IPS
Unit 11
Chemical Equations
Section 1
Chemical Reaction
- change in which one or more substances are
converted into new substances
Reactants - original substances
( what you start with)
Products –
new substances produced
(what you end with)
Chemical Equations
• Chemical equations are a way to describe a
chemical reaction using chemical formulas and
symbols
• Chemical equations quickly convey information
such as the states of matter of the reactants
and products
• Chemical equations show the conversion of
reactants into products
• Chemical equations
use symbols to show
the conversion from
reactants to
products.
Characteristics of Chemical Equations
• Equations must represent the known facts
• Equations must contain the correct formulas for the
reactants and products
• Diatomic Molecules – molecule containing only two
atoms
H2, N2, O2, F2, Cl2, Br2, I2
• The Law of Conservation of Mass must be satisfied
mass can neither be created nor destroyed
Word Equations
• chemical equations shown with words
Example:
methane and oxygen yields carbon dioxide and water
Formula Equations
• chemical equations shown with symbols and formulas
Example:
CH4 + O2  CO2 + H2O
Balancing Chemical Equations
• When balancing a chemical reaction you may add
coefficients in front of the compounds to balance
the reaction, but you may NOT change the
subscripts
• Coefficients represent the number of units/
molecules of each substance taking part in a
reaction
• Changing the subscripts changes the compound.
Subscripts are determined by the valence electrons
(charges for ionic or sharing for covalent)
HgO
O2 + Hg
• If you put a coefficient of 2 before the HgO on the
left, the oxygen atoms will be balanced, but the
mercury atoms become unbalanced.
• To balance the equation, also put a 2 in front of
mercury on the right. The equation is now balanced.
2HgO
O2 + 2Hg
How to Write Balanced Chemical Equations
1. Start with a formula equation (write one if
needed). Make sure to show correct reactants and
products.
 Write one if needed
 Write the correct formulas (one at a time)
 Don’t forget the diatomics!
 Do not balance yet!
water yields hydrogen and oxygen
H2O  H2 + O2
How to Write Balanced Chemical Equations
2. Balance the formula equation according to the Law
of Conservation of Mass.
 Balance different types of atoms one at a time
 If you see the same polyatomic ion on each side
of the equation, keep it together and balance it
as one thing
 First balance the atoms of elements that appear
only once on each side of the equation
 Save H atoms and O atoms until all the other
elements have been balanced
2 H2O

2 H2
H- 2 4
H- 2 4
O- 1 2
O- 2
+
O2
Practice Problems
Mg + HCl  MgCl2 + H2
Practice Problems
Potassium Bromide plus Chlorine gas yields
Potassium Chloride and Bromine gas
KBr + Cl2  KCl
+
Br2
Practice Problems
Aluminum plus bromine yields aluminum bromide
Al + Br2  AlBr3
Practice Problems
C3H8 + O2  CO2 + H2O
Practice Problems
Sodium phosphate plus iron (III) oxide yields
sodium oxide and iron (III) phosphate
Na3PO4+ Fe2O3 Na2O + FePO4