PRIVATE ISTEK ACIBADEM HIGH SCHOOL
2011-2012 9th GRADE CHEMISTRY COURSE
CHEMICAL CHANGES
 All substances have properties that we
can use to identify them.
 There are billions of billions kinds of
changes taking place around us.
 There are two basic types of properties
that we can associate with matter.
 These properties are called physical
properties and chemical properties
Physical Properties and Chemical
Properties
 Physical Properties- a property that can
be observed and measured without
changing the material’s composition.
 Examples- color, hardness, m.p., b.p.
 Chemical Properties- a property that
can only be observed by changing the
composition of the material.
 Examples- ability to burn, decompose,
ferment, react with, etc.
Physical Changes and Chemical
Changes
 Physical change will change the visible
appearance, without changing the
composition of the material.
 Boil, melt, cut, bend, split,
crack
 Chemical change - a change where a
new form of matter is formed.
 Rust, burn, decompose, ferment
 Physical changes are the changes in the
physical properties of matter.
 A physical change takes place without
any changes in molecular composition.
 Physical changes do not result in the
formation of new substances.
 An example of a physical change is the
melting of ice. If a sample of ice is
heated, it changes into liquid form of
water at 0°C. But there is no change in
the composition of water.
 The boiling of water also involves a
physical change from the liquid to the
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gaseous state, but the composition of
water is still unchanged.
A chemical change alters the
composition of the original matter. For
example, if hydrogen and oxygen gas
are mixed together, and a spark is
passed through the mixture, an
explosion takes place, and water is
formed.
Water has completely different
properties than both hydrogen and
oxygen gas. Not many reactions are as
fast as the formation of water. The
rusting iron, ripening a fruit, or fading
of dye in cloth are examples of slow
reactions
In a chemical change, the molecular
structure of matter changes. Some
molecules are broken down and some
molecules are formed. One or more
samples are converted to new samples
with different composition.
For example, if the iron filings are
exposed to moisture, the iron will soon
be changed into rust. The properties of
rust are totally different from those of
iron and oxygen, and it is concluded
that a new substance has been formed
Example: Classify each of the statements
as a physical or chemical property, and
explain your answer
 Cl2 is a greenish colored gas at room
temperature.
 Liquid oxygen is attracted by a
magnet.
 Gold Is highly resistant to corrosion.
 Hydrogen cyanide is an extremely
poisonous gas
Identify each of the following as a physical
or chemical change.
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a. hard-boiling an egg
b. magnetizing iron
c. breaking bread into pieces
d. digestion of food
e. dissolving sugar in water
f. souring milk.
In chemical reactions:
a. Atoms are conserved: The total
number of atoms of each type remains
unchanged in a chemical reaction.
b. Mass is conserved: This is known as
the principle of "Conservation of
mass". The mass of the reactants is
equal to the mass of the products.
c. The chemical bonding changes: The
arrangement of atoms is different in the
reactants from what it is in the
products of a chemical equation.
2H-O-H —►2H-H + O = O
d. The number of molecules, and volume
of the substances may not be conserved
in chemical reactions.
e. The electrical charge is conserved: The
total charge of reactants must be the
same as that of products.
Chemical Equations
 The representation of a chemical
reaction is important for the chemists
all over the world to communicate with
one another about chemical reactions.
 Therefore a standard way to represent
chemical reactions has been developed.
Writing Chemical Equations
 A chemical reaction is a process in
which new chemical substances, called
products are produced from a set of
original substances, called reactants.
 A symbolic representation of a
chemical reaction is called a chemical
equation. In a chemical equation the
formulas of the reactants are written on
the left side and those of the products,
on the right.
 The two sides are joined by an arrow
(→).
In writing a chemical equation;
a. We have to know what the reactants
and products are. At this stage you are not
expected to predict the products of a
reaction. Later on you will gain experience
to be able to make intelligent guesses.
b. We have to write the correct molecular
formula for each reactant and each
product.
c. We must show that atoms are
conserved.
 These three steps are illustrated in the
following example. When propane gas
burns completely, the products are
carbon dioxide and water. Let us write
an equation to represent this
combustion reaction.
Balancing Chemical Equations
 To be valid an equation must conserve
atoms. In other words the number of
atoms of each kind on both sides of the
arrow must be the same. To do this a
suitable set of coefficients are placed in
front of the formula of each of the
reactants and products.
