Chapter 23
Chemical Reactions
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All chemical reactions
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A chemical reaction is a change in which one or
more substances are converted into new substances.
Reactants - the substances you start with and are
located on the left side of the reaction arrow
Products- the substances you end up with and are
located on the right side of the reaction arrow
What occurs when a chemical reaction takes
place?
Heat is produced/removed
 Light is produced
 Bubbles are produced
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Father of Modern Chemistry
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The French chemist Antoine Lavoisier established
that the total mass of the products always equals
the total mass of the reactants.
This is known as The Law of Conservation of Mass
Lavoisier is known today as the father of modern
chemistry for his more accurate explanation of the
conservation of mass and for describing a common
type of chemical reaction called combustion.
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In a chemical reaction
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A chemical equation is a
way to describe a chemical
reaction using chemical
formulas and other symbols.
What are coefficients?
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A coefficient is a number used to balance
equations in order to maintain the Law of
Conservation of Mass.
It is placed IN FRONT of a compound and
is distributed to all atoms within the
compound.
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What is a balanced equation?
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The formulas in a chemical equation must
accurately represent the compounds that
react.
Fixing this equation requires a process
called balancing.
The balancing process involves changing
coefficients in a reaction to achieve a
balanced chemical equation, which has the
same number of atoms of each element on
both sides of the equation.
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Balancing Equations
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http://www.bozemanscience.com/begi
nners-guide-balancing-equations
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Steps for Balancing
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Step 1 Write a chemical equation for
the reaction using formulas and
symbols.
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Step 2 Count the atoms in reactants and
products. If polyatomic ions are on both
sides of the arrow…leave it together!
Mg
O2  Mg
O
1
2
1
1
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Steps for Balancing
Step 3 Choose coefficients that balance
the equation.
 Remember, never change subscripts of
a correct formula to balance an
equation. Coefficients go IN FRONT
of compounds and get distributed to
the atoms in the compound.
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Steps for Balancing
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Step 4 Recheck the numbers of each
atom on each side of the equation and
adjust coefficients again if necessary.
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Again…..Never!!
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Change a subscript to balance an
equation.
– If you change the formula you are
describing a different reaction.
– H2O is a different compound than
H2O2
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Never put a coefficient in the middle of a
formula
– 2 NaCl is okay, Na2Cl is not.
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Example
2 H 2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
This is the answer
Not this
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Examples
CH4 + O2  CO2 + H2O
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Examples
AgNO3 + Cu  Cu(NO3)2 + Ag
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Techniques
If an atom appears more than once on a
side, balance it last.
 If you fix everything except one element,
and it is even on one side and odd on the
other, double the first number, then
move on from there.
 C4H10 + O2  CO2 + H2O
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Types of Chemical Reactions
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https://www.youtube.com/watch?v=gbiRwAVTV8
Types of Reactions
There are too many reactions to
remember
 Fall into categories.
 We will learn 5 types.
 Will be able to predict the products.
 For some we will be able to predict
whether they will happen at all.
 Must recognize them by the reactants
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#1 Combustion Reaction
A compound composed of only C H and
maybe O is reacted with oxygen
If the combustion is complete, the
products will be CO2 and H2O.
If the combustion is incomplete, the
products will be CO and H2O.
or just C and H2O.
O2 will always be the second reactant
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#1 Combustion Reaction
CH4 + O2  ______ + ______
 Write and Balance
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#2 Synthesis Reactions
Combine - put together
 2 elements, or compounds combine to
make 1 compound.
 Ca +O2 CaO
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SO3 + H2O  H2SO4
We can predict the products if they are
two elements.
 Mg + N2 
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Write and balance
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Al + O2 
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Remember that the first step is to write
the formula
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Then balance
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#3 Decomposition Reactions
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decompose = fall apart
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one reactant falls apart into two or
more elements or compounds.
electricity
NaCl    Na + Cl2
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 CaCO3  CaO + CO2
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#3 Decomposition Reactions
Can predict the products if it is a
binary compound
 Made up of only two elements
 Falls apart into its elements
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electricity
H2O   
#3 Decomposition Reactions
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HgO  
#4 Single Replacement
One element replaces another
 Reactants must be an element and a
compound.
 Products will be a different element
and a different compound.
 Na + KCl  K + NaCl
 F2 + LiCl  LiF + Cl2
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#4 Single Replacement
Metals replace metals (and hydrogen)
 Al + CuSO4 
 Zn + H2SO4 
 Think of water as HOH
 Metals replace one of the H, combine
with hydroxide.
 Na + HOH 
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#4 Single Replacement
We can tell whether a reaction
will happen
 Some are more active than other
 More active replaces less active
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The activity series chart lets us
Know whether or not a reaction will
occur
#4 Single Replacement
There is a list on page 732
 Higher on the list replaces
lower.
