Download Equations in the AP Examinations

EQUATIONS IN THE AP EXAMINATIONS
(last revision: 08/06/07)
As of May 2007, Equations need to be balanced. Balancing can be done
either by trial-and-error as in the past or by a more sophisticated method that you
will need to apply to redox equations.
Omit spectator ions, i.e., ions that stay in solution during the reaction and form
no precipitates (see worksheet “Solubility Rules”). Strong acids that are mostly
dissociated (HCl, HNO3, H2SO4) should be written in their ionized form, e.g., HCl
= H+ + Cl-. Weak acids that are mostly undissociated (HC2H3O2 and similar
organic acids) must be written as undissociated molecules.
Each question consists of two parts:


One equation that must be correctly balanced.
A question about that reaction. This question may involve numbers of
electrons transferred, oxidation numbers, observable changes during
reactions, formulas/names of precipitates, etc.
Since this change is a very recent one, my suggestions as how to approach this
new format can be only tentative.
I will therefore include my previous strategies.
1) ACID-BASE REACTIONS:
There are no changes of oxidation states in acid-base reactions.
a) acid and base solutions:
H2SO4 (aq) + Ca(OH)2 (aq)  CaSO4 (s) + 2 H2O should be written as:
H+ + SO42- + Ca2+ + 2 OH-  CaSO4 + 2 H2O
Some acids are unstable:
HCO3- / CO32- + H+  (H2CO3)*  H2O + CO2 (Don’t include H2CO3!)
HSO3- / SO32- + H+  (H2SO3)*  H2O + SO2
(Don’t include H2SO3!)
A star superscript after a compound in parentheses (…)* is often used to indicate
instable compounds or intermediates.
b) solution and anhydride:
N2O5 + 2 OH-  2 NO3- + H2O
or
6 H+ + Cr2O3  2 Cr3+ + 3 H2O
c) anhydrides:
P2O3 + 3 MgO  Mg3(PO3)2
d) amphoteric hydroxides:
Some hydroxides (Sn4+, Zn2+, Al3+, Cr3+)
dissolve in acids as well as excess base.
Al(OH)3 + 3 H+  Al3+ + 3 H2O
Al(OH)3 + 3 OH- (xs.)  Al(OH)63- (also: AlO33- + 3 H2O
e) Lewis acid-base reactions:
Compounds with lone electron pairs react with
electron-deficient compounds.
BF3 + NH3  BF3 • NH3 (F3B—NH3)
f) hydrolysis of nonmetal halides or oxyhalides:
PCl5 + 4 H2O  8 H+ + 5 Cl- + PO43-
Yields two acids.
SO2Cl2 + 2 H2O  4 H+ + SO42- + 2 Cl-
g) hydrolysis of certain transition metal halides:
UF6 + 2 H2O  UO2F2 + 4 H+ + 4 F-
Yields oxyhalides and
acid.
2) DOUBLE REPLACEMENT:
compound+ compound  compound + compound
AB
+ CD
 AD
+ BC
2 AgNO3(aq) + BaCl2(aq)  2 AgCl(s) + Ba(NO3)2(aq)
(Ag+ + Cl-  AgCl)
Driving force: Formation of precipitate.
3) SINGLE REPLACEMENT:
element + compound  compound + element
a) cationic single replacement: A + BC  AC + B
(A, B = metals or
hydrogen)
H2 + CuO  H2O + Cu
2 Al + Cr2O3  Al2O3 + 2 Cr
Mg + 2 H+  Mg2+ + H2
b) anionic single replacement: A + BC  AB + C
(A, C = nonmetals)
F2 + 2 I-  2 F- + I2
Cl2 + 2 Br-  2 Cl- + Br2
Driving force: Check Table of Standard Reduction Potentials, often hydrogen is
formed.
4) REDOX REACTIONS:
a) oxidizing agents:
oxygen:
halogens:
Elements in (unusually) high oxidation states.
O2 (0), peroxides O22- (-1)  O2-
X2 (0)
e.g. Cl2
XO- (+1)
e.g. ClOXO2 (+3)
e.g. ClO2XO3- (+5)
e.g. ClO3XO4 (+7)
e.g. ClO4(oxidation strength increases with higher oxidation number)
chromates and dichromates:
CrO42-, Cr2O72-  Cr3+
manganese dioxide and (per)manganates:
MnO2  Mn2+
MnO42-, MnO4-  MnO2 (basic)
MnO42-, MnO4-  Mn2+ (acidic)
oxyacids (esp. when concentrated), common: HNO3, H2SO4 and HClO4
HNO3  NO
H2SO4  SO2
HClO4  Cl-
b) reducing agents:
alkali and alkaline earth metals
Na, K, Ca  Na+, K+, Ca2+
aluminum
Al  Al3+
transition metals in low oxidation states
Fe2+ Fe3+
hydrogen
H2  H2O
carbon monoxide
CO  CO2
sulfite and thiosulfate ion
S2O32- / SO32-  SO42-
Cu+  Cu2+
c) miscellaneous:
Halogens often oxidize and reduce themselves
in water (= disproportionation),
e.g. Cl2 + H2O
 H+ + Cl- + ClOHalogens in low AND high oxidation states
reach one oxidation state
(= comproportionation),
e.g. I- + IO3- + H+  I2 + H2O
5) (THERMAL) DECOMPOSITIONS:
a) Carbonates (esp. alkaline earth):
CaCO3  CaO + CO2
b) Hydrogencarbonates:
2 NaHCO3  Na2O + H2O + CO2
c) Azides (esp. Na, Ag):
decompose explosively and form
nitrides and nitrogen, e.g.
