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CHEMICAL REACTIONS
UNIT
Notes
CHEMICAL REACTIONS
Think: When you hear the words “Chemical
Reactions”, what comes to your mind?
Often times, people picture a scientist in a
lab working with chemicals.
In real life, chemical reactions are taking
place all around us when we breathe, eat,
when fruit ripens, when we cook, and when
plants go through photosynthesis.
CHEMICAL REACTIONS
A chemical reaction is a process that
results in some chemical change to 1 or
more substances.
If a substance changes in a way other than
a physical change, a chemical reaction
must have taken place.
SIGNS THAT A CHEMICAL REACTION HAS
TAKEN PLACE
1.
2.
3.
4.
5.
Release of a gas
Change in color/odor
Presence of a precipitate
Energy change (heat, light, etc.)
Change of identity of substances
REACTANTS & PRODUCTS
Reactants: substances that undergo a
chemical change
Products: substances that form as a result of
a chemical change
Reactants → Products
*Note: Products and reactants contain the
same types of atoms. Remember the law of
conservation of mass: Mass cannot be created
or destroyed
ENERGY AND CHEMICAL REACTIONS
In order for chemical bonds to be
broken, enough energy to overcome the
attractive forces of the reactants must
be added.
Chemical Energy: the energy stored
within atoms and molecules that can be
released when a substance reacts
The total energy before a reaction is
equal to the total energy after a reaction.
2 T YPES OF REACTIONS IN TERMS OF
ENERGY
1. Exothermic
Reaction: a
chemical reaction
that gives off
energy in the form
of heat
2. Endothermic
Reaction: a
chemical reaction
in which energy is
absorbed from the
surrounding area.
ENDOTHERMIC VS. EXOTHERMIC
 http://app.discoveryeducation.com/search?Ntt=endothermic+
exothermic+reactions
 https://www.youtube.com/watch?v=TKcrZlaYv7c
CHEMICAL EQUATIONS
Chemical Equation: An equation that
uses chemical formulas and symbols to
show the reactants & products in a
reaction.
In a chemical equation: the reactants,
on the left side of the arrow, form
products, on the right side of the arrow.
The symbol “→” means “gives” or
“yields”
BALANCING CHEMICAL EQUATIONS
An equation is balanced when the
number of atoms of each element on the
right side equals the number of atoms of
each element on the left side.
A balanced chemical equation follows
the Law of Conservation of Mass
RULES FOR BALANCING CHEMICAL
EQUATIONS
1. You CANNOT balance an equation by changing
the chemical formulas.
 You must leave the subscripts in the formulas
alone!!
2. An equation can be balanced only by: putting
numbers, called coefficients, in front of the
chemical formulas.
 The coefficients tell how many molecules of
that compound are present.
3. The numbers of atoms for each element must
be the same on each side of the equation.
STEPS USED TO BALANCE A CHEMICAL
EQUATION
1. Determine number of atoms for each
element.
2. Pick an element that is not equal on both
sides of the equation.
3. Add a coefficient in front of the formula with
that element and adjust your counts.
4. Continue adding coefficients to get the same
number of atoms of each element on each
side.
EXAMPLE #1
 Mg + O 2 → MgO
Mg=
Mg=
O=
O=
Whenever you have a coefficient of
1, it is NOT written!!
EXAMPLE #2

CuSO 4 + Al → Al 2(SO 4) 3 + Cu
LAW OF CONSERVATION OF MASS
 The law of conservation of mass is
followed in chemical equations: The
total mass of the reactants is the
same as the total mass of products.
LAW OF DEFINITE PROPORTIONS
The law of definite proportions: A
compound ALWAYS contains the
same elements in the same
proportion, regardless of how the
compound is made or how much of
the compound is formed.
RATES OF CHEMICAL REACTIONS
There are 6 factors that affect the rate of
chemical reactions:
1. Increase in temperature: Why? The particles
are moving faster and have more chances to
collide into each other to make a reaction.
2. Increase in Surface area: Why? More of the
substance is exposed, so the particles have
more opportunities to come into contact
with each other.
 Examples: crushing, chopping up
RATES OF CHEMICAL REACTIONS
3. Increase in concentration: Why? Increasing
the number of particles increases chance of
collision resulting in a reaction.
4. Increase in pressure: Why? Particles are
squeezed into a smaller volume, so there is
less space and more collisions occur
between particles.
RATES OF CHEMICAL REACTIONS
5. Nature of the reactants:
a. Size and Shape: Why? Heavier particles
move slower than lighter particles at the
same temperature & have less opportunity
to collide with other molecules.
b. Nature of reactant: Why? Some
molecules must fit together in a certain
way in order to react.
RATES OF CHEMICAL REACTIONS
6. Adding a catalyst: A substance that alters
the rate of a reaction, but is left unchanged
in the reaction.
a. Inhibitors: catalysts that slow down
reactions.
b. Enzymes: biological catalysts that speed
up a biochemical reaction.