 If the element by itself is higher,
it happens,
 if element by itself is lower, it
doesn’t
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#4 Single Replacement
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Note the *
H can be replaced in acids
by everything higher
 Only the first 4 (Li - Na)
react with water.
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#4 Single Replacement
Al + HCl 
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#4 Single Replacement
Pb + KCl 
#4 Single Replacement
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What does it mean that Ag
is on the bottom of the list?
#4 Single Replacement
Nonmetals can replace other nonmetals
 Limited to F2 , Cl2 , Br2 , I2
 The order of activity is that on the
periodic table.
 Higher replaces lower.
 F2 + HCl 
 Br2 + KCl 
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#5 Double Replacement
Two things replace each other.
 Reactants must be two ionic compounds
or acids.
 Usually in aqueous solution
 NaOH + FeCl3 
 The positive ions change place.
 NaOH + FeCl3 Fe3+OH- + Na+Cl NaOH + FeCl3 Fe(OH)3 + NaCl
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#5 Double Replacement
Will only happen if one of the
products
– doesn’t dissolve in water and forms
a solid
– or is a gas that bubbles out.
– or is a covalent compound usually
water.
 Polyatomic ions don’t change from
side to side
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Complete and balance
assume all of the reactions take place.
 CaCl2 + NaOH 
 CuCl2 + K2S 
 KOH + Fe(NO3)3 
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How to recognize which type
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Rates of Reaction
A dynamic explosion is an example of
a rapid chemical reaction.
 Most chemical reactions
proceed more slowly,
but
all chemical
reactions
release or
absorb energy.
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Rates of Reaction
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As previously learned, atoms and molecules
must bump into each other before a product
can be formed.
They also need a minimum amount of
energy to kick start the reaction.
This minimum amount of energy needed to
start a reaction is called activation energy.
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Rates of Reaction
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http://ed.ted.com/lessons/activationenergy-kickstarting-chemicalreactions-vance-kite
Rates of Reaction
When most chemical reactions take
place, some chemical bonds in the
reactants are broken, which requires
energy.
 In order for products to be produced,
new bonds must form. Bond formation
releases energy.
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Rates of Reaction
Chemical reactions that release energy
are called exergonic (ek sur GAH nihk)
reactions.
 In these reactions less energy is
required to break the original bonds
than is released when new bonds
form.
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As a result, some form of energy, such as
light or heat is given off by the reaction.
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Rates of Reaction
When the energy given off in a
reaction is primarily in the form of
heat, the reaction is called an
exothermic reaction.
 The burning of wood and
the
explosion of dynamite
are
exothermic reactions.
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Rates of Reaction
Sometimes a chemical reaction
requires more energy to break bonds
than is released when new ones are
formed.
 These reactions are called endergonic
reactions.
 The energy absorbed can be in the
form of light, heat or electricity.
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Rates of Reaction
When the energy needed is in the form
of heat, the reaction is called an
endothermic reaction.
 Some reactions are so endothermic
that they can cause water to freeze.
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Rates of Reaction
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The rate of reaction is the speed at which
reactants are consumed and products are
produced in a given reaction.
There are 6 things that affect rates of
reaction: Temperature, Concentration,
Surface Area, Agitation, Pressure and
Catalysts/Inhibitors.
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Temperature
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With increasing temperature, atoms
and molecules move faster and kinetic
energy increases.
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Concentration
The closer atoms and molecules are to
each other, the greater the chance of
collision.
 Increasing the amount (concentration)
of a substance increases the chance
of atoms/molecules bumping into each
other increases the reaction rate.
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Surface Area
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When atoms/molecules bump into each other, only
their surfaces collide.
When a substance is divided into smaller
piecesincreases the surface area  more
collisions can occur increases the rate of reaction
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Agitation
Agitation is a fancy word for stirring
 Agitation is a physical process that
allows reactants to mix increasing
collisionsincreasing rate of reaction
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Pressure
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Increasing the pressure of gases, molecules have
less room to move aroundincreases the
concentration increases the amount of
collisions increases the rate of reactions
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Catalysts and Inhibitors
Some reactions proceed too slowly to
be useful.
 To speed them up, a catalyst can be
added.
 A catalyst is a substance that speeds
up a chemical reaction without being
permanently changed itself.
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Catalysts and Inhibitors
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At times, it is worthwhile to prevent certain
reactions from occurring.
Substances called inhibitors are used to
slow down a chemical reaction.
One thing to remember when thinking
about catalysts and inhibitors is that they do
not change the amount of product produced.
They only change the rate of production.
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