Transcript Introduction to Chemical Reactions and Equations

Introduction to Chemical
Reactions and Equations
Recognizing Chemical Reactions
A CHEMICAL REACTION is a process in
which one or more substances are
changed into new* substances.
“New” simply means substance that were not there before the reaction!
Chemical Equations
A chemical reaction cannot be seen; it
occurs on the SUBMICROSCOPIC
LEVEL!
Since reactions can’t be seen, we use a
CHEMICAL EQUATION to represent a
CHEMICAL REACTION.
A CHEMICAL EQUATION is a WRITTEN
EXPRESSION that illustrates what
happens during a chemical reaction.
All CHEMICAL EQUATIONS have
THREE BASIC PARTS:
REACTANT(S)
One or more
substances that
enter the reaction,
separated by
PLUS SIGNS.
PRODUCT(S)
The arrow is called a
“YIELD SIGN”…it’s
like an equal sign in a
math equation.
One or more new
substances that are
formed during the
reaction, separated by
PLUS SIGNS.
Type of Equations
FORMULA EQUATIONS are written using
the chemical symbols and formulas for
substances.
EXAMPLE
Mg +
S

MgS
WORD EQUATIONS are written
using the names of the elements
and compounds involved.
EXAMPLE
Potassium + Oxygen  Potassium Oxide
Equations MUST be BALANCED!
 Since a chemical equation illustrates what
happens to atoms on the SUBMICROSCOPIC
level, it must show exactly what happens to ALL
ATOM INVOLVED!
The LAW OF CONSERVATION OF
MATTER, says MATTER CANNOT BE
MADE OR DESTROYED.
• Because of this law, we must write
BALANCED equations: equations that
have the SAME NUMBER of EACH
ATOM on BOTH SIDES of the equation.
Consider this equation:
Na
+
Cl2

NaCl
Are there the SAME NUMBER of SODIUM and
CHLORINE atoms on BOTH SIDES of the equation?
NO! There are 1 sodium and 2 chlorine atoms on
the reactant side, but only 1 sodium and 1 chlorine
on the product side…this is NOT BALANCED!
Balancing an Equation
Balancing an equation is easy.
We can only add numbers BEFORE the
formulas to adjust the number of atoms on
each side….we CANNOT change
subscripts!
These “numbers” are called
COEFFICIENTS
Coefficients MULTIPLY the number of
each kind of atom in a formula!
• Let’s balance our equations now:
2 Na
+
Cl2

2 NaCl
1. Count the number of EACH atom on both sides:
There is 1 Na atom and 2 Cl atoms on the left side.
There is 1 Na atoms and 1 Cl atom on the right side.
2. Start with the first unbalanced atom and try adding a
coefficient to balance it.
3. Continue adding coefficients as needed until all atoms
are balanced in number on both sides.
*****USE A PENCIL!!******
Try balancing these equations:
K
+
O2

K2O
Al
+
S

Al2S3
C
+
Cl2

CCl4
4K
+
O2
2 Al
+
3S
C
+
2 Cl2



2 K2O
Al2S3
CCl4
How do you know when a chemical
reaction has occurred?
There are several factors that identify a
chemical reaction:
•ENERGY is given off as HEAT or LIGHT, such as
burning, or an explosion.
•A GAS is formed.
• A PRECIPITATE forms [a solid that settles to the bottom].
•A change in COLOR or ODOR may indicate
a reaction, but not always [can be physical].