Chapter 4.1
CHEMICAL REACTIONS
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Chemical Reactions
 Chemical Change– the rearrangement of the atoms
in a substance that creates new products with
different chemical properties
 Chemical Reaction-- the process in which one or
more substances undergo a chemical change
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Change in energy Energy can be released (heat, light, sound)
or absorbed
Occur at different rates: a reaction can be slow or fast (a
catalyst is used to speed up reactions)
Demos
Q: How do we know a chemical change has occurred?
Observe:
Demo 1– lead nitrate + potassium iodide
Demo 2– baking soda + citric acid solution
Clues to a Chemical Reaction
 Change in colour
 Precipitate forms
 Energy is released or absorbed
 Gas is produced
 Difficult to reverse
REMEMBER: A change in state or dissolving a
substance is a physical change-- NOT a chemical
change
How can we speed up a chemical reaction?
 Increase the temperature
 Increase the surface area
 Increase reactant concentration
 CATALYST: A substance that makes a chemical
reaction go faster WITHOUT being consumed.
Chemical Equations
 Using words or symbols and formulas to represent a
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
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chemical reaction
Starting materials= “reactants”
New substances= “products”
An arrow is read as “produces”
States of matter are in brackets
(s)= solid (l)= liquid (g)= gas (aq)= dissolved in water
REACTANTS 
PRODUCTS
Word Equations
 Words represent the chemical reaction
REMEMBER: any compound that is not molecular,
check the criss cross rule first.
Example:
Lithium + Aluminum Chloride  Aluminum + Lithium Chloride
Skeleton Equations
 An unbalanced equation that represents the chemical
formulas of reactants and products
Ex. Na + O2  Na2O
Rules:
 Write the symbol of a metal NOT in a compound
 If the non-metal is part of hockey stick and puck, write it
as a diatomic molecule (H2, N2, O2, F2, Cl2, Br2, I2)
 Write the formula of other compounds using rules you
have learned
Law of Conservation of Mass
 In a chemical reaction…
The total mass of reactants EQUALS the total mass
of products because elements cannot be created nor
destroyed in a chemical reaction. They can only be
rearranged!
Formula Equations
 Formulas represent reactions
 Include states of matter
 Coefficients show how the ratios of different
substances in the chemical reaction
Ex. 2 N2 (g) + 5 O2 (g)  2 N2O5 (s)
Balancing Chemical Formulas
Some tips:
 Balance polyatomic ions first if the same ion is on
both sides of the equations
 Balance all other elements other than H and O
 Balance H
 Balance O
Examples
1.
FeCl3 +
NaOH 
2.
Cu +
3.
C5H12 +
O2 
CO2 +
H2O
4.
C 2H 6 +
O2 
CO2 +
H 2O
AgNO3 
Fe(OH)3 +
NaCl
Cu(NO3)2 +
Ag
Some Helpful Videos
 Elephant toothpaste (catalyst example):
http://www.youtube.com/watch?v=4N0m95PExHY
 Balancing chemical equations:
http://www.youtube.com/watch?v=HB6cG7bQew0
 Practice balancing chemical equations:
http://www.skyweb.net/science/balancing_chemical_equations_ex
amples.htm
Homework
- Balancing chemical equations worksheet
- p. 155 # 3, 5