Transcript Types of Chemical Reactions

Chemical Reactions Alter
Arrangements of Atoms
Section 7-1
Atoms interact in chemical
reactions.
 How
does a chemical reaction produce
a new substance?
 By
changing the way the atoms are
arranged. Bonds between atoms are
broken and new bonds form. This
happens when particles collide.
Physical Changes
 A physical
change is a change in the
state (phase) of a substance.
 What are the three states of matter?
 Solid,
Liquid, and Gas
What happens when a substance
undergoes a physical change?
 Its
physical form changes but the
substance itself remains unchanged.
What is electrolysis?

The process by which water breaks down into
hydrogen and oxygen molecules.
 How can you tell that splitting water to form
hydrogen and oxygen is a chemical change?
How does this change occur?

New substances form with new and different
properties. Bonds break and new bonds form.
Chemical Vs. Physical
 How
is a chemical change different from
a physical change?
 During
a chemical change the original
substances turn into different substances.
In a physical change, the original
substances still keep their original
properties.
Reactants and Products
– the substances present at
the beginning of a chemical reaction.
 Reactant
– the substances formed by the
chemical reaction.
 Product
Reactants and Products
 What
must happen for a reactant to be
changed into a product?
 The
bonds in the reactants must be broken
and new bonds must form in the products.
Evidence of a Chemical
Reaction
1.
2.
3.
4.
Color Change
Formation of a precipitate
Formation of a gas
Temperature change –gives off heat or
light
Evidence of a Chemical
Reaction

How can temperature change in a
chemical reaction be observed without
using a thermometer?

Seeing a flame or feeling warmth
(production of heat or light).
3 Types of Chemical
Reactions:
Reaction – a new compound
is formed by the combination of simpler
reactants
 Decomposition Reaction – a reactant
breaks down into simpler products
 Combustion Reaction – one reactant is
always oxygen, the other often contains
carbon or hydrogen.
 Synthesis
3 Types of Chemical
Reactions:
 Synthesis
Reaction means “making a
substance from simpler substances.”
 Decomposition Reaction means
“separation into parts.”
 Combustion Reaction is the process of
burning with oxygen.
3 Types of Chemical
Reactions:

1.
2.
How are synthesis reactions different
from decomposition reactions?
Synthesis reactions make more
complex products from simpler
reactants.
Decomposition reactions break down
complex reactants into simpler
products.
Synthesis Reaction
Two or more simple substances
combine to form a new, more
complex substance.
A + B  C
2 reactants
1 product
Synthesis Reaction Example
2Na +Cl2  2NaCl
Decomposition Reaction
A complex substance breaks
down into two or more simpler
substances.
C  A + B
1 reactant
2 products
Decomposition Reaction
Example
C12H22O11  11 H2O +12C
Sugar heat Water
Carbon
H2CO3  H2O + CO2
Carbonic Acid
The rates of chemical
reactions can vary.
 The
three physical factors that affect the
rate of a chemical reaction are
concentration, surface area, and
temperature.
 The chemical factor that affects reaction
rate is called a catalyst
Concentration
Concentration measures the
number of particles present in a
certain volume.
Surface Area
 Why
does a reaction proceed faster
when the reactants have a greater
surface area?
 More
particles are available to collide and
react when the surface area is greater.
Temperature
 The
rate of a reaction can be increased
by making the particles move faster.
 Why are collisions between particles
important in chemical reactions?
 Particles
of reactants must collide to react.
Catalysts
– a substance that increases
the rate of a chemical reaction but is not
itself consumed by the reaction
 Characteristics – remains unchanged
 Example – Enzymes
 Non-Example – something that slows
the reaction or is changed/consumed by
the reaction.
 Definition
Review
 How
might the chewing of food be
related to the rate of a chemical
reaction, digestion, that occurs in your
body? Explain your answer.
Review
 How
might the chewing of food be
related to the rate of a chemical
reaction, digestion, that occurs in your
body? Explain your answer.
 Chewing
increases the surface area of
food which speeds up chemical reactions
involved in digestion.
Ch. 7.2 Notes
The mass of reactants and
products are equal.
The mass of reactants and
products are equal.

