1. Balancing Chemical Equations
Unit 2 – Lecture 8

2. Chemical Equations
A shorthand way to describe a chemical reaction using chemical symbols and formulae
Three Parts to a Chemical Equation
Reactants
substances present before the reaction
Yield Arrow
indicates the direction of a reaction
some rxns are reversible [double-sided arrow]
Products
substances present after the reaction

3. Chem. Equations – cont’d
Chemical symbols give a “before-and-after” picture of a chemical reaction Reactants Products MgO + C CO + Mg magnesium oxide to form carbon monoxide reacts with carbon and magnesium or… magnesium oxide reacting with carbon yields carbon monoxide and magnesium

4. Chemical Equations – cont’d
Coefficient – (large) # in front of an element
applies to all elements in the group to the right of it
ex: 4C
4 carbon atoms
ex: 5KI
5 potassium atoms & 5 iodine atoms
ex: 4SO4
4 Sulfur atoms & 16 Oxygen atoms (4x4)

5. Chemical Equations – cont’d
Subscript – (small) # to the lower right of an element
applies only to the element it is attached to
if outside of parentheses [(XY)3] applies to everything inside of parentheses
ex: H2
2 hydrogen atoms
ex: CO2
1 carbon atom, 2 oxygen atoms
ex: (SO4)3
3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)

6. Chemical Equations - cont’d
Try this:
Al2(SO4)3
Aluminum?
2 atoms
Sulfur?
3 atoms
Oxygen?
12 atoms (4x3)

7. Chemical Equations reactants and products
Chemical Equations
reactants and products
4 Al (s) O2 (g) ---> 2 Al2O3 (s)
coefficients
The letters (s), (g), and (l) are the physical states of compounds.

8. Symbols Used in Equations
Solid (s)
Liquid (l)
Gas (g)
Aqueous solution (aq) (dissolved in water)
Catalyst H2SO or Pt
Escaping gas ()
Change of temperature/ heat energy ( or + 3kJ or – 3kJ) *there is no subtraction…a negative sign means released/exothermic

9. Balancing Chem Equations
Law of Conservation of Matter
matter is neither created nor destroyed in a chemical reaction, it is conserved
biology: energy is lost, matter is RECYCLED
(changed from one thing to another)
ie, if you begin a reaction with 5 atoms of Hydrogen, you must end up with 5 atoms of Hydrogen

10. Balancing – cont’d Al + S Al2S3 2Al + 3S Al2S3
Balanced = same # of each element on each side
Al S Al2S3
2Al S Al2S3
Unbalanced
Balanced

11. Balanced or Unbalanced?
C3H8 + 5O H2O + 3CO2
Carbon?
= 3 vs 3
Hydrogen?
= 8 vs 8
Oxygen
= 10 vs 10 (4 + 6)
Is it balanced?
Yes.

12. Balanced or Unbalanced?
Fe3O H Fe H2O
Iron
= 3 vs 1
Oxygen
= 4 vs 1
Hydrogen
= 2 vs 2
Is it balanced?
No.
So, now what?

13. Balancing a Chemical Reaction
Unequal? Balance Coefficients. [can’t change subscripts]
you can treat Polyatomic Ions as one if they appear on both sides of the equation like (SO4) in parentheses
change your greatest inequality first
leave solitary elements for last

14. Practice
Fe3O H Fe H2O Fe O H 3 4 2 1 2

15. Practice
Complete the two problems on the front of your sheet.
____H2 + ____O2  ____H2O
____Cl2 + ____NaBr  ____Br2 + ____NaCl 2 2 1 2 1 1 2