Chemical reactions
Classifications
Reactions in solution
Ionic equations
Learning objectives
Distinguish between chemical and physical change
Describe concepts of oxidation and reduction
Classify reaction according to types of reactants and
products
Distinguish among strong, weak and non-electrolytes
Identify common acids and bases by from chemical
formula
Predict formation of precipitates by application of
solubility rules
Write total and net ionic equations from balanced
molecular equations
One approach to classification
Oxidation – reduction: focusing on
electrons
Oxidation is loss of electrons
Reduction is gain of electrons
Oxidation is always accompanied by
reduction
The total number of electrons is kept constant
Oxidizing agents oxidize and are
themselves reduced
Reducing agents reduce and are
themselves oxidized
Redox in chemistry
All reactions involve rearrangement of
atoms and molecules
Some reactions involve rearrangement of
atoms and molecules and electrons
– Photosynthesis, respiration, combustion...
These are called redox reactions
Any reaction involving elements must be
redox
Combination reactions
Element + element  compound (redox)
– S + O2 → SO2
– Metal + nonmetal  binary ionic compound
– Nonmetal + nonmetal  binary covalent compound
Compound + element  compound (redox)
– CO + O2 → CO2
Compound + compound  compound
– SO2 + H2O →H2SO3
Decomposition reactions
Compound  element + element (redox)
– HgO → Hg + O2
Compound  element + compound
(redox)
– PCl5 → PCl3 + Cl2
Compound  compound + compound
– CaCO3 → CaO + CO2
Single replacement (displacement)
Element displaces another element from
compound (redox)
– Zn + CuSO4 → ZnSO4 + Cu
Double replacement (displacement)
Compounds exchanging partners
– Usually ionic compounds in solution
Identify ions and swap them
KCl + AgNO3 → KNO3 + AgCl(s)
Very often a solid is produced
Acid – base neutralization:
special case of double replacement
BASE
ACID
SALT
WATER
KOH(aq) + HNO3(aq) = KNO3(aq) + H2O(l)
Product is liquid water not a solid
Combustion
Element or compound reacting with
oxygen (redox)
– CH4 + O2 → CO2 + H2O
Associated with production of heat and light
Often involves hydrocarbons (CxHy)
CO2 and H2O are products
Sorting solution reactions:
dissolved species
Electrolytes:
– Ionic compounds produce ions in solution
(NaCl, NH4NO3 etc.)
Non-electrolytes:
– Covalent compounds do not produce ions in
solution (CH3OH, C6H12O6 etc.)
Electrolytes: distinguishing by
strength
Strong electrolytes are characterized by
complete dissociation in water
Weak electrolytes dissociate to a much smaller
extent.
Strong, weak or non electrolyte?
All soluble salts are strong electrolytes
Strong acids and bases are strong
electrolytes
Weak acids and bases are weak
electrolytes
Insoluble compounds are non-electrolytes
Molecular compounds are non-electrolytes
Classification of electrolytes
Strong
electrolytes
Weak
Nonelectrolytes electrolytes
ACIDS:
HCl, HBr, HI
HClO4, HNO3, H2SO4
ACIDS:
HF, H3PO4,
CH3CO2H
SALTS:
KBr, Na3PO4
SALTS:
None
BASES:
NaOH, Ba(OH)2
BASES:
NH3
Molecular
covalent
compounds:
H2O,
CH3OH,
C12H22O11
(sucrose)
Most organic
compounds
Four classes of substance with
solution reactions
Solubility in water
Yes
No
Acid or base
Yes
Weak or strong
No
Ionic or covalent
weak
Weak electrolyte
strong
ionic
Strong electrolyte
cov
Nonelectrolyte
Insoluble
substance
Recognizing acids and bases
Acids usually have H at the beginning of
the formula – HCl
Bases usually have OH in the formula –
NaOH
– Not in organic compounds though - CH3OH
Focus on double replacement
Driven by removal of ions from solution
1. Formation of an insoluble solid (precipitate)
2. Formation of non-ionized molecules (acid – base)
3. Formation of a gas
1. Predicting precipitation reactions
Does one of the new cation-anion
combinations produce insoluble salt?
How do I know?
– Initial combinations are all soluble
– Use solubility rules to investigate
– If yes, a precipitate is produced
Solubility rools
Group I and ammonium compounds are
generally soluble
Nitrates and acetates are generally soluble
Chlorides, bromides and iodides are generally
soluble {except Pb(II), Ag(I) and Hg(I)}
Carbonates and phosphates are generally
insoluble (except group I)
Hydroxides and sulphides are generally
insoluble (except groups I and II)
3. Production of a gas
If product is a gas that has a low solubility
in water, reaction in solution is driven to
produce the gas
Tums relief
Any carbonate with an acid
NaHCO3(s) + HCl(aq) = NaCl(aq) + H2O(l) +
CO2(g)
Writing balanced molecular equations
for double replacement reactions
Use correct formulae
– Metal ion charge predicted
from group number
– Use table for correct
formula and charge for
polyatomic ions
Identify as solid (s), gas
(g), liquid (l) or dissolved
(aq)
Balance: atoms (groups)
on left = atoms (groups)
on right
Balancing double replacement equations
It’s very much a matter of states – show
them!
Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s)
Balance polyatomic ions as units:
– Pb2+, K+, I-, NO3-
Left hand side
Right hand side
1 Pb2+
1 Pb2+
2 NO3-
2 NO3-
2 K+
2 K+
2 I-
2 I-
Molecular equation for reaction of
Na2SO4 + Ba(NO3)2
Total ionic equations
Dissolved substances:
– Strong electrolytes show as ions
– Weak or non- electrolytes show as molecular formula
All others show as molecular formula
Pb(NO3)2(aq) + 2KI(aq) = 2KNO3(aq) + PbI2(s)
Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) = 2K+(aq) + 2NO3-(aq)
+ PbI2(s)
Net ionic equations
Spectator ions are those ions that do not
undergo a change; they do not participate
in the chemical change and are the same
on both sides of the equation
Remove all spectator ions from the
equation
Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) = 2K+(aq) +
2NO3-(aq) + PbI2(s)
Net ionic equations
Pb2+(aq) + 2I-(aq) = PbI2(s)
Mass and charge must still balance, although
overall charge may not be neutral in a net ionic
equation
Net ionic equation for reaction of
Na2SO4 + Pb(NO3)2