Chemical Reactions
Chapter 7
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What type of change is
happening in the picture?
When charcoal burns, it
changes into other
substances while
producing heat and light.
Burning is a chemical
change
Describing Reactions
7.1
One way to describe a change of state is
to describe what is present before and
after the change.
 In a chemical reaction, the substances
that undergo change are called
reactants.
 The new substances formed as a result of
that change are called products.

◦ In the picture, the reactants are the carbon in
the charcoal, and the oxygen in the air.
◦ The product is CO2 gas.
Chemical Equations
Reactants  Products
 To describe burning of charcoal:
Carbon + Oxygen  Carbon Dioxide
C + O2  CO2

A chemical equation is a representation of
a chemical reaction in which the reactants
and products are expressed as formulas
Using Equations to Represent
Reactions
What happens to the products in a
chemical reaction?
 During a chemical reaction, the mass of
the products is always equal to the mass
of the reactants.
 This is called the Law of Conservation
of Mass, that mass is neither created nor
destroyed.
 This law was established by Antoine
Lavoisier

Conservation of Mass
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Equation reads “one
carbon atom reacts with
one molecule of oxygen
and forms one molecule
of carbon dioxide”.
If you have 6 C atoms,
they will react with 6 O2
molecules to form 6 CO2
molecules
The equation has the
same number of atoms
on each side of the
equation
Conservation of Mass

Some chemical reactions are powerful enough to
propel a space craft.
◦ Rocket fuels contain a compound called
hydrazine, N2H4
◦ When hydrazine burns in the presence of
oxygen, the reaction produces nitrogen, water
vapor, and heat.
◦ You can describe this reaction by writing a
chemical equation:
◦ N2H4 + O2  N2 + H2O
Balancing Equations
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N2H4 + O2  N2 + H2O
If we count the atoms on both sides, we will see
that the # of atoms on the left are not equal to
the # of atoms on the ride side of the equation.
This equation is NOT balanced
In order to show that mass is conserved during a
reaction, a chemical equation must be balanced.
You balance a chemical equation by changing the
coefficients (# in front of the formula)
Balancing Equations
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No coefficient- is an
understood “1”
First, count the number
of atoms of each
element on each side of
equation
Starting with metals,
change coefficients in
front of formulas until
balanced.
Try
◦ Na + H2O  NaOH + H2

HCl + CaCO3  CaCl2 + CO2 + H2O
Steps to Balancing Equations

Write a balanced equation for the reaction
between copper and oxygen to produce
copper (II) oxide, CuO.
◦ Steps:
 Write equation with reactants on the left and
products on the right
◦ Cu + O2  CuO
 Balance atoms
◦ 2Cu + O2  2CuO
MATH PRACTICE

Balance the following chemical equations:
◦ H2O2  H2O + O2
◦ Mg + HCl  H2 + MgCl2
Math Practice
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How many shoes do you have?
Shoes are counted in pairs.
How many eggs are in a dozen? Bottle
rockets in a gross?
Pair, dozen, gross are all UNITs we use to
count.
How do chemists count particles?
DEMO
Counting Chemicals
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Chemists need to be able to count atoms or
molecules.
These units are too small to be counted, so
chemists have another way to count them.
Chemists use a unit called the mole (No, not
that kind of mole!)
A mole is an amount that contains 6.02 x
1023 particles of that substance
◦ Aka “Avogadro’s #’
◦ Particles: atoms, molecules or ions
◦ Ex: 1 mole of Fe (Iron) contains 6.02 x 1023 atoms
of iron.
Counting Moles
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Does a dozen eggs weigh the same as a
dozen oranges?
Demo
◦ A mole of carbon has a different mass than a mole
of sulfur
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The mass of one mole of substance is called a
molar mass
The molar mass for chemicals is the same as
the atomic mass.
◦ Ex: Carbon’s mass is 12 amu or 12 grams per one
mole of carbon
12 grams C
or
1 mole C
1 mole C
Molar Mass
12 grams C

For a compound, add the atomic masses of
its components atoms
◦ EX: CO2
◦ 1 atom of Carbon x 12 g
◦ + 2 atoms of Oxygen x 16 g
44 grams of CO2 per one mole of CO2

“Molar mass”
is the same as
“formula
mass”
You can use this molar mass to convert moles
of a substance to mass and vice versa.
44.0 g CO2
1 mol CO2
or
1 mol CO2
44.0 g CO2
Mole Mass Conversions

Suppose you have 55 g of CO2. You can
use the g/mol conversion factor to
calculate how many moles:
◦ 55.0 g CO2
1 mol CO2
1.25 mol CO2
44g CO2
◦ You can also convert from moles back to grams
◦ 2 mol CO2
44g CO2
1 mol CO2
88.0 g CO2
Mole-Mass Conversions
Types of Reactions
7.2
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How do you classify matter?
◦ (solid, liquid, gas – remember?)

