Chemical Reactions
Chapter 16
Chemical Changes in
Matter
• Chemical Reaction: when one or
more substances are changed to
new substances.
– Reactants: the substances that are
about to react
– Products: what the new substances
produced are called
• Reactants…..reaction….products
• Mg + O → MgO
Chemical Changes in
Matter
• Examples of Chemical Reactions:
– Photosynthesis: one of the most important
chemical changes on earth
– CO2 + H2O + sunlight →C6H12O6 +O2
– Carbon dioxide + water and sunlight produces
glucose sugar and oxygen
– Respiration: takes place in cells that burn sugar to
make energy
– C6H12O6 +O2+enzymes → CO2 + H2O
– Glucose sugar + oxygen and enzymes produces
carbon dioxide and water
Chemical Changes in
Matter
• Law of Conservation of Mass: in a
chemical reaction, matter is not created
or destroyed, but is conserved
• This law means that the starting mass
of the reactants equals the final mass
of the products
• Atoms are rearranged but never lost or
destroyed
Equation Analogy
Imagine that you work at a skateboard shop and you are in
charge of assembling the skateboards. Every skateboard requires
one deck (the board), two trucks (the mounted axles), and four
wheels.
1. Your boss asks you to make five skateboards. How
many trucks do you need?
2. The following diagram shows the “recipe” for one
skateboard. What do you notice about the relative
amounts of each part on either side of the arrow?
Chemical Changes in
Matter
• Fireworks and the law of
conservation of mass
• Powder + Fire → Light + heat
• Nothing is lost, all the atoms are still
present, the chemical change that
has taken place has changed them
back to elements, and new
compounds
Chemical Changes in
Matter
• Chemical Equation: an expression
that describes a chemical reaction
using chemical formulas and other
symbols
• Coefficients: numbers that represent
the number of units of each
substance or compound taking part
in a reaction
• 6CO2 + 6H2O + sunlight →C6H12O6
+6O2
Chemical Changes in
Matter
• The coefficients are a big part of the
conservation of mass. They show
how the reaction is balanced on both
sides. This means that the same
number of each atom are on each
side of the equation. Thus, matter is
neither created or destroyed
• This is called a balanced chemical
equation
Chemical Reaction up in
the air..
• The Ozone layer: a gas called
Ozone, O3, found in the Earth’s
upper atmosphere, helps keep some
harmful ultraviolet radiation from
reaching the Earth’s surface.
• Ozone Isn’t helpful, near the surface
of the earth, it is a pollutant found in
automobile exhaust and from
making steel.
Chemical Reaction up in
the air..
• So What’s the problem?
• The level of ozone is decreasing,
particularly the poles. Less ozone in
the stratosphere, or upper
atmosphere means more UV
radiation can reach the Earth.
• Page 447, figure 16-5
• So why did we put a hole in the sky?
Chemical Reaction up in
the air..
• Well…….most scientists believe that
a group of compounds called
Chlorofluorocarbons (CFC’s)
migrate through the Earth’s
atmosphere and decompose. This
decomposition releases chlorine
atoms that destroy the ozone
Chemical Reaction up in
the air..
• CFC’s? Are used in air-conditioning
systems and in making some types
of polymer foams
• Other sources of chlorine released
into the air are produced by naturally
by plants, ocean like, fungi, forest
fires, and volcanoes.
Types of Chemical
Reactions
• Synthesis Reactions: two or more
substances combine to from another
substance.
• Formula: A + B → AB
• Examples:
– The reaction between hydrogen and
oxygen fuels the main stage of the space
shuttle
– 6CO2 + 6H2O + sunlight →C6H12O6 +6O2
– Na + Cl → NaCl
The Synthesis of Carbon Dioxide
The Synthesis of Carbon Dioxide
The Synthesis of Carbon Dioxide
Types of Chemical
Reactions
• Decomposition Reactions: the
opposite of synthesis reactions.
One substance breaks down, or
decomposes into two or more
simpler substances
• Formula: AB → A + B
• Most decomposition reaction require
the use of heat, light or electricity
• Example:
– 2KClO3 → 2KCl + 3O2
Types of Chemical
Reactions
• Displacement Reactions: occur
when one element replaces another
in a compound
• Formula: A + BC → AC + B
• A replaces B
• In a double replacement reaction, an
insoluble compound is formed, this
is called a Precipitate.
Energy and Chemical
Reactions
• In all chemical reactions, energy is either
release or absorbed. This energy can take
many forms:
–
–
–
–
Light
Heat
Sound
Electricity
• When most chemical reactions take place,
bonds in the reactants must be broken. To
break chemical bonds, energy must be
provided.
Energy and Chemical
Reactions
• In order for products to be produced,
NEW bonds must be formed. Bond
formation releases energy
• Exothermic Reactions: Some form of
energy is given off by the reaction.
The reaction product will often feel
hot, this is because less energy is
required to break original bonds than
is released when new bonds from.
Energy and Chemical
Reactions
• Example of Exothermic reactions:
– The burning of wood
– Dynamite exploding
– Rusting of Metal
• Catalyst: a substance that speed up a
chemical reaction without itself being
permanently changed.
• Proteins know as Enzymes act as
catalysts in living organisms.
Energy and Chemical
Reactions
• Without enzymes, it would take you
body 100 years to digest a single
meal!
• Enzymes allow reactions to occur at
faster rates and lower temperatures
than would other wise be possible.
Energy and Chemical
Reactions
• Inhibitors: used to prevent certain
reactions from occuring.
• Inhibitors are combined with one of
the reactants to stop is from
undergoing the original reaction
• Food preservatives BHT and BHA are
inhibitors. They prevent the spoilage
of foods.
• Catalytic Converters: page 457
Energy and Chemical
Reactions
• Endothermic Reactions: energy must be
provided for the reaction to take place
• With some endothermic reactions, so
much heat is absorbed that their
containers feel cold to the touch.
• More energy is required to break bonds
than to form new ones in the chemical
reaction
• Example: Iron ore, page 458
Exothermic and Endothermic Processes
Energy is either absorbed or released during chemical and
physical changes. During an endothermic change, energy is
absorbed from the surroundings. During an exothermic
change, energy is released to the surroundings. Two different
physical changes are described below. Read each description
and decide if the change is exothermic or endothermic.
1. A plastic bottle of water is placed in a freezer. After
several hours, the water has frozen solid.
2. A chef places a stick of solid butter in a saucepan,
and heats the saucepan over low heat on a stove.
After several minutes, the stick of butter has melted
1. A plastic bottle of water is placed in a freezer. After
several hours, the water has frozen solid.
Exothermic. The liquid water releases energy as
it changes into ice.
2. A chef places a stick of solid butter in a saucepan,
and heats the saucepan over low heat on a stove. After
several minutes, the stick of butter has melted.
Endothermic. The solid butter absorbs energy
as it changes into liquid.
Comparing and Contrasting
a.
b.
releases energy to the surroundings
absorbs energy from surroundings