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Chemical Reactions
Background
• We learned about Physical and Chemical
properties and changes.
– Physical properties were those like mass, volume,
density, melting point, boiling point, conductivity,
malleability, state/phase of matter etc.
• Changes within these properties can be undone and the
properties of the substances are not altered.
– Chemical properties were those like oxidation,
flammability, and reactivity.
• Changes within these properties CANNOT be undone.
Change is permanent and the product now has new
properties of its own.
Remember…..
Chemical Reactions occur because of the forces between atoms and valence
electrons. The groups on the periodic tables all are organized based on Chemical
and Physical Properties. Chemically, we are speaking of valence electrons.
IONIC BOND- When a metal bond to a NON-metal, this is usually due to an IONIC
bond where the NON-metal has taken a free electron (valence electron) from a
metal. If you look…groups 1 and 2 (all metals) have 1 or 2 valence electrons. The
NON-metals are in groups 13-18. They have between 3-8 valence electrons. They
NEED the electrons to complete their “octets” (means 8 by the way) or shells. BY
stealing away 1 or 2 from a Metal, they begin to complete their shells and leave the
Metal’s shell full.
Covalent Bond- When a NON-metal and a NON-metal are close enough for
electromagnetic force to attract valence electrons, the atoms SHARE electrons to
complete their “octets” or shells. *See next slide.
Remember—we did Covalent in
class!
Organize Your Thoughts
Chemical
reactions
Chemical
equations
• Balancing equations
• Predicting products
from reactants
Packard, Jacobs, Marshall, Chemistry Pearson AGS Globe, page 175
Chemical
equations
• Synthesis
• Decomposition
• Single replacement
• Double replacement
• Combustion
Signs of Chemical Reactions when atoms
bond….
There are five main signs that indicate a chemical reaction has taken place:
release
input
change in color
change in odor
production of new
gases or vapor
input or release
of energy
difficult to reverse
In other words……..
ubbles
production of new
gases or vapor
change in odor
difficult to reverse
input or
release
of energy
change in color
difficult to reverse
What? New Substance!?
• I thought you said Matter cannot be created nor
destroyed, only changed???
– New substance is referring to a substance that has its
own chemical properties (different from those that
created it).
– Example: Sometimes when 2 substances are mixed,
the result is a PRECIPITATE. This is a SOLID that is
the product of 2 reacting substances.
– A kidney stone is a real life example! There
Aren’t really stones in your body!
Chemical Equations
• Reactants – the substances that exist before a chemical
change (or reaction) takes place.
• Products – the new substance(s) that are formed during
the chemical changes.
• CHEMICAL EQUATION indicates the reactants and products of
a reaction.
REACTANTS  PRODUCTS
For Example--
2H2 + 02  2H20
Do you remember 6th Grade? Combine your 6th Grade
Knowledge with your 8th Grade Knowledge….
Use these
to fill in
the
Reactants
and the
Products
Did you get it correct?
Common Processes using a chemical
equation:
Look! They are the reverse of each other!!!
Some Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction
(s)
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
(l)
A reactant or product in the liquid state
A reactant or product in an aqueous solution
(aq)
(dissolved in water)
(g)
A reactant or product in the gaseous state
Chemical Equations
aluminum oxide
Reactants
4 Al(s) + 3 O2(g)
Product
2 Al2O3(s)
The letters (s), (g), and (l) are the
physical states of compounds.
The numbers in the front are called
stoichiometric coefficients.
Characteristics of Chemical Equations
• The equation must represent known facts.
• The equation must contain the correct
formulas for the reactants and products.
• The law of conservation of mass must be
satisfied. (Equation must be balanced).
Chemical Equations
4 Al(s) + 3 O2(g)
2 Al2O3(s)
aluminum oxide
sandpaper
This equation means:
4 Al atoms + 3 O2 molecules yields 2 molecules of Al2O3
Visualizing a Chemical Reaction
2 Na
+
Cl2
2 NaCl
Chemical Equations
Because the same atoms are present
in a reaction at the beginning (reactants)
and at the end (products), the amount
of matter in a system does not change.
The Law of Conservation of Matter
100%
Kotz web
Chemical
Factory
100%
20%
80%
Chemical Equations
Because of the law of the conservation of matter, an equation must be balanced.
It must have the same number of atoms of the same kind on both sides.
*Remember from class?
On both sides:
C=1
H=4
O=4
Word Equations
• A WORD EQUATION describes chemical change using the
names of the reactants and products.
Write the word equation for the reaction of methane gas
with oxygen gas to form carbon dioxide and water.
methane
+
oxygen
Reactant
CH4
+
2 O2
carbon dioxide
+
water
Product
CO2
+
2 H2O
NEW! Very IMPORTANT!
*Synthesis (also known as Composition) and Decomposition reactions are considered
REDOX reactions (more on this later)
Synthesis Reaction –
Synthesis means put together!
2 Na
+

