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Chapter 21: Chemical Reactions
Page 632
Section 1: Chemical Change/Conservation of mass
Section 2: Chemical Equations
Section 3: Classifying Chemical Reactions
Section 4: Reactions and Energy
Physical or Chemical Change?
• Matter can undergo two kinds of changes—
physical and chemical.
• Physical changes in a substance affects only
physical properties, such as its size and
shape, or whether it is a solid, liquid or gas.
• Chemical changes produce new substances
that have properties different from those of
the original substances.
Physical or Chemical Change?
• A process that produces chemical change is a
chemical reaction.
Evidence of Chemical
Reaction
(p462)
Some indicators of chemical changes include:
• color changes,
• energy (such as heat or light)
production or loss,
• production of bubbles (which
indicates the production of a gas),
• precipitation of a new solid when
two solutions are mixed.
Chemical Changes
Burning of gasoline, wood, or
charcoal are examples of chemical
changes.
A chemical change is referred to as
a chemical reaction.
The burning of charcoal (carbon) in oxygen is a
chemical reaction and can be expressed as the
chemical equation
carbon + oxygen  carbon dioxide
reactants
product
Chemical Equations
• To describe a chemical reaction, you must
know which substances react and which
substances are formed in the reaction.
• The substances that react are called reactants
(ree AK tunts).
• Reactants are the substances that exist before
the reaction begins.
• The substances that form as a result of the
reaction are called the products.
Using Words
• One way you can
describe a chemical
reaction is with an
equation that uses
words to name the
reactants and
products.
• Reactants are on the left side of an arrow, separated from
each other by plus signs and products are on the right side
of an arrow, separated from each other by plus signs..
Chemical Changes
chemical equations may be
expressed in words, but are most
commonly expressed using
chemical formulas.
carbon + oxygen  carbon dioxide
C +
O2 
CO2
reactants

product
Symbols used in equations (P635)
Symbol
Meaning

Produces or forms
+
Plus
(s)
Solid
(l)
Liquid
(g)
Gas
(aq)
Aqueous, substance dissolved
in water
Heat

