AP Chemistry Exam Question 4 (Equations) Strategies
Question 4 (Equations) Strategies
AP Chemistry Exam – Question 4 (Equations) Strategies
1. Answer all the questions.
2. In all cases, a reaction occurs.
3. Each equation is worth 5 points; 1 point for correct reactants; 2 points for correct
Products, 1 for balancing, 1 for answering the associated question. NEVER leave any of the
equations blank – you should be able to at least get the reactant point for each of the equations!
4. If you are unsure of the products for a reaction, and are looking for the reactant point
only, BE SURE TO SELECT A REACTION WHERE THE REACTANT(S)
CLEARLY HAVE NO SPECTATORS!
Note: Every time a solid is indicated, write it in a solid form, not ionic.
Examples:
2002 Exam question 4(e):
“Solid sodium hydrogen carbonate (sodium bicarbonate) is strongly heated.”
NaHCO3 à ??? Scores 1 reactant point
2002 Exam question 4(f):
“An excess of hydrochloric acid is added to solid zinc sulfide.”
HCl + ZnS à ??? Scores 0 points. (Cl- is a spectator in the complete equation.)
5. Balancing the equations is required.
6. Indicating physical states (i.e. solid, liquid, gas, aqueous, etc.) is unnecessary.
Indicating a catalyst, or energy (light, heat, etc.) is unnecessary.
7. Equations MUST be written in NET IONIC FORM. (i.e. spectator ions must be
eliminated from the final answer.)
8. Solids DO NOT have spectators.
(e.g. Solid sodium chloride added to water is written: NaCl + H2O à )
Weak acids must be written as molecular formulas.
(e.g. Oxalic acid in water is written: H2C2O4)
Strong acids must be written as ions.
(e.g. Hydrochloric acid in water is written H+ + Cl-)
Memorize the six strong acids [i.e. HCl, HBr, HI, H2SO4 (first proton only), HNO3, HClO4]. Be sure to eliminate spectator ions – if appropriate!
9. Each year the same “types” of reactions are used. The ability to recognize reaction types
should assist you in determining products. Types include:
(A) REDOX Reactions
Type indicators: strong oxidizers and/or strong reducers
Strong oxidizers:
Cr2O72-
MnO4-
H2O2
O3 (ozone)
Halogens (F2, Cl2, Br2, I2)
Reactants in reactions with large positive Eo (reduction potentials)
Strong reducers:
Groups I and II metals (Alkali and Alkaline Earth metals)
Products in reactions with large negative Eo (reduction potentials)
Note: Groups I and II metals (except Be and Mg) will reduce H in water to
hydrogen gas producing hydroxide ion (Ca in HOT water only!)
Once a REDOX reaction has been identified, remember: A species cannot be oxidized without another species being reduced – an oxidizer will become reduced, and a reducer will become oxidized. It will be necessary to have memorized the common oxidation states of the metals, and when writing products, use the common oxidized and reduced states. Watch for “common compounds” to be formed.
Examples:
2002 Exam question 4(g):
“Acidified solutions of potassium permanganate and iron (II) nitrate are mixed
together.” (Hint: “acidified” à REDOX reaction)
Start: K+ + MnO4- + H+ + Fe2+ NO3- à ???
Mn (in MnO4-) has an oxidation state of 7+ which will become reduced to 2+.
Fe2+ will become oxidized to Fe3+. K+ and NO3- are spectators. H2O will be formed from
the H+ and the oxygen in MnO4-.
Finish: MnO4- + H+ + Fe2+ à Mn2+ + Fe3+ + H2O
Don’t forget! The “Reduction Potential Table”, provided with this section of the test,
gives the activity series of the metals. This is particularly useful for single replacement
reactions (which are almost always REDOX reactions!). In this case, the PRODUCTS
of the reactions with high negative Eo (reduction potentials) are the most reactive metals.
Review “Single Replacement Reactions” from first year chemistry.
2003 Exam question 4(g):
“A bar of strontium metal is immersed in a 1.0 M copper (II) nitrate solution.”
Strontium is higher on the activity series than copper.
Sr + Cu2+ à Sr2+ + Cu
(B) Acid/Base Reactions
1. Know the six strong acids. Memorize the six strong acids [i.e. HCl, HBr, HI, H2SO4 (first proton only), HNO3, HClO4].
2. Know the rules for acid strength of weak acids. (See notes on Equilibrium and Acids/Bases)
a) binary acids
b) oxoacids
3. Non-metal oxides are ACIDIC and produce H3O+ in aqueous solution.
4. Know the strong bases (i.e. any compound that completely ionizes in solution to produce OH- REGARDLESS OF SOLUBILITY).
a) Group I (and heavier Group II) metal hydroxides [e.g. NaOH, KOH,
Ca(OH)2]
b) amide ion (NH2-)
c) hydride ion (H-)
d) methide ion (CH3-)
e) oxide ion (O2-)
f) nitride ion (N3-)
5. Metal oxides are BASIC and produce OH- in aqueous solution.
Example:
2002 Exam question 4(c):
“Solid cesium oxide is added to water.”
