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Chemical Equations and
Reactions
Describing Chemical Reactions
Chemical Reactions
 when
a substance changes
identity



Reactants - original
Products - resulting
Reactants  Products
 Law
of Conservation of Mass - mass is
never created or destroy, just rearranged

total mass of reactants = total mass of products
Chemical Reactions
 chemical


equation
Written form of a chemical reaction
uses symbols and formulas
Hints of Chemical Rxn
 heat

can also happen with
physical changes
 gas

or light
bubbles
means a gas is product
 precipitate

solid is being created
 color
change
Writing Chemical Equations
 Most
pure elements - write symbol
 Diatomic molecules - found in pairs when
not bonded to other elements

HOFBrINCl! H2, O2, F2, Br2, I2, N2, Cl2
 Other


exceptions
sulfur: S8
phosphorus: P4
Word Equations
 uses
names instead of formulas
 helps you to write formula equation
Symbols Used in Equations
yields (produces)
reversible
Things above arrow show
Special requirements:
or heat: heated
MnO2 or Pt: catalyst
25°C :specific Temp
requirement
2 atm: specific Pressure
after a formula:
(s) solid
(l)
liquid
(aq) aqueous:
dissolved in
water
(g)
gas
Coefficients
4Fe(s) + 3 O2(g)  2 Fe2O3(s)
Whole numbers in front of formula
 Distributes to numbers of atoms in formula
 specifies the number of moles in the reaction
 used to balance the equation

Example
 Description:
Solid sodium oxide is added to water at
room temperature and forms sodium
hydroxide.
 Word Equation:
sodium oxide + water  sodium hydroxide
 Formula
Equation:
Na2O + H2O  NaOH
Equations cannot tell us:
 if

the reaction will actually occur
depends on many factors affecting energy
 the



speed (rate) of the reaction
depends on chemical kinetics
can be altered by temperature, surface area,
addition of a catalyst
can be very slow, almost unnoticeably
 how

the bonding actually changes
formula may not clearly show bonding