Unit 2 – Lecture 8
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

Chem. Equations – cont’d
Chemical symbols give a “before-and-after” picture of
a chemical reaction
Reactants
MgO
+
Products
C
magnesium oxide to form
reacts with carbon
CO
+ Mg
carbon monoxide
and magnesium
or… magnesium oxide reacting with carbon
yields carbon monoxide and magnesium
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)

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)
Chemical Equations - cont’d
Try this:
 Al2(SO4)3
 Aluminum?
 2 atoms
 Sulfur?
 3 atoms
 Oxygen?
 12 atoms (4x3)
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
Balancing – cont’d
 Balanced = same # of each element on each side
Al
+ S
2Al
+
3S
Al2S3
Al2S3
Unbalanced
Balanced
Balanced or Unbalanced?
 C3H8 + 5O2
4H2O + 3CO2
 Carbon?
= 3 vs 3
 Hydrogen?
 = 8 vs 8
 Oxygen
 = 10 vs 10 (4 + 6)

Is it balanced?
Yes.
Balanced or Unbalanced?
 Fe3O4 + H2
 Iron
= 3 vs 1
 Oxygen
 = 4 vs 1
 Hydrogen
 = 2 vs 2

Fe + H2O
Is it balanced?
No.
So, now what?
Balancing a Chemical Reaction
 Set up two columns: labeled “reactants” &“products.”
 Identify & record reacting elements on the left.
 Record elements in the SAME ORDER on the right.
 Count & Record # of each reacting element.
 Count & Record # of each element produced.
 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
Practice

Fe3O4 +
H2
Fe +
H2O
Practice
 On your worksheet, complete the two problems on the
front of your sheet.
2 2 + ____O
2 2O
____H
1 2  ____H
1 2 + ____NaBr
2
1
2
____Cl
 ____Br
2 + ____NaCl
Homework
 Page 6 –
 Balancing Chemical Equations w/s
 1-9 on-level
 all for honors