Chemical reactions are also classified into
different types:
◦
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Synthesis
Decomposition
Single-replacement
Double-replacement
Combustion
Classifying Reactions

Synthesis reactions are reactions in which
2 or more substances react to form a single
substance
◦ A + B  AB (like a MARRIAGE)
◦ Ex: 2 Na + Cl2  2 NaCl
◦ Video
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Decomposition reactions are reaction in
which one substance is broken down into two
or more simpler substances (opposite of
synthesis)
◦ AB A + B (like a DIVORCE)
◦ Ex: 2H2O  2 H2 + O2
◦ Video
Synthesis & Decomposition
Here is another example of a synthesis reaction
Another view of a decomposition reaction

Single Replacement is a reaction where 1
element takes the place of another element
in a compound
◦ A + BC  B + AC (like a LOVE TRIANGLE)
◦ Ex: Cu +2Ag(NO3)  2Ag + Cu(NO3)2
◦ Video
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Double Replacement two different
compounds exchange positive ions and form
2 new compounds
◦ AB + CD  AD + CB (like DO-Si-DO and change
partners)
◦ Ex: Pb(NO3) + 2KI  PbI2 + 2 KNO3
◦ Video
◦ With precipitation
Single and Double Replacment
Single Replacement Reactions
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Combustion reaction one in which a
substance reacts rapidly with oxygen,
producing heat and light
◦ Ex: CH4 + 2O2  CO2 + 2 H2O
◦ Always react with oxygen and usually produces CO2
, gas and water
 Products in combustion are ALWAYS carbon dioxide and
water. (although incomplete burning does cause some
by-products like carbon monoxide)
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Many reactions can be classified by more
than one type.
Combustion
Oxidation-reduction reaction: a reaction where
electrons are transferred from one reactant to
another, aka redox
 Ex: Calcium reacts with oxygen to produce calcium
oxide
Oxidation
2Ca +
O2  2CaOCan
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occur
without
◦ The product (CaO
is composed
of ions where the reactants
were neutral atoms) oxygen
◦ When calcium reacts with oxygen, each neutral atom loses
electrons to form Ca+2 ions
Ca  Ca2+ + 2e-
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When an element loses electrons during a
chemical reaction it is called oxidation
The calcium lost electrons so it was oxidized
Oxidation-Reduction (Redox)
Reduction is the process where an
element gains electrons
 Each neutral oxygen atom gains two
electrons becoming O2- ion
O + 2e-  O2 The oxygen gained electron and has been
reduced

OILoccur
RIG together
◦ Oxidation and reduction always
Oxidati
◦ If one element loses electrons
another element
on
HAS to gain them
Is
Loss
Reducti
on
Oxidation-Reduction (Redox)
Is
Gain
•
1.
2.
3.
4.
5.
State the type, classify and balance the
following reactions:
_Pb(NO3)2 + _HCl  _PbCl2 + _HNO3
_C2H6 + _O2  _CO2 + _HOH
_Ca + _HCl  _CaCl2 + _H2
_Hg + O2  HgO
_SO2 + _O2  _SO3
Do you see a
Redox reaction?
Mixed Practice
Energy Changes in
Reactions
7.3
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Where does the heat come from when you
light a propane grill?
◦ C3H8 + 5O2  3CO2 +4H2O + Heat
◦ This equation shows that the heat released in
the reaction came from the reactants.
◦ Chemical energy is the energy stored in the
chemical bonds of a substance
◦ energy changes in chemical reactions are
determined by changes that occur in the
chemical bonding
◦ Chemical reactions involve the breaking of
chemical bonds in the reactants and the
formation of chemical bonds in the products
Chemical Bonds and Energy

Breaking chemical bonds requires energy.
◦ Where could this energy come from when using
a propane grill?
 Grills have a lighter which produces a spark,
giving enough energy to break the bonds and
start the reaction

The formation of chemical bonds releases
energy. (resulting in heat and light that
you see)
Breaking & Forming Bonds

Physical changes can release or absorb
energy
◦ Exothermic: releases
◦ Endothermic: absorbing
◦ During a chemical reaction energy is either
released or absorbed
◦ A chemical reaction that releases energy to its
surroundings is called an exothermic
reaction
◦ A chemical reaction that absorbs energy from
its surroundings is an endothermic reaction
Endothermic and Exothermic
Reactions
•As you go from left to right in each graph, what happens
to the reactants?
•They react to form the products
•What point on each graph represents the highest
energy?
•The energy is highest at each curve’s peak
•What do the double-headed arrows represent?
•The difference in chemical energy between the reactants
and products
•Which type of reaction has products with a greater
amount of energy that the reactants?
• endothermic
Reaction rates
7.4
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A reaction rate is the rate at which reactants
change into products over time
◦ Rate just means a change over time, like distance
over time= speed

Reaction rates tell you how fast a reaction is
going
◦ How fast reactants are consumed, products are
formed or energy released/absorbed.
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Factors that affect reaction rates are:
◦
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Temperature
Surface Area
Concentration
Stirring
Catalysts
Reaction Rates
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How does temperature affect reaction rates?
◦ Increasing temp causes particles to move faster
and collide, # of collisions increases then rate
increases
◦ Decreasing the temperature will decrease the
reaction rate

Surface area is the amount of area exposed
◦ An increase in the surface area increases the
exposure of reactants to one another allowing more
collisions
 And therefore allowing an increase in the reaction rate
 (newspapers)
Temperature and Surface Area
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Stirring increases the reaction rate by
increasing the number of collisions between
the particles of the reactants.
◦ (washing machine vs. soaking)
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Concentration is the number of particles in a
given volume. (ex sugar in tea)
◦ The more particles of reactant, the higher the
reaction rate

Gas concentration changes with pressure
(less room)
◦ The greater the pressure of the gas, the greater it’s
concentration and the faster it’s reaction rate
Stirring and Concentration
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A catalyst is a substance
that affects the reaction
rate without being used
up in the reaction.
They can be used to
speed up or slow down
reactions
Graph shows how a
catalyst can lower the
amount of energy
needed to cause a
reaction
Catalysts