Cesium oxide is a metal oxide, producing OH- in aqueous solution.
Cs2O + H2O à 2 Cs+ + 2 OH-
6. Don’t forget!
a) An acid reacts with carbonates and bicarbonates to produce carbon dioxide and water.
b) An acid reacts with a sulfide to produce hydrogen sulfide (H2S) gas.
c) Ammonium ion reacts with hydroxide ion to produce NH3 and H2O.
7. Don’t forget! The reaction of a strong acid with a strong base will ALWAYS be:
H+ + OH- à H2O
Example:
2003 Exam question 4(d):
“A 0.02 M hydrochloric solution is mixed with an equal volume of 0.01 M
calcium hydroxide solution.”
H+ + OH- à H2O
(C) Precipitation Reactions
Know the rules for aqueous solubility:
(aka Survival Solubility Rules, covers 95% of the questions)
Generally soluble in water:
Compounds containing Na+, K+, NH4+, NO3-, CH3COO-, ClO3-
Generally insoluble in water (but soluble with alkali metals and ammonium ion):
Compounds containing the anions O2-, OH-, CO32-, PO43-, SO32-, CrO42-, S2-
Also know:
(1) 3 insoluble chlorides, bromides and iodides (in water):
e.g. PbCl2 (soluble in HOT water)
Hg2Cl2
AgCl
(2) 4 insoluble sulfates (in water): PbSO4
SrSO4
Hg2SO4
BaSO4
(3) 7 soluble sulfides (in water): Li2S MgS
Na2S CaS
K2S SrS
(NH4)2S BaS
(i.e. alkali and most alkaline earth cations and ammonium)
(D) Complex Ion Formation
Type indicator: Transition metal, aluminum, tin or lead and a common ligand.
Common ligands: chloride ion (Cl-), cyanide ion (CN-), hydroxide ion (OH-),
ammonia (NH3), thiocyanate ion (SCN-), water (H2O)
The coordination number (i.e. the number of ligands which attach to the metal ion) is
commonly TWICE the charge of the metal ion.
Complex ion formation is an example of a Lewis Acid/Base reaction.
Examples:
2003 Exam question 4(e):
“Excess concentrated aqueous ammonia is added to solid silver chloride.”
Silver is a transition metal with a 1+ oxidation state; ammonia is a common ligand.
AgCl + NH3 à Ag(NH3)+ + Cl-
2002 Exam question 4(d):
“Excessive concentrated hydrochloric acid is added to a 1.0 M solution of cobalt (II)
chloride.”
Cobalt is a transition metal with a 2+ oxidation state. Chloride ion is a common ligand.
Co2+ + 4 Cl- à CoCl42+
(E) Decomposition Reaction (usually 1 of the 8 reactions each year)
The most common decomposition reaction used is the decomposition of a metal carbonate with heat. Carbon dioxide and a metal oxide will be formed. In the case of a bicarbonate, carbon dioxide, water, and a metal carbonate will be formed.
Sometimes decomposition will be a REDOX reaction.
Example:
2003 Exam Question 4(c):
“A solution of hydrogen peroxide is exposed to strong sunlight.”
2 H2O2 à O2 + 2 H2O
(F) Combustion Reaction (usually 1 of the 8 reactions each year)
The most common application is the burning of a hydrocarbon or an organic compound in
air. Unless otherwise stated, assume complete combustion with carbon dioxide and water
as the only products.
NOTE: This type of problem is where your knowledge of organic chemistry is most
likely to be tested. Remember – each equation has two product points. Even if you don’t
know the formula of the reactant, knowing that it is a combustion reaction and indicating
carbon dioxide and water as products will score 2 out of 3 points on the equation!
Example:
2000 Exam Question 4(b):
“Butanol is burned in air.”
Assume you do not know the formula for butanol. The answer:
???? à CO2 + H2O scores 2 (out of 3) points
(The correct equation is C4H9OH + 6 O2 à 4 CO2 + 5 H2O)
(G) Combination (Synthesis) Reaction
It can be difficult to predict products for these types of reactions.
For reactions of metal and non-metal elements, the product will be the ionic compound
formed from the metal and non-metal.
Example:
2003 Exam Question 4(f):
“Magnesium ribbon burns in air.”
2 Mg + O2 à 2 MgO
For reactants containing only non-metals, try combining one molecule of each of the
reactants, and see if this gives a whole number multiple of a familiar compound. Keep in
mind what constitutes acidic and basic compounds. (See 8(B) above and “Notes on
Equilibrium and Acids/Bases”.)
Example:
2003 Exam Question 4(f):
“Solid dinitrogen pentoxide is added to water.”
N2O5 + H2O à H2N2O6
H2N2O6 is two nitric acid (HNO3) molecules! Nitric acid is a strong acid, so it will exist
as ions in an aqueous solution. So:
N2O5 + H2O à 2 H+ + 2 NO3-
- 4 -