∆
The reactants are heated

Light

The reactants are exposed to
light
Elec

The reactants are exposed to
electric current
Conservation of Mass (P632)
• According to the law of conservation of
mass, the mass of the products must be the
same as the mass of the reactants in that
chemical reaction.
• This principle was first stated by the French
chemist Antoine Lavoisier (1743-1794).
• He showed that chemical reactions are much
like mathematical equations.
• In math equations, the right and left sides of
the questions are numerically equal.
Conservation of Mass
• Chemical
equations are
similar to math
equations, the
mass must be
equal on the two
sides.
Example (P633)
• 10 grams of solid Mercury (II) Oxide when
heated produced 9.3 grams of liquid
mercury and 0.7 grams of oxygen gas.
• HgO  Hg + O2
• 10.0 gm  9.3 gm + 0.7 gm
• Mass Balanced Chemical Equation
Law of Conservation of Mass
Page 463
• When a chemical reaction takes place, the total
mass of reactants equals the total mass of
products.
• If 18g of hydrogen react completely with 633 g of
chlorine, how many grams of HCl are formed?
The equation for the reaction is
•
H2 + Cl2  2 HCl
18g + 633g  ______ g
Assignment (p463:1,2 and 637:6)
• In the following reaction, 24g of CH4 (methane) react
completely with 96g of O2 (oxygen gas) to form 66g of CO2.
How many grams of H2O are formed?
CH4 + 2O2  CO2 + 2 H2O
24g + 96 g  66g + ?
• In the following reaction, 54 g of Al react completely with
409.2 g of ZnCl2 to form 196.2 g of Zn metal. How many
grams of AlCl3 are formed?
2 Al + 3 ZnCl2  3 Zn + 2 AlCl3
54g + 409.2g  196.2g + ?
• In making soap, 890 g of fat react completely with 120 g of
sodium hydroxide to produce 92 g of glycerin and soap. How
much soap is produced?
fat + sodium hydroxide  glycerin + soap
Conservation of Matter
• Not only must the mass be equal on both sides but the
type and number of atoms of each type must be equal.
Balancing Chemical Equations
• When you write the chemical equation for
a reaction, you must observe the law of
conservation of mass.
• When you count the number of carbon,
hydrogen, oxygen, and sodium atoms on
each side of the arrow in the equation, you
find equal numbers of each kind of atom.
Balancing Chemical Equations
• This means the equation is balanced and the
law of conservation of mass is observed.
Balancing Equations
•HgO  Hg + O2
Balancing Chemical Equations
• Not all chemical equations are balanced
so easily. Trial and error
• The following unbalanced equation shows
what happens when silver tarnishes.
Count the Atoms
• When balancing chemical equations,
numbers are placed before the formulas
as you did for Ag.
• These are called coefficients. However,
never change the subscripts written to the
right of the atoms in a formula.
• Changing these subscripts change the
identity of the compound.
How to Balance an Equation (p639)
• Magnesium burns with such a brilliant white light that it is
often used in emergency flares. Burning magnesium leaves
a white powder called magnesium oxide. To write a
balanced equation for this follow these 4 steps.
• STEP 1 Write formula and symbols to describe the
reaction. Remember that oxygen is one of the diatomic
gases.
BrINClHOF
• Solid Magnesium metal + oxygen  Magnesium oxide
•
Mg(s) + O2(g)  MgO(s)
• STEP 2 Count the atoms in reactants and products.
• Mg
• O
Balancing Continued
• Step 3 Choose coefficients that balance
the equation.
• NEVER CHANGE SUBSCRIPTS
• ____ Mg(s) + ____ O2(g)  ____ MgO
• STEP 4 Recheck the number of each type
of atom on each side of the equation.
Practice (page 640)
• When lithium metal is treated with water, hydrogen
gas and lithium hydroxide are produced.
• STEP 1 Write the formulas and symbols
• STEP 2 Count the atoms in reactants and products
• STEP 3 Choose coefficients that balance the
equation. Begin with atoms that occur only once,
then polyatomic ions, then O and H
• STEP 4 Recheck
You Practice
• Iron metal reacts with Chlorine gas to form
solid Iron(III)Chloride.
• Write the formula for each reactant and
product.
• Iron Metal
• Chlorine gas
• Iron(III)Chloride
• Write the equation
• Balance the equation
Chapter 21 Assignments
•
•
•
•
•
•
•
•
•
•
Practice Problems (test extra credit)
Conservation of Matter Problems
Writing equations worksheets
Note taking worksheet
Chapter Review page 656: 11-19, 29, 30
Thursday 4/28 Types of reactions
Friday 4/29 Energy and Chemical Reactions
Monday 5/2 Chapter Review
Tuesday 5/3 Lab – John Adams Hall
Wednesday 5/4 Test
Chapter 21 Section 3
Classifying Chemical
Reactions
Page 641
Section 3: Classifying Chemical Reactions
(4 or 5 classes)
• Synthesis (composition)
– Combustion
• Decomposition
– Electrolysis
• Single Displacement
• Double Displacement
• You can’t learn these by going over them
in class once! You must study them!
Synthesis Reactions
• In a synthesis reaction two or more substances
combine to form a new compound.
• Synthesis reactions are also known as
composition reactions.
• Synthesis reactions may be represented by the
general equation
Substance-1 + substance-2  new substance
A
+
B
 AB
Synthesis Reactions
• Metals react with oxygen to
form metal oxides.
2 Mg(s) + O2(g) 2 MgO(s)
04m15vd1
Combustion Reactions
Page 641 (subclass)
•In a combustion
reaction a substance
combines with
oxygen, releasing a
large amount of
energy in the forms
of heat and light.
carbon + oxygen  carbon dioxide + heat
C(s) + O2(g)  CO2(g) + heat
Combustion (cont)
• The burning of hydrocarbons such as
methane, propane, and gasoline are
examples of combustion.
• Combustion of a hydrocarbon always
produces carbon dioxide and water.
• Propane burns to form carbon dioxide and
water.
•
C3H8 +
O2  CO2 + H2O
Hydrocarbons
• Methane
CH4
• Ethane
C 2H 4
• Propane
C 3H 8
• Butane
C4H10
Decomposition
Reactions
• In a decomposition reaction a single
compound reacts to form two or more simpler
substances.
• It is the opposite of a synthesis reaction.
• Usually requires energy (heat, light, electricity)
• Decomposition reactions may be represented
by the general equation
Substance-1  substance-2 + substance-3
AB

