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