Download Chemical Reactions - Wando High School

Chemical Reactions
Chemical Change
How
do you know if a
chemical change has
taken place?
All chemical changes are
results of chemical
reactions.
Chemical Reactions
Chemical
Reactions: occur
when bonds are broken and a
new product is formed.
(Chemical Change)
Involve the rearrangement of
atoms
Chemical reactions
 Chemical
formulas are used to write
chemical equations.
 The chemical compounds used to
start the reaction are Reactants.
 The chemical compounds at the end
of the reaction are Products.
Reaction Symbols
 There are several types of
Reaction symbols used.
 + plus (added to)
  forms (produces or yields)
– gas
 s – solid
 l – liquid
g
 aq—aqueous
solution
Law of Conservation of Mass
 Matter
cannot be created or destroyed. It
is always conserved in a reaction.
 Atoms cannot simply appear or
disappear.
 The amount used at the start of the
reaction is equal to the amount at the
end of the reaction.
#
of atoms of REACTANTS =
# of atoms of PRODUCTS
Take a look at this…
Does
it look balanced?
We must the get the O2 to balance..
Is
this balanced now?
And now….Is it balanced?
YES!
Practice Counting Atoms
MgCl2 Mg= 1 atom of magnesium
Cl= 2 atoms of chlorine
H2SO4
H= 2
S= 1
O= 4
More counting!
N= 1
NH4Cl H= 4
Cl= 1
Mg=
1
Mg(NO3)2 N= 2
O= 6
(NH4)3PO4 N= 3 P= 1
H= 12 O= 4
Balanced Equations
 In
order to conserve atoms, we
must balance the number and type
of atoms.
 CAN’T
 Can
CHANGE SUBSCRIPTS!
only change the numbers of atoms on
each side of the equation by adding
COEFFICIENTS!
 Coefficients: the numbers that come in
front of a chemical formula.
Let’s Try to Figure out Coefficients
How
many do you have
of each?
3 H20
2 H2O3
4 Fe2O3
6 NaCl
NO2
Steps to Balancing
First, count the number and
type of each atom on both
sides
 If
there are different numbers of
atoms for an element on both
sides, you must add
coefficients to compounds to
change the number of atoms
 Second,
figure out what
coefficient is needed in order to
make the numbers of atoms
equal.
 REMEMBER: IF YOU CHANGE
A COEFFICIENT, ALL
ELEMENTS IN THAT
COMPOUND ARE AFFECTED.
Magnesium
+ Oxygen (g)
Magnesium Oxide
Mg + 02
MgO
Now try on your own…

C + Cl2

Al2O3
CCl4
Al + O2
Helpful Hints to Balancing
If
an element appears in
more than one compound
on the same side of the
equation, leave that element
until LAST to balance
Other Helpful Hints
If there is an even number of
atoms of an element on one
side of the equation and an odd
number on the other, place a
coefficient of 2 in front of the
ODD compound
Energy in Reactions
Energy
is either
released or absorbed
Exothermic or
endothermic
Exothermic v. Endothermic
Exothermic
rxns produce
energy (RELEASE!) so the
temperature rises!
Endothermic rxns require a lot
of energy (ABSORBS!) so the
temperature drops!
Exothermic Reactions
LESS
energy is required to
break the bonds in the
reactants than is released
when bonds are formed to
make new products
More energy is released when
new products form
Endothermic
MORE
energy is required to
break the bonds of the
reactants than is released from
the formation of new bonds
Energy is being pulled into the
reaction from outside
THE END! :0)