A
+ B
Decomposition
Reactions
• There are many types of decomposition
reactions.
• The decomposition of a substance by an
electric current is called electrolysis.
•
elec
2 H2O(l)  2 H2(g) + O2(g)
Decomposition
Reactions
• Metal carbonates decompose with heat to
yield a metal oxide and carbon dioxide gas.
heat
CaCO3(s)  CaO(s) + CO2(g)
Single-Displacement Reactions
• Displacement reactions are also called
replacement reactions.
• In a single-displacement reaction one element
replaces a similar element in a compound.
• Single-replacements reactions may be represented
by the general equations
metal + compoundnew compound + different metal
or
Y + XB  YB + X
Single-Displacement Reactions
• Hydrogen in water may be displaced by a metal
to produce hydrogen and a basic solution.
2Na(s) + 2 H2O(aq)  2 NaOH(aq) + H2(g)
02m17vd1
What Replaces Activity Series (P643)
• An element will replace any element below it on
the activity series.
•
•
•
•
•
•
•
•
•
•
•
•
•
Lithium
Potassium
Calcium
Sodium
Aluminum
Zinc
Iron
Tin
Lead
Hydrogen
Copper
Silver
Gold
Li
K
Ca
Na
Al
Zn
Fe
Sn
Pb
H
Cu
Ag
Au
Double-Displacement
Reactions
• Displacement reactions are also called
replacement reactions.
• In a double-displacement reaction the ions of two
compounds exchange places in an aqueous
solution to form two new compounds.
• Double-replacements reactions may be
represented by the general equation
AB(aq) + CD(aq)  AD(s)+ CB(aq)
Double-Displacement Reactions
The “driving force” is the formation of an insoluble
compound — a precipitate.
Lead nitrate + potassium iodide potassium nitrate + lead iodide
Pb(NO3)2(aq) + 2 K I(aq) 2 KNO3(aq) +PbI2(s)
Double Displacement
• Ba(NO3)2(aq) + K2 SO4(aq) BaSO4(s) +2 KNO3(aq)
• This type reaction takes place only when one of
the products formed is insoluable in water.
Reaction Types (P641-643)
Combustion
A +O2  CO2 + ? + heat
Synthesis or composition (2 or more to 1)
A + B  AB
Decomposition
AB A + B
Single Displacement
A + BC  AC + B
Double Displacement
AB + CD  AD + CB
Energy in Chemical Reactions
Section 4 page 646
• Often, energy is released or absorbed during
a chemical reaction.
• For example, energy of a welding torch is
released when hydrogen and oxygen combine
to form water.
Chemical Energy Released
• Where does this energy come from?
• In reactions that release energy, the
products are more stable, and their bonds
have less energy than those of the reactants.
• The extra energy is released in various
forms—light, sound, and heat.
Energy Absorbed
• In reactions that absorb energy, the reactants
are more stable, and their bonds have less
energy than those of the products.
Energy Absorbed
• As you have seen,
reactions can
release or absorb
energy of several
kinds, including
electricity, light,
sound, and heat.
• Electrical energy is needed to break water into
its components.
• Electrolysis
Energy Absorbed/Released
• Endothermic (en doh THUR mihk) reactions
absorb heat energy.
• Exothermic (ek soh THUR mihk) reactions
release heat energy.
Heat Released
• Burning (combustion)
is an exothermic
chemical reaction in
which a substance
combines with oxygen
to produce heat along
with light, carbon
dioxide, and water.
Heat Absorbed
• An example of an endothermic
physical process that absorbs
heat energy is the cold pack
shown.
• The heavy plastic cold pack
holds ammonium nitrate and
water.
• The two substances are
separated by a plastic divider.
• When you squeeze the bag, you
break the divider so that the
ammonium nitrate dissolves in
the water.
• The dissolving process absorbs heat energy.
Energy in the Equation
• The word energy often is written in equations as
either a reactant or a product.
• Energy written as a reactant helps you think of
energy as a necessary ingredient for the reaction to
take place. Endothermic
• Similarly, in the equation for an exothermic
reaction, the word energy often is written along
