South Pasadena  AP Chemistry
[Keep for Reference]
Predicting Reactions
CLASSIFYING REACTIONS
The reaction section of the AP exam can seem daunting because of the large variety of equations presented.
When you look at many examples of these reactions, you begin to recognize certain types of reactions.
Recognizing the different types of reactions is a good first step to predicting what might occur. One useful
scheme was suggested by an experienced AP teacher in a newsletter from the College Board. Here is my
version of that classification scheme.
D
Double Replacement reactions
When you see two ionic compounds (including acids), the compounds switch partners to form two new
compounds. This will lead to a reaction when the products are a gas, a precipitate, or a weak electrolyte.
a gas
memorize the double replacement products that decompose into gases
(H2CO3, H2SO3, HNO2, NH4OH)
an insoluble compound (precipitate) memorize the solubility rules
a weak electrolyte
memorize the strong acids so you recognize a weak acid
memorize H2O and NH4OH as other common weak electrolytes
NOTE: Watch for important stoichiometry clues (e.g. “equimolar”) for formation of acids salts like HPO42.
NOTE: Watch for complex ion formation when double replacement does not form new compounds. For
example, “Excess concentrated sodium hydroxide solution is added to a precipitate of zinc hydroxide”,
Zn(OH)2 + 2NaOH  Zn(OH)42 + 2 Na+
R
Redox (Oxidation-Reduction) reactions
Anytime you see a neutral element, copper wire, oxygen gas, nitrogen gas, etc. Redox must take place.
Memorize the common strong oxidizers (often compounds with lots of oxygen atoms, MnO4, Cr2O72, etc.)
and memorize what they turn into. Look for some chemical to oxidize.
NOTE: Watch for keyword “acidified” or an acid included in the reactants. The H+ forms H2O with the
oxygen atoms in the oxidizing agent so the central ion can change to a lower oxidation state.
Memorize the common reducers and what they turn into.
Memorize the equations for the oxidation and reduction of water during the electrolysis of water.
L
Lewis Acid-Base reactions
“Have Pair Will Share”—Lewis Base. The formation of a coordinate covalent bond between NH3 and BF3 to
form H3N:BF3 is a classic example of a Lewis acid-base reaction. (NH3 is the Lewis base with its lone pair.)
Oxides of metals (basic anhydrides) and oxides of nonmetals (acid anhydrides) are also examples.
Examples:
CO2 + H2O  H2CO3
oxides of nonmetals with water form acids
CaO + H2O  Ca(OH)2
oxides of metals with water form bases
CaO + CO2  CaCO3
a Lewis base + a Lewis acid form a salt
C
Complex Ion (Coordination Compound) reactions
Complex ions are usually formed from a transition metal ion surrounded by ligands (polar molecules or
negative ions). As a “rule of thumb” you place twice the number of ligands around an ion as the charge on the
ion. Examples: the dark blue copper-ammonia complex, Cu(NH3)42+ (ammonia is a test for Cu2+ ions) and
AgCl2- (solid AgCl dissolves when excess, concentrated HCl is added).
Memorize the common ligands.
NOTE: Aluminum forms complex ions as do some of the post transition metals.
NOTE: The names are very impressive, but easy… the ions above are the tetraamminecopper(II) ion and the
dichlorosilver(I) ion. Zn(OH)42 is the tetrahydroxyzinc(II) ion, the charge is the sum of the central
ion and the ligands (2+) + 4(1-).
NOTE: Acid-base reactions may change NH3 into NH4+ (or vice versa) which will alter its ability to act as a
ligand.
NOTE: Visually, a precipitate may go back into solution as a complex ion is formed. For example, Cu2+ plus a
little NH4OH will form the light blue precipitate, Cu(OH)2. With excess ammonia, the complex,
Cu(NH3)42+, forms. If a little HCl is added, the NH3 becomes NH4+ and no longer acts as a ligand; the
Cu(OH)2 will re-form.
NOTE: Keywords such as “excess” and “concentrated” may indicate complex ions. AgNO3 + HCl forms the
white precipitate, AgCl. With excess concentrated HCl, the complex ion, AgCl2, forms and the
solution clears.
NOTE: The odd complex ion, FeSCN2+, shows up once I awhile simply because it is commonly used in an
equilibrium lab.
O
Organic chemistry reactions
These are especially useful for doing reactions from older tests. Organic reactions are no longer used on the AP
exam. Organic nomenclature including functional groups is used quite often on the AP exams.
Oxidation
Complete combustion of hydrocarbons results in CO2 and H2O. Milder oxidation may change
a primary alcohol into an aldehydes, a secondary alcohol into a ketone. No reaction occurs
with a tertiary alcohol.
Substitution Br2 + a hydrocarbon results in HBr + a bromo-hydrocarbon. One of the bromine atoms
substitutes for a hydrogen.
Esterification organic acid + alcohol results in an ester and H2O. This is called a condensation reaction.
Esters often have nice odors (e.g. wintergreen or fruit odors).
Addition
A double bond is broken and atoms are attached to the two carbon atoms. If H2O is added, an
H and an OH add to the carbon atoms (an alcohol is formed). If Br2 is added, a bromine atoms
is added to each of the carbon atoms.
In general:
NOTE: Some reactions come from industrial processes such as the formation of bleach (OCl ion) from Cl2
and NaOH. Note these as you come across them and add them to your “bag of tricks”.
NOTE: You can write (or think about) chemical in different ways for different reactions. Ammonia may be
NH3(aq) for complex ions or NH4+ + OH for double-replacement or acid-base reactions. Water may
be H2O or you may think of it as H+ and OH for hydrolysis or redox reactions. HNO3 may be an acid
(a source of H+ ions) or it may be an oxidizer (forming NO + H2O or NO2 + H2O).