1. Balancing Chemical Equations

2. Chemical reactions can be written in 2 ways: word equation:
Nitrogen monoxide + oxygen  nitrogen dioxide
symbolic equation:
2NO(g) + O2(g)  2NO2(g)
State of matter
(aq) =aqueous/
dissolved in water
(s) = solid
(l) = liquid
(g) = gas
Coefficients
indicate the ratio of compounds in the reaction
here, there is twice as much NO and NO2 than there is O2

3. The Law of Conservation of Mass
In chemical reactions, atoms are neither created nor destroyed
Mass of reactants = Mass of products

4. Counting Atoms
To balance chemical equations, you must be able to count atoms.
How many atoms of each element are in the following molecules?
1. H2SO4
2. CaF2
3. NaCl
4. (NH3)3P04
5. 3H2O

5. Writing and Balancing Chemical Equations
A skeleton equation shows only the formulas of the elements/compounds
Shows atoms, but not quantities of atoms
K(s) + O2 (g)  K2O(s)
A balanced chemical equation shows all atoms & their quantities
Balancing ensures that the number of each atom is the same on both sides of the reaction arrow
Always use the smallest whole number ratio
4K(s) + O2 (g)  2K2O(s)

6. Answer: 2 Fe + 3 Br2  2 FeBr3 Balance the following: Fe + Br2  FeBr3
Left Right
Balance Fe:
Balance Br:
x3 x2
Br =
Balance Fe again: x2
Fe =
Answer: 2 Fe + 3 Br2  2 FeBr3

7. Hints for Writing Chemical Equations
Most elements travel as single atoms except:
Diatomics - The “special seven” (hockey stick & puck)
H2, N2, O2, F2, Cl2, Br2, I2
Polyatomics - P4 and S8
See page 208

9. Tips to balance chemical equations:
Trial and error will work, but can be very inefficient
Balance compounds first, elements last
Balance one compound at a time
Only add coefficients; NEVER change subscripts!
Balance metals first, then nonmetals, then H & O
If H and O appear in more than one place, attempt to balance them LAST
Polyatomic ions (such as SO42–) can often be balanced as a whole group
Always double-check after you think you are finished!

10. Balance the following chemical equations
Balance the following chemical equations. ____ N2 +____ O2  ____ N2O ____ KI + ____ Cl2  ____ KCl + ____ I2 ____ C2H6 + ____ O2  ____ CO2 + ____ H2O ____ Mg(NO3)2 + ____ K3PO4  ____ Mg3(PO4)2 + ____ KNO3

11. Balance the following: 1) ____ AlBr3 + ____ K  ____ KBr + ____ Al 2) ____ FeO + ____ PdF2  ____ FeF2 + ____ PdO 3) ____ P4 + ____ Br2  ____ PBr3 4) ____ LiCl + ____ Br2  ____ LiBr + ____ Cl2

12. CHALLENGE: 5. Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4 6
CHALLENGE: 5. Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4 6. C2H6 + O2  CO2 + H2O