with the products.
• Energy = heat, light, electricity, sound
Reaction Rate
• A rate tells you how much something
changes over a given period of time.
• Chemical reactions have rates, too.
• The rate of reaction tells how fast a reaction
occurs after it has started.
• To find the rate of a reaction, you can measure
either how quickly one of the reactions is
consumed or how quickly one of the products is
created.
Slowing Down Reaction
• A substance that slows down a chemical reaction
is called an inhibitor.
• And inhibitor makes the formation of a certain
amount of product take longer.
• Rust inhibitors. Fruit fresh (lemon juice)
• Some inhibitors completely stop reactions.
Speeding Up Reactions
• A catalyst is a substance that speeds up a
chemical reaction.
• Catalysts do not appear in chemical equations,
because they are not changed permanently or used up.
Catalytic Converters
• Catalysts are used in the exhaust systems of
cars and trucks to aid fuel combustion.
• Catalysts speed the
reactions that change
incompletely burned
substances that are
harmful, such as carbon
monoxide, into less
harmful substances,
such as carbon dioxide.
Chapter 21 Schedule
• Monday --4-12
– Balancing Chemical Equations
• Tuesday – 4-13
– Reaction Types
• Wednesday -- 4-14
– Chemical Reactions and Energy
• Thursday – 4/15
– Lab-Evidence for Chemical Reactions
– Chapter Review P656-657:11-19,23,25,26,29,30
• Friday – 4/16
– Notetaking worksheets
• Monday
4/19
– Chapter review
• Tuesday 4/20
– Chapter 21 Test
Rapid Release
• Sometimes energy is released rapidly.
• For example, charcoal lighter fluid
combines with oxygen in the air and
produces enough heat to ignite a charcoal
fire within a few minutes.
Slow Release
• Other materials also combine with oxygen
but release heat so slowly that you cannot
see or feel it happen.
• This is the case
when iron
combines with
oxygen in the air
to form rust.
How Fast?
• Fireworks explode in
rapid succession on a
summer night.
• Old copper pennies
darken slowly while
they lie forgotten in a
drawer.
How Fast?
• Not all chemical
reactions take place
at the same rate.
• Some reactions, such
as fireworks or
lighting a campfire,
need help to get going.
• Others seem to start
on their own.
Activation Energy— Starting a Reaction
• Before a reaction can start, molecules of the
reactants have to bump into each other, or collide.
• The collision must be strong enough.
• This means the reactants must smash into
each other with a certain amount of energy.
• To start any chemical reaction, a minimum
amount of energy is needed.
• This energy is called the activation energy
of the reaction.
Temperature Changes Rate
• You can keep the food you buy at the store
from spoiling so quickly by putting it in
refrigerator or freezer.
• Food spoiling
is a chemical
reaction.
• Lowering the
temperature of the
food slows the rate
of this reaction.
Temperature Affects Rate
• Most chemical
reactions speed
up when
temperature
increases.
• Molecules collide more frequently at higher
temperatures that at lower temperatures.
• This means they are more likely to react.
Concentration Affects Rate
• The closer reactant atoms and molecules are to each
other, the greater the chance of collisions between
them and the faster the reaction rate.
• The amount of substance present in a certain volume
is called the concentration of that substance.
• If you increase the concentration, you increase the
number of particles of a substance per unit of volume.
Surface Area Affects Rate
• The exposed surface area of reactant
particles also affects how fast the reaction
can occur.
• Only the atoms or molecules in the outer
layer of the reactant material can touch the
other reactants and react.
• To start a campfire use wood cut into fine
sticks called kindling wood because it is has
more surface area but less weight.