Unit 7 - Lake Dallas Independent School District
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Transcript Unit 7 - Lake Dallas Independent School District
Chemical Reactions
Balancing Chemical Equations
Types of Reactions
Remember – A chemical change occurs when
substances react together and form NEW products.
Examples of this include:
rusting
Combustion
Tarnishing
Fermentation
oxidation
Acid rain reacting with calcium in rocks
Evolution of heat and light
Production of a gas
Formation of a precipitate
A precipitate is a solid produced as a result of a chemical
reaction in solution and that separates from the
solution
Unexpected Color change
Change in energy (absorption or evolution)
Formation of water
Production of an odor
Chemical reaction can be written in words or as
formulas. When they are written using formulas and
symbol this is called a chemical equation.
Chemical reactions must contain correct reactants
(the beginning substances) and products (the ending
substances)
Chemical equations must be BALANCED in order
to follow the LAW of Conservation of Mass, which
states that mass can neither be created or
destroyed.
Remember… what you start with has to equal with what
you end with!
To balance a chemical equation you must make sure
there is the same number of each type of atom on both
sides of the arrow.
Only change the coefficients (the number in front
of the compounds or element), never change
subscripts!
Diatomic molecules (di=2, atomic=atoms)
(always exist as 2 when they are by themselves)
N2
O2
F2
Cl2
Br2
I2
H2
Remember that when balancing an equation each side
has to equal. (Law of Conservation of Mass).
First:
Write out the equation: ___N2 + ___H2
___NH3
Count how many atoms of each element are on each side.
Reactants
Products
N=2
N=1
H=2
H=3
This equation is NOT balanced.
You will now have to add coefficients to make each
side balanced.
Symbol
Explanation
“Yields”; indicates result of reactions
Indicates a reversible reaction
(s)
A reactant or product in the solid state, also indicates a precipitate
(l)
A reactant or product in the liquid state
(aq)
A reactant or product in an aqueous solution (dissolved in water)
(g)
A reactant or product in the gaseous state
or
Reactants are heated
Pressure at which a reaction is carried out (in this case 2 atm)
Temperature at which reaction is carried out (in this case 0oC)
Formula of catalyst, in this case manganese dioxide, used to alter the rate of reaction
Synthesis
2 elements or simple compounds combine to form ONE
product.
Examples:
Generic equation: A + X AX
8 Fe + S8 8 FeS
CaO + H2O Ca(OH)2
Decomposition
ONE compound is broken down into simpler parts
Examples:
Generic equations:
AX A + X
AXY AX +Y
2 H2O 2 H2 + O2
Single Replacement
An element and a compound combine to form a new
element and compound.
Metals can replace other metals and hydrogen in an
acid.
Halogens can replace halogens.
Must look at activity series to determine if the reaction
will occur (it is located on your reference sheet)
Generic equations:
AX + B BX + A
AX + Y AY + X
2 Na + MgSO4 Mg + Na2SO4
Cl2 + 2NaBr Br2 +
2 NaCl
Double Replacement
2 compounds produce 2 NEW compounds.
These compounds are in aqueous solution so the ions
switch places with each other.
Must consider solubility rules to determine which
product is the precipitate, if any.
Generic Equation:
AX + BY AY + BX
Pb(NO3)2 + 2 KI PbI2 + 2 KNO3
Combustion
Oxygen combines with a compound to produce carbon
dioxide and water.
This releases large amounts of energy (heat).
Generic equation:
CxHy + O2 CO2 + H2O