
It’s balanced

12. Practice Ca(OH)2 + H3PO4  Ca3(PO4)2 + H2O
Inventory: 1Ca, 5H’s, 6O’s and 1P on left and 3Ca’s, 9O’s, 2H’s, and 2 P on right.
Balance Ca first  need 3 on left
3Ca(OH)2 + H3PO4  Ca3(PO4)2 + H2O
Balance PO4’s next  need 2 on left
3Ca(OH)2 + 2H3PO4  Ca3(PO4)2 + H2O
Inventory again for H’s and O’s: 6O’s+8O’s(14) and 6H’s+6H’s(12) on left and 8O’s+1O’s(9) and 2H’s on right

13. Practice To get 12H’s on right we can use 6 as a coefficient for water
3Ca(OH)2 + 2H3PO4  Ca3(PO4)2 + 6H2O
Inventory: 3Ca’s, 2P’s, (6+6=12)H’s, and (6+8=14)O’s on left and 3Ca’s, 2P’s, (6x2=12)H’s and (8+6=14)O’s on right
It’s balanced

14. Practice ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g)
____B4H10(g) _____ O2(g) > ___ B2O3(g) _____ H2O(g) 5 3 4 2 11 4 10

15. Practice NH4NO3 ---> N2O + 2 H2O
Carbon monoxide reacts with nitrogen monoxide to form carbon dioxide and nitrogen gas (pairs)
Equation: CO + NO  CO2 + N2
CO + NO  CO2 + N2 2 2 2

16. Practice
Phosphorus trichloride reacts with water to form hydrochloric acid and phosphorous acid.
Equation:
PCl3 + H2O  HCl + H3PO3
Magnesium nitride reacts with water to form magnesium hydroxide and ammonia
Mg3N H2O  Mg(OH) NH3
PCl3 + H2O  HCl + H3PO3 3 3
Mg3N2 + H2O  Mg(OH)2 + NH3 6 3 2