Chemical Equations
Chemical Equations
 Shorthand method, using symbols and
numbers, to describe a chemical reaction.
 Example: One mole of oxygen gas reacts
with two moles of hydrogen gas to yield
two moles of water vapor.
O2 (g) + 2 H2 (g)  2 H2O (g)
Reactants  Products
Balancing Chemical
Equations
Law of Conservation of Mass
 In a closed system, mass is conserved.
 This means:
 Mass and matter is neither created or
destroyed in a chemical reaction.
 In a chemical reaction, atoms are not
destroyed, they are rearranged to make new
substances.
Balancing an Equation
 ALL EQUATIONS MUST BE BALANCED in
order to describe reactions properly!
(Law of Conservation of Matter)
C3H8 + O2  CO2 + H2O
(not balanced)
3 carbons on reactant side, 1 carbon on product
side.
8 hydrogens on reactant, 2 hydrogens on
product.
2 oxygens on reactant, 2 + 1 oxygens on product.
C3H8 + 5O2  3CO2 + 4H2O
(!!balanced!!)
3 carbons – 3 carbons
8 hydrogens – 8 hydrogens
10 oxygens – 6 + 4 oxygens
Identifying Chemical
Equations
Synthesis
 Two or more substances react and form
one (or more) new substance(s).
 The number of new products is less than
the number of reactants.
2 H2 + O2  2 H2O
Decomposition
(Synthesis in Reverse)
 One (or more) substance(s) breaks down
into more than one new substances.
 The number of new products is more than
the number of reactants.
2 H2O2  2 H2O + O2
Single Replacement /
Displacement
 An element replaces part of a compound.
 Metal replaces metal, or, nonmetal replaces
nonmetal
Na + LiCl  NaCl + Li
Double Replacement /
Displacement
 Two compounds trade pieces of
themselves.
 Metal trades with metal, or, nonmetal trades
with nonmetal.
HCl + NaOH  NaCl + H2O
Combustion
 A reaction with oxygen (O2), usually
releasing light and heat, and often (but not
always) forming carbon dioxide and water
vapor.
Substance + O2  new substance or
substances
CH4 + 2 O2  CO2 + 2 H2O
2 H2 + O2  2 H2O (also a synthesis react.)
REDOX REACTIONS
Reactions as Electron Transfers
 When electrons were discovered, certain chemical
equations could be classified as transfers of electrons
between atoms.
 Oxidation reactions – any reaction where an element
loses electrons.
 Oxidized = lost electrons.
 2 Ca + O2  2 CaO
 Ca  Ca2+ + 2e Reduction reactions – any reaction where an element
gains electrons.
 2 Na + F2  2 NaF
 F + e-  F-
 Oxidation-Reduction always occurs
together.
 Oxygen DOES NOT need to be present
for oxidation to occur.
 Complete transfer of electrons does not
always have to occur
 Redox can form ionic and covalent bonds.