Chemical Reactions
Atoms are the building blocks for
everything
 Elements are made of atoms
 Compounds are made of elements

Chemical Reactions

Let's start with the idea of a reaction. In
chemistry, a reaction happens when two
or more molecules interact and something
happens.

There are a few key points you should
know about chemical reactions.
 1.
A chemical change must occur. You start
with one compound and turn it into another.
 2. A reaction could include ions, molecules, or
pure atoms. A reaction can happen with
anything, just as long as a chemical change
occurs (not a physical one).
Compounds
A
substance formed from elements,
but it does not have the same
characteristics as the individual
elements
 Ex.
Iron oxide (rust)
Review
Chemical Formulas
 Chemists
us chemical formulas to
represent the composition (make-up)
of the elements in compounds
 A short-hand (text message) way of
writing a chemical compound
 NaCl-
salt or Sodium Cloride
 H20- water
Electron shells
a)
b)
c)
Atomic number = number of Electrons
Electrons vary in the amount of energy
they possess, and they occur at certain
energy levels or electron shells.
Electron shells determine how an
atom behaves when it encounters
other atoms
Electrons are placed in shells
according to rules:
1) The 1st shell can hold up to two
electrons,
2) and each shell thereafter can hold up
to 8 electrons.
Valence
 An
atom is stable when it has 8
electrons in its outer shell. This makes
them “fat and happy”
Electron Dot Structures
Symbols of atoms with dots to represent the valenceshell electrons
1
2
13
14
15
16
17
18
H
He:

Li Be


B 


C


Na Mg


Al

N




: F  :Ne :
O




 Si 
P
 S








:Cl  :Ar :


Why are electrons important?
Elements have different electron
configurations
 different electron configurations
mean different levels of bonding
 The configuration determines
how it bonds with other
elements
Chemical Bond
 When
an element comes together or
combines with another element to
form a compound
 An attempt to fill outer shell (make it
fat and happy) with 8 electrons
Types of Bonds
 Ionic-
giving or receiving of electrons
to another element
 Covalent- sharing of electrons in the
outer shell (valence). Covalent bonds
form molecules
IONIC BOND
bond formed between
two ions by the
transfer of electronsmetals combine with
nonmetals
Ions
 Formed
when electrons (-) are given
away from element resulting in a
positive charge, because the number
of protons (+) is greater than the
number of electrons (-) in the atom
 Positive
ions are called cations
Ions
 Formed
when electrons (-) are received
from an element resulting in a negative
charge, because the number of
protons (+) is less than the number of
electrons (-) in the atom
 Negative
ions are called anions.
Formation of Ions from Metals

Ionic compounds result when metals react with
nonmetals

Metals lose electrons to match the number of valence
electrons of their nearest noble gas

Positive ions form when the number of electrons are
less than the number of protons

Group 1 metals 
ion 1+
Group 2 metals 
ion 2+
Group 3A (13) metals 
ion 3+
Ions from Nonmetal Ions
In ionic compounds, nonmetals in 15, 16, and 17 gain
electrons from metals
Nonmetals add electrons to achieve the octet (8)
arrangement
Negative ions form when the number of
electrons are
more than the number of protons
Group 5(15) nonmetals 
ion 1-
Group 6 (16) nonmetals  ion 2Group 7 (17) nonmetals  ion 3-
Learning Check
Give the ionic charge for each of the following:
A. 12 p+ and 10 e1) 0
2) 2+
3) 2B. 50p+ and 46 e1) 2+
2) 4+
3) 4-
C. 15 p+ and 18e1) 3+
2) 3-
3) 5-
COVALENT BOND
bond formed by the
sharing of electrons
Covalent Bond
 Between
nonmetallic elements of
similar electronegativity.
 Formed by sharing electron pairs
 Examples; O2, CO2, C2H6, H2O, SiC
Chemical Change
 Change
from one substance to another
 Burning
paper
Physical Change
 Change
is physical or outward
appearance. Does not change chemical
make-up
 Wrinkled
paper
 Reactants
the elements or
compounds that are put together to
form a chemical change
 Na + Cl  NaCl
reactants
 Products-
the end result of reactants
coming together
 Na + Cl  NaCl
products
Chemical Reactions Types
 Endothermic-energy
absorbed
 Exothermic- energy is released
Rates of Reactions
 Catalyst-
speeds up a chemical
reaction
 Inhibitor- slows down a chemical
reaction (takes longer)
Law of Conservation of Mass

“In an ordinary chemical reaction, matter
is neither created nor destroyed.”

This means that you have neither gained nor
lost any atoms during the reaction. They may
be combined differently, but they’re still there.
Law of Conservation of Mass
 The
mass of the products must equal
the mass of the reactants (what I put
into a reaction, I must get out)
 Mass
is neither created or destroyed.
 If
the product has a mass less than the
reactants what happed? Was their a
gas, smoke or other byproducts?
 Example
Burning paper- does the burnt
paper (ashes) weigh as much as the
original piece? Why?
Mass of
Smoke =7 g
How to Balance Equations


Draw boxes around all
the chemical formulas.
Make a Chemical
Inventory (Look at the
subscripts next to each
atom to find the number
of atoms in the
equation.)




Write numbers in front
of each of the boxes
(coefficient) until the
inventory for each
element is the same
both before and after
the reaction.
Multiply the coefficient
Never change subscripts
Try to leave single
elements for last

Whenever you change a
number, make sure to
update the inventory otherwise, you run the
risk of balancing it
incorrectly. When all the
numbers in the
inventory balance, then
the equation can balance
Finished

Since both sides of
the inventory match,
the equation is now
balanced! All other
equations will balance
in exactly the same
way, though it might
take a few more steps
in some cases.
Some practice problems:
1.__NaCl + __BeF2 --> __NaF + __BeCl2
2. __FeCl3 + _Be3(PO4)2 --> _BeCl2 + _FePO4
3. __AgNO3 + _LiOH --> _AgOH + __LiNO3
4. __CH4 + __O2 --> __CO2 + __H2O
5. __Mg + __Mn2O3 --> __MgO + __Mn
Solutions
2 NaCl + 1 BeF2 --> 2 NaF + 1 BeCl2
2 FeCl3 + 1 Be3(PO4)2 --> 3 BeCl2 + 2 FePO4
1 AgNO3 + 1 LiOH --> 1 AgOH + 1 LiNO3
1 CH4+ 2 O2 --> 1 CO2 + 2 H2O
3 Mg + 1 Mn2O3 --> 3 MgO + 2 Mn
Practice at home
 www.chemistry-
drills.com/balance.html