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Chemical Reactions
Part 1: Physical and Chemical Change
Types of Change
There are two ways that a substance can change:
Physical
Change
Changes that may alter the
appearance but do NOT
produce new substances
Chemical
Change
Changes that produce one
or more new substances
Types of Change
There are two ways that a substance can change:
Physical
Change
Bending, ripping, mixing
and changing
physical states
Chemical
Change
Burning, rusting, baking
Types of Change
Ripping
Burning
Physical Change
Chemical Change
No new substances:
it’s still paper!
New substances:
smoke and ash
Types of Change
Mixing the
dough
Rolling the
dough
Baking the
dough
Physical
Change
Physical
Change
Chemical
Change
4 Signs of a Chemical Change
A change in color
A change in energy
Formation of a gas
Formation of a solid
Chemical Reactions
A chemical change occurs when a substance
changes into one or more new substances.
This change is also called a
chemical reaction.
During a chemical reaction,
atoms rearrange
themselves to form new
substances.
Chemical Reactions
Part 2: Chemical Equations
Chemical Reactions
Reactions can be written as chemical equations.
A chemical equation uses chemical formulas
and symbols to describe a chemical reaction.
Let’s look at one
chemical reaction:
magnesium (Mg) burns
brightly in oxygen (O2)
Chemical Reactions
+
Magnesium
Oxygen
Magnesium Oxide
Reactants
Products
Substances that react at
the start of a reaction
Substances formed at
the end of a reaction
Chemical Equations
A chemical equation shows how the atoms
rearrange during a reaction in a simple way.
+
Magnesium
2 Mg
Oxygen
+
O2
Magnesium
Oxide
à
2 MgO
Chemical Equations
Reactants
2 Mg
+
produces
O2
à
2 MgO
Products
Chemical Equations
These cannot be changed!
chemical formulas
22 Mg
Mg
+
O2
à
MgO
2 MgO
Chemical Equations
These cannot be changed!
chemical formulas
22 Mg
Mg
+
O2
à
coefficients
These can be changed!
MgO
2 MgO
Law of Conservation of Mass
Chemical equations MUST be balanced.
This is because they must follow
the Law of Conservation of Mass.
“In a chemical
reaction, mass is
not created or
destroyed, it is
simply rearranged.”
Law of Conservation of Mass
The numbers of reactants on one side MUST equal
the number of products on the other side.
2 Mg
+
O2
à
2 MgO
2 Mg
2 O
2 MgO
4 atoms
4 atoms
How to Balance Equations
To balance equations, use the MeNOH method!
Metals – balance all metals first
Nonmetals – balance nonmetals (not O,H)
Oxygen – balance any oxygen atoms
Hydrogen – balance any hydrogen atoms
Balancing Equations
1.
SiCl4 à
Si
+
Cl2
1.
SiCl4 à
Si
+
2 Cl2
Balancing Equations
2.
Na +
O2 à
Na2O
2. 4 Na +
O2 à
2 Na2O
Balancing Equations
3.
H2 +
N2 à
NH3
3. 3 H2 +
N2 à
2 NH3
Balancing Equations
4.
Mg +
HCl à
H2 +
MgCl2
4.
Mg + 2 HCl à
H2 +
MgCl2
Balancing Equations
5.
H2O2 à
H 2O +
O2
5. 2 H2O2 à
2 H 2O +
O2
Chemical Reactions
Part 3: Types of Chemical Reactions
Types of Reactions
There are 5 major types of chemical reactions:
1. Synthesis
2. Decomposition
3. Combustion
4. Single Displacement
5. Double Displacement
1. Synthesis
A synthesis reaction is when 2+ substances
combine to form a single substance.
Example: synthesis of water:
+
Hydrogen
(H2)
Oxygen
(O2)
Water
(H2O)
Decomposition
A decomposition reaction is when 1 substance
breaks down into 2+ simpler substances.
Example: decomposition of water:
+
Water
(H2O)
Hydrogen
(H2)
Oxygen
(O2)
Combustion
A combustion reaction is when a
hydrocarbon (CXHX) reacts with oxygen to
produce CO2 and H2O and heat and light
energy.