3 NaN3  Na3N + 4 N2
Driving force: Entropy gain (gas!) and enthalpy gain by formation of NN bond
6) LIGAND EXCHANGE REACTIONS:
Molecules or anions with lone pairs can have a higher affinity to hydrated metal
ions than water and subsequently replace one or all water molecules. (examples:
NH3, CN-, CO)
Cu(H2O)42+ + 4 NH3  Cu(NH3)4+ + 4 H2O
Fe(H2O)63+ + 6 CN-  Fe(CN)63- + 6 H2O
AgCl + 2 CN-  Ag(CN)2- + ClAmmine ligands (NH3) can be removed by acids:
Ag(NH3)2+ + 2 H+  Ag+ + 2 NH4+
7) ORGANIC REACTIONS:
a) Addition to multiple bonds:
Alkynes and alkenes add halogens, acids and water.
HCCH + Cl2  HClC=CClH
H2C=CH2 + HBr  H3C—CH2Br
H2C=CH—CH3 + H2O  H3C—CHOH—CH3
b) Substitution of single bonds:
Alkanes and aromatic hydrocarbons substitute halogens for one or more
hydrogen.
CH4 + 4 Cl2  CCl4 + 4 HCl
C6H6 + Br2  C6H5Br + HBr
c) Ether formation: alcohol + alcohol  ether + water (catalysis: H+)
C2H5OH + C2H5OH  C2H5—O—C2H5 + H2O
d) Ester formation: alcohol + acid  ester + water (catalysis: H+)
CH3CH2COOH + CH3CH2CH2CH2OH  CH3CH2—COO—CH2CH2CH2CH3 +
H2O
EXAMPLES FROM AP EXAMS:
2007 (example):
(i) A strip of magnesium is added to a solution of silver (I) nitrate.
Mg + 2 Ag+ Mg2+ + 2 Ag
(ii) Which substance is oxidized in the reaction? Mg is oxidized.
2007 (a):
(i) A solution of sodium hydroxide is added to a solution of lead (II) nitrate.
Pb2+ + 2 OH-  Pb(OH)2
(ii) If 1.0 L volumes of 1.0 M solutions of sodium hydroxide and lead (II) nhitrate
are mixed together, how many moles of product(s) will be produced. Assume the
reaction goes to completion.
0.5 moles of lead (II) hydroxide Pb(OH)2 will be formed.
(Since OH- is used up at twice the rate, it is the limiting reactant.)
2007 (b):
(i) Excess nitric acid is added to solid sodium carbonate.
2 H+ + CaCO3  Ca2+ + CO2 + H2O
(ii) Briefly explain why statues made of marble (calcium carbonate) displayed
outdoors in urban areas are deteriorating.
The air in urban areas contains nonmetal oxides such as SO2, SO3, and
NOx. These react as acid anhydrides and create acid rain which has a
deteriorating effect on calcium carbonate.
2007 (c)
(i) A solution containing silver (I) ion (an oxidizing agent) is mixed with a solution
containing iron (II) ion (a reducing agent).
Ag+ + Fe2+  Ag + Fe3+
(ii) If the contents of the reaction mixture described above are filtered, what
substance(s), if any, would remain on the filter paper.
Silver metal (Ag).
1974/2
A sample of pure 2-butene is treated with hydrogen bromide gas.
Answer:
2-butene is an alkene and undergoes addition to the double bond.
CH3—CH=CH—CH3 + HBr  CH3—CH2—CHBr—CH3
1988/1
A solution of potassium iodide is added to an acidified solution of potassium
dichromate.
Answer:
a) potassium K+ is a spectator ion  omit!
b) Cr2O72- is an oxidizing agent  iodide is oxidized, dichromate
reduced
I- + H+ + Cr2O72-  I2 + Cr3+ + H2O
1989/2
Solutions of silver nitrate and lithium bromide are mixed.
Answer:
a) lithium Li+ and nitrate NO3- are spectator ions  omit!
b) silver bromide is insoluble.
Ag+ + Br-  AgBr
1990/5
Hydrogen sulfide gas is bubbled through a solution of potassium hydroxide.
Answer:
a) potassium K+ is a spectator ion  omit!
b) H2S is an acid, OH- a base
H2S + OH-  S2- + H2O
1993/4
Excess chlorine gas is passed over hot iron filings.
Answer:
a) chlorine acts as an oxidizing agent, iron as a reducing agent
b) synthesis reaction: metal + nonmetal  salt
Fe + Cl2  FeCl3