AP TOPIC: Equation Writing
Question 4 in Section B, Part II of the AP Chemistry examination requires the candidates to write five chemical equations from a
choice of eight sentences that describe chemical reactions in words.
In general, it is worth noting the following points.
Learn inorganic nomenclature rules. Without knowledge of these rules this question is almost impossible.
Where appropriate all compounds that produce ions in solution should be written in their ionic form and spectator ions
should be ignored, i.e. where appropriate, write the net-ionic equation.
There are no reactions in the question that do not occur.
There is no requirement to balance the equations you write, however, it may be helpful to do so.
It is possible to get partial credit by writing reactants carefully and guessing products.
Double Replacement (Metathesis).
These reactions involve two reactants forming two products. They usually involve acids, bases and salts.
Formulae begin with H and have a hydrogen ion (H +) that they donate in reactions.
Organic acids often have ionizable hydrogen ions written at the end of their formulae and include the -COOH or -CO2H
group. E.g. ethanoic acid CH3COOH. Organic acids are weak.
Learn the strong acids. HCl, HBr, HI and acids where the number of oxygen’s present exceeds the number of hydrogen’s by
two or more, e.g. HNO3, H2SO4.
Sulfuric acid, H2SO4, is more accurately represented as only losing one H + ion, i.e. as H+ + HSO4-, rather than 2H+ + SO42-,
since the second dissociation of the HSO4- ions is only very slight in comparison to the loss of one H + from H2SO4 to form H+
Carbonic acid, H2CO3, is weak not very stable and is better represented as CO2 + H2O. If it were to be represented as an acid
it should be shown as a monoprotic acid, forming H + and HCO3-.
Sulfurous acid, H2SO3, is generally thought of as being a monoprotic acid forming H + and HSO3-, or alternatively, if a
product, as a weak acid and not dissociating at all.
Formulae end in OH (hydroxide ions, OH- present) except for ammonia and organic bases that contain Nitrogen that has a
lone pair of electrons allowing it to act as a base. Do not confuse organic alcohols that also have formulae ending in OH as
hydroxide ions. An alcohol can be recognized as having a carbon chain preceding the –OH group, e.g. C2H5OH or
CH3CH2CH2OH, for ethanol and propan-1-ol respectively.
Learn the strong bases. Confined to Group I & II hydroxides and ammonium hydroxide, i.e. LiOH, NaOH, KOH, Ca(OH)2,
Sr(OH)2, Ba(OH)2 and NH4OH.
Ammonium hydroxide, NH4OH, is not very stable and is better represented as NH3 + H2O.
A salt is a compound where the hydrogen ion(s) in an acid have been replaced by metal ions or the ammonium (NH 4+) ion.
Learn the solubility rules for salts.
ACID + BASE SALT + WATER (NEUTRALIZATION)
ACID + CARBONATE SALT + WATER + CARBON DIOXIDE
ACID + METAL SALT + HYDROGEN
Equimolar means the reactants react in a 1:1 ratio and is often a clue to only some of the ionizable hydrogen’s in an acid
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If one of the products in a reaction is a salt that is thought of as being slightly soluble (commonly Ca(OH) 2, Sr(OH)2 and
Ba(OH)2) credit may be given for writing either the dissociated or undissociated form.
Solutions of sodium phosphate and calcium chloride are A dilute solution of lithium hydroxide is mixed with dilute
mixed. hydrobromic acid.
Ca2+ + PO43- Ca3(PO4)2 H+ + OH- H2O
Hydrogen sulfide gas is bubbled through a solution of silver
H2S + Ag+ H+ + Ag2S
SIMPLE REDOX (a simple change in oxidation state).
A reaction where electrons are transferred from one species to another. These reactions fall into a number of different sub-
Reactions where an element reacts with a compound. Common examples include a more reactive metal displacing another
less reactive one from solution (See Standard Electrode potential table for clues), halogen displacement and hydrogen
displacement from both acids and water.
A reaction with oxygen. Common examples are organic compounds (hydrocarbons and carbohydrates) forming CO 2 and H2O
and metals burning in air to produce the corresponding oxide.
A reaction where elements and/or compounds combine together. Usually fall into one of four types.
Element (metal) + Element (non-metal) compound
Non-metal oxide + water acid
Metal oxide + water base
Metal oxide + non metal oxide salt
The reverse of combination, where the compound decomposes to give elements, or a combination of elements and new
compounds Clue, only one reactant usually being heated.
Base metal oxide + water
Acid non-metal oxide + water
Salt containing oxygen (often carbonate) metal oxide + non-metal oxide (often CO2)
Magnesium metal is added to a solution of Iron (III)
chloride Sulfur trioxide is added to water
Mg + Fe3+ Fe2+ + Mg2+ SO3 + H2O H+ + HSO4-
Solid magnesium carbonate is heated
MgCO3 MgO + CO2
NON-SIMPLE REDOX (a non-simple change in oxidation state).
In these reactions an oxidizing agent will cause the oxidation of another compound and in the process, itself will be reduced. The
opposite is true of reducing agents.
If a reaction takes place in acid solution it means H + ions are reactants and water will be one of the products. Water is
often formed when the oxidizing agent contains oxygen atoms.
