CHEMICAL REACTIONS
YouTube - Bill Nye The Science Guy on Chemical
Reactions (Full Clip)
CHEMICAL REACTIONS
• Pretend that the element symbols are the
alphabet of chemistry
• Molecular and ionic compounds would be
the words of chemistry
• Chemical equations are the sentences of
chemistry.
• In a chemical equation, there are numerous
pieces of information that are hidden.
• You can also think of a chemical equation as
being a recipe…..
CHEMICAL REACTIONS
A recipe contains:
• ingredients--what kind and
how much of each?
• products--how many dozen
chocolate chip cookies are you
going to make?
• and how do you get from
ingredients to products?--cook
at 350o for 12 minutes
CHEMICAL EQUATIONS
• Chemical equations have ingredients also,
we just call them REACTANTS
• Chemical equations have products also,
we call them PRODUCTS
• Chemical equations tell us how to get from
reactants to products--just like a recipe
CHEMICAL EQUATIONS
This is a balanced chemical equation:
• 2NaOH + H2SO4  Na2SO4 + 2H2O
• The two chemicals on the left of the arrow
are the reactants (NaOH and H2SO4).
• The two chemicals on the right are the
products (Na2SO4 and H2O)
CHEMICAL EQUATIONS
• The arrow in the middle tells us how to get
from one side to the other side. Since no
information is given, we can assume that
this reaction will go from the reactants to
the products as soon as the reactants are
mixed together (no baking required)
Chemical Reactions
Evidence of a Chemical Reaction
• Chemical Reaction is the Rearrangement of Atoms
• There are several ways to tell whether a chemical
reaction has happened. These methods include:
–
–
–
–
Color Changes
Produce (Formation) of a Solid
Formation of a Gas
Temperature change
• Absorption (colder)
• Emission (warmer)
– Light emission
– Sound emission
Chemical Reactions
Evidence of a Chemical Reaction
Color Change
Chemical Reactions
Evidence of a Chemical Reaction
Formation of a Solid
Chemical Reactions
Evidence of a Chemical Reaction
Formation of a Gas
Chemical Reactions
Evidence of a Chemical Reaction
Emission of Light
Chemical Reactions
Evidence of a Chemical Reaction
Emission or Absorption of Heat
Energy and Reactions
• Energy must be added to break bonds.
• Forming bonds releases energy.
Reaction Model
When gasoline burns, energy in the form of heat and
light is released as the products of the isooctaneoxygen reaction and other gasoline reactions form.
Energy and Reactions,
• Energy is conserved in chemical reactions. Energy
cannot be created or destroyed. Energy is conserved in
chemical reactions
• Chemical energy is the energy released when a
chemical compound reacts to produce new
compounds.
The total energy that exists before the reaction is equal
to the total energy of the products and their
surroundings.
• An exothermic reaction is a chemical reaction in which
heat is released to the surroundings.
• An endothermic reaction is a chemical reaction that
absorbs heat.
Reaction Model
Exothermic or Endothermic?
CHEMICAL EQUATIONS
Before we start balancing equations, we are
going to look at several types of
equations. Each type of chemical
equation represents a different type of
chemical reaction.
You can think of chemical reactions as being
similar to what happens at a senior
prom….
CHEMICAL REACTIONS
Decomposition Reactions
• One reactant decomposes to form two or
more products
• (One couple has a fight at the prom and
leaves as two individuals)
Pattern: AB  A + B
Example:
2HgO  2Hg + O2
CHEMICAL REACTIONS
Synthesis Reactions
• Two or more reactants combine to form a
larger product
• (Two individuals meet and leave as a couple)
Pattern: A+ B  AB
Example:
2H2 + O2  2H2O
CHEMICAL REACTIONS
Single Displacement Reactions
• One element replaces a different element
in a compound
• (John and Mary go to the Prom as a
couple, Fred comes by himself--Fred and
Mary leave together)
Pattern: A + BC  B + AC
Example
• Zn + 2HCl  ZnCl2 + H2
Double Displacement Reactions
• Two compounds trade partners
• (Fred and Mary go to the prom and meet
John and Martha. Fred and Martha leave
and John and Mary leave together)
Pattern: AB + CD  AD + CB
Example:
• BaCl2 +
Na2SO4  BaSO4 + 2NaCl
CHEMICAL REACTIONS
Combustion Reactions
• A chemical combines with oxygen (“burning”)
• (John and Mary go to the Prom. Mary looks at
Fred with interest--John goes postal and
everything changes)
Pattern: hydrocarbon + O2  CO2 + 2H2O
Example: CH4 + 2O2  CO2 + 2H2O
YouTube - Five Major Chemical Reactions
YouTube - The Five Major Class of Chemical Reactions
Ch.6 Review
1.
2.
3.
What is a substance that undergoes a
chemical change called?
What is the substance produced by a
chemical change called?
What happens to the atoms of a substance
that undergoes a chemical reaction?
4. What are two kinds of chemical
reaction having to do with energy?
5. Which gets warmer?
6. Which gets colder?
What type of reaction are each of the
following?
1. 2HCl + Cu CuCl2 + H2
2. 2C2H6 + 7O2  6H2O + 4CO2
3. CaCl2 +
K2SO4  CaSO4 + 2KCl
4. 2CO  2C + O2
Types of Reactions?