Example: combustion of methane gas:
+
Methane
(CH4 )
+
Oxygen
(O2 )
Carbon Dioxide
(CO2)
Water
(H2O)
Single/Double Displacement
In displacement reactions, elements
replace other elements in a compound.
Example: displacement of Fe with CuSO4
+
Iron
(Fe )
+
Copper
Sulfate
(CuSO4 )
Iron
Sulfate
(FeSO4)
Copper
(Cu)
Summary
SYNTHESIS
A
DECOMPOSITION
A
COMBUSTION
H
H
C
+
B
A
B
H
H
A
+ O2
+
1 DISPLACEMENT
A
2 DISPLACEMENT
A D
B
+
B
C
C
+
B
B
CO2 + H2O
+
A C
A C
+
B
B
D
Summary
SYNTHESIS
DECOMPOSITION
COMBUSTION
Mg + O2  MgO
NaHCO3  Na2CO3 + H2O
CH4 + O2  CO2 + H2O
1 DISPLACEMENT
Cu + AgNO3  CuNO3 + Ag
2 DISPLACEMENT
KI + PbNO3  KNO3 + PbI
Classify These Reactions!
1. 2 HgO  2 Hg + O2
2. 4 Sb +
3 O2  Sb4O6
3. Fe2O3 + 3 Mg 
4. C3H8 + 5 O2 
2 Fe + 3 MgO
3 CO2 +
4 H2O
5. NaCl + CuSO4  NaSO4 + CuCl
Classify These Reactions!
1. 2 HgO DECOMPOSITION
2 Hg + O2
2. 4 Sb +
3 OSYNTHESIS
2  Sb4O6
3. Fe2O3 + 13 DISPLACEMENT
Mg  2 Fe + 3 MgO
4. C3H8 + 5 COMBUSTION
O2  3 CO2 +
4 H2O
2 DISPLACEMENT
5. NaCl + CuSO
4  NaSO4 + CuCl
Chemical Reactions
Part 4: Rates of Chemical Reactions
Introduction
Some reactions are quick, others are slow.
Rusting (slow!)
Combustion (fast!)
The area of chemistry that studies the
speed of reactions is called kinetics.
Rate of Reactions
The speed of a reaction is called its rate.
Specifically, the rate of a reaction
measures how fast the reactants are
changed to products in a certain time.
To understand reaction
rates, we must first learn
about Collision Theory…
Collision Theory
Collision Theory states that for substances
to react, they must physically collide.
Reaction
However, not ALL collisions cause a reaction!
1. Reactions Need Energy
Firstly, for two reactants to collide with
success, they must have enough energy.
No Reaction
To give the reactants more energy,
you must increase the temperature.
1. Reactions Need Energy
Firstly, for two reactants to collide with
success, they must have enough energy.
Reaction!
To give the reactants more energy,
you must increase the temperature.
2. Reactions Need Orientation
Secondly, for two reactants to collide with
success, they must be lined up properly.
No Reaction
If the reactants are orientated
correctly, the collision will be successful.
2. Reactions Need Orientation
Secondly, for two reactants to collide with
success, they must be lined up properly.
Reaction!
Note: Orientation is by random chance only!
It cannot truly be controlled.
Increasing the Rate of Reaction
The rate of reaction can be increased.
Collision Theory states that as the
number of collisions (per second)
increases, so will the rate of reaction.
The number of collisions
can be increased in 4 ways:
h the temperature
h the concentration
h the surface area
Use of a catalyst
Effect of Temperature
Increasing the temperature increases the rate.
As temperature increases, the reactants
gain more kinetic energy. This increases
the number of successful collisions.
Low Temperature
High Temperature
Effect of Concentration
Increasing the concentration increases the rate.
Increasing the number of molecules
of one or more reactants makes the
collisions more likely and more frequent.
Low Concentration
High Concentration
Effect of Surface Area
Increasing surface area increases the rate.
Breaking a reactant into smaller pieces
(or a powder) makes more of it exposed
to collide with another reactant.
Low Surface Area
High Surface Area
Catalysts
Using a catalyst increases the rate.
A catalyst is a substance that speeds up
the reaction without being consumed. It
helps make the reactants collide easier.
Catalysts work by
lowering the energy the
reactants need to react.
Reaction!
Any Questions?