If a reaction takes place in basic solution it means OH- ions are a reactant and water is one of the products.
Elements in their highest oxidation states can only be reduced and elements in their lowest oxidation states can only be
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Learn as many of the common oxidizing agents and reducing agents (and what they turn into) as possible.
Disproportionation. Simultaneous oxidation and reduction of one species.
E.g. 3NO2 + H2O 2H+ + 2NO3- + NO
Change & #
Reducing Change & # of
Oxidizing agent Turns into of electrons Turns into
agent electrons lost
MnO4- +7 +2 Halide ions Free Halogens -1 0
(in acid) (+5) (F-, Cl-, Br-, I-) (F2, Cl2, Br2, I2) (-1) x 2
MnO4- +7 +4 Free metals (e.g. 0 Various
MnO2 Metal cations
(in neutral or basic) (+3) Zn) (- Various)
MnO2 +4 +2 +4 +6
Mn2+ SO32- or SO2 SO42-
(in acid) (+2) (-2)
Cr2O72- +6 +3 +3 +5
Cr3+ NO2- NO3-
(in acid) (+3) x 2 (-2)
HNO3 +5 +4 Free Halogens Hypohalite ions 0 +1
(Conc.) (+1) (in dilute basic) (XO-) + halide (X-) (-1) x 2
HNO3 +5 +2 Free Halogens 0 +5
NO Halite ions (XO3-)
(Dilute) (+3) (in Conc. basic) (-5) x 2
H2SO4 +6 +4 Highly charged
SO2 metal cations (e.g Various
(hot & Conc.) (+2) metal cations
Highly charged metal Lower charged H2O2 -1 0
Various O2 + H2O
cations metal cations (in basic solution) (-1) x 2
Free Halogens Halide ions 0 -1 +2 +2.5
(F2, Cl2, Br2, I2) (F-, Cl-, Br-, I-) (+1) x 2 (-0.5) x 2
H2O2 -1 -2 H+ (usually in the 0 +1
(in acid) (+1) x 2 form of water) (-1) x 2
Sulfur dioxide gas is bubbled through a solution of acidified Hydrogen peroxide is added to an acidified solution of
potassium manganate (VII) (permanganate) potassium dichromate (VI)
SO2 + MnO4- + H+ H2O + SO42- + Mn2+ H2O2 + H+ + Cr2O72- Cr3+ + O2 + H2O
Concentrated hydrochloric acid is added to solid manganese
H+ + Cl- + MnO2 Mn2+ + Cl2 + H2O
HYDROLYSIS (reaction with water)
A reaction with water.
Anion of weak acid + water weak acid + hydroxide ion
Cation of weak base + water weak base + hydrogen ion
Non-metal halide + water (H+ ion + more electronegative element) + other elements combined
PCl3 + H2O HCl + H3PO3
PCl5 + H2O HCl + H3PO4
SCl4 + H2O HCl + H2SO3
The oxidation number of the phosphorous and sulfur in each case does not change
Hydrides of group I release hydrogen gas
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Lithium hydride is added to water Phosphorous tribromide is added to water
LiH + H2O Li+ + OH- + H2 PBr3 + H2O H+ + Br- + H3PO3
Gaseous hydrogen chloride is bubbled into water
HCl + H2O H3O+ + Cl-
Reactions where organic compounds (those involving carbon chains) react in many different ways.
Combustion or oxidation. Products are usually CO2 + H2O
Addition. Compounds with multiple bonds have a halogen, hydrogen halide or hydrogen added
Substitution. Commonly hydrogen atoms are displaced (replaced) by another atom
Esterification. Carboxylic acid + alcohol ester + water
Learn rules for nomenclature, learn functional groups
Methane is mixed with an excess of bromine gas
CH4 + Br2 CBr4 + HBr Ethanoic acid is refluxed with ethanol and a catalyst for
Propanol is burned completely in air CH3COOH + C2H5OH CH3CO2C2H5 + H2O
CH3CH2CH2OH + O2 CO2 + H2O
COMPLEX ION FORMATION
These reactions involve a transition metal in aqueous solution reacting with (commonly) ammonia ligands or hydroxide ligands.
The breaking down of complexes can be achieved by reacting the complex ion with acid.
Aluminum and zinc will act like transition metal ions and form complexes
The word excess often appears in these questions
Learn nomenclature of complexes and common reactions
Most first row transition metal complexes exist in solution as hexaaqua ions
A solution of ammonium thiocyanate is added to a solution of iron (III) chloride
[Fe(H2O)6]3+ + SCN- [Fe(SCN)(H2O)5]2+
Excess dilute nitric acid is added to a solution containing tetraamminecadmium (II) ions
H+ + [Cd(NH3)4]2+ Cd2+ + NH4+
Aluminum pellets are treated with excess sodium hydroxide
Al + OH- Al(OH)4-
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ion of iron (III) chlo ride
[Fe(H2 O)6 ]3+ + SCN- [Fe(SCN)(H2 O)5 ]2+
Excess dilute nitric acid is added to a solution containing tetraamminecadmiu m (II) ions
H+ + [Cd(NH3 )4 ]2+ Cd 2+ + NH4 +
Aluminu m pellets are treated with excess sodium hydro xide
Al + OH- Al(OH)4 -
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