Cl2
Na
2 NaCl
Cl
Na+ Cl -
Cl
Cl - Na+
Na
General form: A
element or
compound
+
B
element or
compound

AB
compound
Synthesis Reactions
Photosynthesis
6 CO2 + 6 H2O
C6H12O6 +
Formation of water
2 H2 + O2
2 H2O
Formation of salt
2 Na + Cl2
2 NaCl
General Form
A + B
C
6 O2
Decomposition Reaction –
Decomposition means to breakdown!
Decomposition reaction
2 H2 O
2 H2
+
O2
H
O
H
+
H
O
H
General form: AB
compound
A
+
B
two or more elements
or compounds
Decomposition Reactions
Hydrogen Peroxide
2 H2O2
2 H2O
+
2 H2
+
O2
Electrolysis of water
2 H2O
electricity
O2
Nitrogen triiodide
2 NI3
N2
+ 3 I2
General Form
AB
A + B
*Both Synthesis and Decomposition reactions (explanations to follow) are considered
Oxidation-Redox reactions. Think back to our last 2 days of bonding. The placement of
electrons had to be altered in order for the molecules to form.
Ionic bonds results in redox
Oxidation (redox) is a type of chemical reaction in which electrons are transferred from one substance
to another. For oxidation to occur, one substance must lose electrons and a second substance must gain
the lost electrons.
The substance gaining electrons is said to be reduced and the substance losing the electrons is said to
be oxidized. Thus an oxidation reaction is called a Redox reaction.
In the Redox Reaction diagram, sodium (Na) is being oxidized and chlorine (Cl) is being reduced.
The term "oxidation", with its obvious root from the word "oxygen", assumes that oxygen has an
oxidation number of -2. WHY -2?? BECAUSE Oxygen must gain 2 electrons in order to fill its shell
remember?
Above, Sodium (Na) must LOSE 1 electron, so therefore its oxidation number is +1
Single and Double Replacement
Reactions
Single-replacement reaction
Mg
+
CuSO4
General form:
A
+ BC

MgSO4

AC
+
+
Cu
B
Double-replacement reaction
CaCO3
+
General form:
AB
+
2 HCl

CaCl2
+
H2CO3
CD

AD
+
CB
Acid-Base
• Neutralization is a type of chemical reaction in which a
strong acid and strong base react with each other to form water
and salt. Have you ever been unlucky enough to be stung by a wasp
or a bee? Bee stings are acidic in nature, which is why a household
remedy for a bee sting is baking soda or sodium bicarbonate, which
is a basic substance. A wasp sting, on the other hand, is mildly
basic, so a household remedy for this will be vinegar, also known as
acetic acid. These simple treatments ease these painful stings by a
process called neutralization.
• Heartburn, as well as an acidic stomach due to eating too much
spicy food, can be relieved by taking an antacid. The antacid is
alkaline/basic and helps neutralize the stomach's acidic
environment. If you look at the medicine for heartburn in the
picture below, you may have used this one before to ease your
heartburn via the process of neutralization.
Evidence of an Acid-Base Reaction is that a SALT and WATER are the Products
Acid- Base Reaction
Combustion
Hydrocarbons (Carbon and Hydrogen combo) Combine to react and produce
Carbon Dioxide and Water. Evidence of a combustion reaction are Carbon
Dioxide and Water as products.