•
•
•
•
•
PbCl2 + AgNO3  Pb(NO3)2 + AgCl
NH3 + HCl  NH4Cl
AlCl3 + Na2SO4  Al2(SO4)3 + NaCl
Zn + S8  ZnS
Al2(SO4)3 + BaCl2  BaSO4 + AlCl3
Neutralization
Acid + Base  water + ionic compound
NaOH + H2SO4  H2O + NaCl
Chemical Reactions
The Chemical Equation
• Symbolic Representation
• Reactants
• Products
CHEMICAL EQUATIONS
When you have finished balancing an
equation, there are three criteria that
MUST be met in order for the equation to
be correctly balanced.
MASS BALANCE:
• you must have the same number of atoms of each
element on both sides of the arrow
CHARGE BALANCE:
• the total charge on the reactants must equal the total
charge on the products
LOWEST INTEGER COEFFICIENT:
• The coefficients in front of each chemical must be the
lowest possible integer ratio
Balancing Chemical Equations
HELPFUL HINTS:
Skeleton Equation – follow nomenclature rules
Elements
– Balance First element that appears in one compound on
both sides of equation
– Balance metals before nonmetals
Free Element – balance last
Coefficient Fractions – multiply to remove
Check elemental totals on each side of the equation
Balancing Chemical Equations
Balance the following equation:
•
C 2H 6 + O 2
-----> CO2 + H2O
• 1) The equation is not balanced
• 2) We have 2 carbons on the left so
we need two carbons on the right.
• 3) Put a two in front of the CO2
Balancing Chemical Equations
C2H6 + O2
-----> 2CO2 + H2O
Now we are down to hydrogen and oxygen.
I am going to look at hydrogen first. There are
six hydrogens on the left side of the arrow. I
need six on the right side of the arrow, but I only
have 2. So I must put a three in front of the
water to give me a total of six hydrogens.
Balancing Chemical Equations
C2H6 + O2
-----> 2CO2 + 3H2O
• Now the carbons and the hydrogens are
balanced.
• I see that I have seven oxygens on the right
(four from 2CO2 and 3 from 3H2O) but only two
on the left. Also, There is no way that I can get
to seven oxygens on the right taking pairs of
oxygens on the left.
Balancing Chemical Equations
• First, rewrite the equation and be sure that you
put in numbers wherever they belong, even the
number 1.
•
1C2H6 + O2
-----> 2CO2 + 3H2O
• The only coefficient which has not been
assigned is the one for oxygen. It would need to
be 7/2 to get the 7 oxygen on the reactant side.
• Multiply all coefficients by 2 to get rid of the
fraction.
• 2C2H6 + 7O2 -----> 4CO2 + 6H2O
Balancing Chemical Equations
• 2C2H6 + 7O2
-----> 4CO2 + 6H2O
• Now is the equation balanced? Check the mass
balance and the lowest integer coefficients.
Balancing Chemical Equations
• Another example:
• H2SO4 + Ba(OH)2 -----> BaSO4 + H2O
• It almost looks balanced, but the hydrogens
don't balance.
• First, check the S (it is the first element which
isn’t hydrogen or oxygen which I come to).
There is one on the left and one on the right.
Balancing Chemical Equations
• Now there is one Ba on the right. Ba comes in
only in Ba(OH)2 on the left side of the arrow, so I
will need one of them.
• 1H2SO4 + 1Ba(OH)2 -----> 1BaSO4 + H2O
Balancing Chemical Equations
• Now I am ready for Hydrogen. I have four
hydrogen on the left (2 from 1H2SO4 and 2 from
1Ba(OH)2). I must have four on the right side.
The only place where I find hydrogen is in water.
So, I will need a two in front of the water to give
me the four hydrogens.
• 1H2SO4 + 1Ba(OH)2 -----> 1BaSO4 + 2H2O
Balancing Chemical Equations
• Now everything is done except oxygen. If we
have done everything properly, oxygen will
balance. There are six oxygens on the left side
and six on the right side. So they do balance.
• So this equation is balanced.
• 1H2SO4 + 1Ba(OH)2 -----> 1BaSO4 + 2H2O
Identify Type of Reaction and Balance
These Reactions:
1. H2 + O2  H2O
2. 2CO  2C + O2
3. NH3 + HCl  NH4Cl
4. PbCl2 + AgNO3  Pb(NO3)2 + AgCl
5. C12H22O11 + O2  CO2 + H2O
Balance These Reactions:
1. Zn + S8  ZnS
2. AlCl3 + Na2SO4  Al2(SO4)3 + NaCl
3. Al2(SO4)3 + BaCl2  BaSO4 + AlCl3
Factors Affecting Reaction Rates
For any reaction to occur, the particles of
the reactants must collide with one another.
Therefore, whatever will help particles
collide with one another will speed up the
reaction rate.
How to Speed up Reactions (and get a date)
• Most reactions go faster at higher
temperatures.
• Greater surface area speeds up
reactions.
• Concentrated solutions react faster.
• Reactions are faster at higher pressure.
• Massive, bulky molecules react slower.
Factors Affecting Reaction Rates,
continued
• A catalyst is a substance that changes the rate of
a chemical reaction without being consumed or
changed significantly.
• Catalysts are not reactants or products,
because they are not used up in the reaction.
• Catalysts are often used in industry to make
reactions go faster.
• Catalysts that slow reactions are called
inhibitors.
Factors Affecting Reaction Rates,
continued
• Enzymes are proteins that serve as
biological catalysts.
• An enzyme is very specific, controlling one
reaction or set of similar reactions.
• Most enzymes are fragile, and stop
working above certain temperatures.
Factors that Affect a Reaction