Careful observations
led to the discovery
of the law of
conservation of
mass.
Describe the Law of
Conservation of Mass
 Lavoisier
found that atoms can never be
created or destroyed in a chemical
reaction.
 Most reactions that seem to gain or lose
mass actually involve reactions with
gases.
 The mass of the reactants is always
equal to the mass of the products.
Why is mass conserved in a
chemical reaction?
Atoms are not created or
destroyed in a chemical reaction.
200g of reactant A and 100g
of reactant b are combined to
form a new substanceProduct C. What is the mass
of product C?
350 g
Why did he use sealed
containers in his experiments?
 By
carefully observing and measuring
all of the reactants and products in
chemical reactions, he found that the
total masses were always equal
 He could measure the total mass of the
air and the mercury instead of just the
mass of the mercury.
Chemical reactions can be
described by ____________
__________________.
How does a chemical
equation show the
conservation of mass?
 By
showing on each side of the
equation the same number of atoms of
each element involved in the reaction.
In order to write a chemical
equation, what information do
you need to know?
 The
reactants and products in the
reaction
 The atomic symbols and chemical
formulas of the reactants and products
in the reaction
 The direction of the reaction
Write the chemical equation
represented by the following
sentence. Carbon (C) reacts
with oxygen (O2) to yield
carbon dioxide (CO2). Label
the reactants and products.
C + O2  CO2
Reactants
Product
Chemical equations must be
balanced.
Is the equation below
balanced? Why or Why not?
H 2 + O 2  H 2O
No, the equation has only one
oxygen atom on the right side and
two oxygen atoms on the left side.
4 steps to balancing
equations:
1.
2.
3.
4.
Write chem. Equation with correct symbols
& formulas
Write the number of atoms of each element
on each side of the equation. (Write the #
very small under the symbol)
Balance atoms by using coefficients(large #
in front of symbol or formula, then change #
under symbol)
Check by counting the numbers of atoms of
each element.
4 steps to balancing
equations:
Cont.
*DO NOT CHANGE SUBSCRIPTS!!!!*
*Oxidation numbers in a compound must add
up to equal zero.
*When adding a subscript to a polyatomic ion,
you must put the polyatomic ion in
parenthesis first.
*The total number of atoms on both sides of the
equation must be the same [(Law of
conservation of mass: mass remains
constant in a chemical reaction) (mass is
neither created nor destroyed; it only
changes form.]
4.
Using Coefficients to Balance
Equations
 Why
can you change coefficients to
balance an equation but not subscripts?
 Subscripts
are part of a chemical formula,
while coefficients just show the number of
molecules involved.
Ch. 7.3 Notes
Chemical reactions involve energy
changes.
I. Chemical reactions release
or absorb energy.
Bond energy
The amount of energy in a
chemical bond between atoms
For each reaction, write
absorbed or released.
a. Breaking
b. Forming
bonds: energy is bonds; energy is
absorbed
released
c. Endothermic:
energy is
absorbed
d. Exothermic:
energy is
released
Exothermic Reactions
 Definition:
a chemical reaction that
releases energy
 Characteristics: often causes an
increase in temperature; energy is
released as heat
 Example: rocket blasting off
 Nonexample: photosynthesis
Endothermic Reaction
 Definition:
a reaction in which energy is
absorbed
 Characteristics: energy must be added
to the reaction, often causes a drop in
temperature
 Example: photosynthesis
 Nonexample: rocket blasting off
II. Exothermic reactions
release energy.
Describe exothermic reactions
 Exothermic
reactions release energy
 Bond energies of the reactants are less
than the bond energies of the products
 Some reactions produce a lot of heat
 All common combustion reactions are
exothermic
 Exothermic chemical reactions also
occur in living things.
The picture below shows a
common chemical reaction.

Is the reaction
exothermic or
endothermic? How
do you know?
The picture below shows a
common chemical reaction.

Exothermic; It is
releasing energy
(heat)
Describe the bond energies in
an exothermic reaction.
 The
bond energies of the reactants are
less than the bond energies of the
products.
III. Endothermic reactions
absorb energy
Describe Endothermic
reactions
 Endothermic
reactions absorb energy.
 The energy difference is usually
absorbed as heat;
 Photosynthesis is probably the most
important series of endothermic
reactions
IV. Exothermic and
endothermic reactions work
together to supply energy.
How does the combination of
endothermic and exothermic
reactions help us use energy
from the sun?

Energy from the Sun
is stored by
endothermic
reactions
(photosynthesis)
and released by
exothermic
reactions
(combustion)
Complete 7.3 Review
Questions
Pg. 219: questions 1-6
Answers to review questions:
1.
2.
Exothermic reactions; products have
higher bond energy than reactants and
energy is released; endothermic
reactions: reactants have higher bond
energy than products and energy is
absorbed.
Exothermic; energy is released as
heat and light
Answers to review questions:
3.
4.
Endothermic: energy from light is
absorbed in order for the reaction to
occur.
They are approximately opposite
reactions in terms of energy as well as
reactants and products
Answers to review questions:
5.
6.
Fossil fuels contain the carbon stored in
plants by photosynthesis hundreds of
millions of years ago. So, energy in fossil
fuels started as energy from the Sun.
Bond energy is greater in the reactants;
when the reaction is reversed, the bond
energy is greater in the products and
energy is released.