 The process of finding these
coefficients is called balancing
equation. There is no set "recipe" for
balancing ordinary chemical equations.
 The "inspection method" or trial and
error method is the first one tried by
chemists in the real world.
 In this method, one usually starts with
the most complicated formula and
choose its coefficient as 1.
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 Then he or she adjusts the coefficients
of the other participants in whatever
way seems appropriate.
 It is suggested to adjust the coefficients
of mono atomic elements near the end
of the balancing act since any change
in their coefficient will not affect the
balance of other elements.
 Only when it fails to work, as in the
case of many oxidation-reduction
reactions, more complex procedures
are used.
Consider the burning reaction of propane.
A useful first step in balancing any equation is
to choose the coefficient of the substance that
contains the largest number of atoms per
molecule as 1, and adjust the other coefficients
according to the coefficient chosen as 1. In our
example the most complex substance is C3H8
(11 atoms per molecule). So we choose its
coefficient 1. But the coefficient one is not
shown in the equation.
 Since the number of each kind of
atoms on both sides of the equation are
the same, then the equation is said to
be balanced equation.
 Multiplying the coefficients of a
balanced equation by a common factor
does not disturb its balance.
 In practice, however, we normally use
the smallest possible set of whole
number coefficients to balance an
equation.
Example:
KClO3  KCl + O2
1KClO3  KCl + 3/2 O2
Exercise:
CH4 + Cl2  CCI4 + HCI
C2H6 + O2  CO2 + H2O
AI(OH)3 + H2SO4  Al2(SO4)3 + H2O
Next step is to look for the element (s) in the
most complex substance that appears only
once on each side of the equation, and to
balance the numbers of this element or these
elements. Here all two elements (C and H)
appear only once on each side. So we can
balance C and H elements. There are 3 C
atoms on the left, we place 3 in front of CO2.
P2H4  PH3 + P4
Fe + H2O  Fe3O4 + H2
Example: What is the molecular formula
of the compound represented by X in the
following balanced equation?
4X + 5O2  4NO + 6H2O
We place 4 in front of H2O to get 8 H atoms
on both sides.
The equation is still not balanced. The total
number of O atoms on the right is 10, 6 in
3CO2 and 4 in 4H2O,
To balance O, we place 5 in front of O2.
To have equal number of N and H atoms
on both sides, 4 X should contain 4 N and
12 H atoms. Then 1X should have 1 N and
3 H atoms or its formula must be NH3.
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 It is helpful to know the physical state
of substances appearing in chemical
equations.
 The symbol (g) after a formula means
that the substance is a gas.
 Liquids are indicated by the symbol
(I), and solids by the symbol (s).
 A substance in water solution is
indicated by the symbol (aq).
Energy Changes in Physical and
Chemical Changes
As you know, water molecules attract each
other strongly because they are polar. The
closer two oppositely charged particles are, the
stronger the attraction between them. Water
molecules are much closer together in liquid
state than in gaseous state. Therefore energy is
needed to overcome the attraction forces when
water evaporates. From this example, we
conclude that a physical change always
involve some energy change.
Most chemical reactions involve the breaking
and formation of chemical bonds. It takes
energy to break a chemical bond but energy is
released when chemical bonds are formed.
Since chemical reactions involve the making
and breaking of chemical bonds, the reactions
either release or absorb energy, depending
upon whether there's more energy in the bonds
of the reactants or in the products of the
reaction.
If the energy stored in the bonds of the
reactants is greater than that in the bonds of
the products, the "excess" energy is released as
the chemical reactions take place. The
reactions in which energy is released are
called exothermic reactions.
The formation of water from hydrogen and
oxygen gas is an example of exothermic
reaction
Generals the heat energy released or absorbed
during a reaction is included in the equation,
as a heat term usually denoted by the letter Q.
In exothermic reactions, Q appears next to the
products on the right hand side of the
equation.
In endothermic reactions, it appears next to the
reactants on the left hand side.
The reverse of an exothermic reactions
endothermic or vice versa.
On the other hand, if the chemical bonds in the
products store more energy than do the bonds
in the reactants, energy is absorbed during
such reactions. The reactions in which energy
is absorbed are called endothermic reactions.
The decomposition of mercury(ll) oxide into
mercury and oxygen is endothermic.
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