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 was 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.
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?
The answer is NO! There are 1 Na and 2 Cl atoms
on the reactant side, but only 1 Na and 1 Cl on the
product side…this is NOT BALANCED!
Balancing an Equation
Balancing an equation is easy but takes
practice. USE A PENCIL!
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 but do not alter the formula
itself!
• Let’s balance our equations now:
Na
+
Cl2

NaCl
1. Count the number of EACH atom on both sides.
•
•
•
There are ____ Na atoms and ___ Cl atoms of the left side.
1
2
There are ____ Na atoms and ____ Cl atoms on the right side.
•
1
1
2. Start with the first unbalanced atoms and try adding a coefficient to
balance it.
3. Continue adding coefficients as needed until all atoms are
balanced in number on both sides.
• Let’s balance our equations now:
2 Na
+
Cl2

2 NaCl
Two atoms of chlorine.
Two atoms of sodium.
*NOTE – Not EVERY element or compound will need a
COEFFICIENT…..if no coefficient is in front of the
element or compounds, it is CONSIDERED TO BE 1.
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
More Practice…and a little more
challenging!
Na3PO4 + CaCl2  NaCl + Ca3(PO4)2
AlBr3 + K2SO4  KBr + Al2(SO4)3
Ca(OH)2 + H3PO4  Ca3(PO4)2 + H2O
5
2 Na3PO4 + 3 CaCl2  6 NaCl + Ca3(PO4)2
2 AlBr3 + 3 K2SO4  6 KBr + Al2(SO4)3
3 Ca(OH)2 + 2 H3PO4  Ca3(PO4)2 + 6 H2O
STOP
How do you know when a chemical
reaction has occurred?
There are several factors that identify a
chemical reaction:
•A CHANGE IN ENERGY OCCURS, such as
HEAT/COLD, LIGHT GIVEN OFF or COMBUSTION.
•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].