2010 Test
Changes
Updates from
The AP College Board August 2010 have stated that the grading policy for Section
I will no longer be determined by the (1/4) points attempted and missed rule.
2007 Test
Changes
As described in
the new 2007, 2008 AP Chemistry Course Description, the AP Chemistry Exam will
have a new format beginning in May 2007. It is important to note that the
content covered by the exam will not change. The weighting of the two
major parts of the exam will change slightly -- Sections I and II will each
contribute 50 percent toward the final grade.
Section I (90 minutes) will not change and will still consist of 75
multiple-choice questions with broad coverage of chemistry topics. However,
there will be three primary changes in the format of Section II of the 2007
exam. While these changes do not introduce new content, students should be made
aware of these changes before taking the exam in May 2007.
The first change in Section II is that students will no longer be asked to
choose between alternative questions. All students will do the same six
questions: three problems, the first of which is an equilibrium problem;
question 4 (reactions); and two essay questions.
A second change in Section II relates to question 4, which assesses students'
knowledge about chemical reactions. Currently students are asked to write
chemical equations for five of eight given sets of reactants. In the new
question 4 format, all students will write balanced chemical equations
for three different sets of reactants and will answer a short question about
each of the three reactions.
The third change in Section II relates to the timing of Part A (during which
calculators are permitted) and Part B (when no calculators are permitted). In
Part A, students will have 55 minutes to answer three problems; in Part B,
students will have 40 minutes to answer question 4 and questions 5 and 6, the
two essay questions.
The table below summarizes the difference between the 2006 and 2007 AP Chemistry
Exam Section II formats. Note that the laboratory question can now appear in
Part A.
Table 1: A Comparison
of the 2006 and 2007 AP Chemistry Exam Section II Formats
One area of interest among AP teachers lies in
the reformatting of question 4; therefore, the remainder of this article will
present the reactions that appeared in the 2006 exam and show how these
reactions might be recast in the new 2007 format.
The current directions for question 4 are as follows.
4. Write the
formulas to show the reactants and the products for any FIVE of the laboratory
situations described below. No more than five choices will be graded. In all
cases, a reaction occurs. Assume that solutions are aqueous unless otherwise
indicated. Represent substances in solution as ions if the substances are
extensively ionized. Omit formulas for any ions or molecules that are unchanged
by the reaction. You need not balance the equations.
Example: A strip of magnesium is added to a solution of silver nitrate.
These instructions will be slightly modified in the 2007 exam booklet; the new
wording (added in boldface) will be very close to this:
4. For each of
the following three reactions, in part (i) write a BALANCED equation and in part
(ii) answer the question about the reaction. In part (i), coefficients should be
in terms of lowest whole numbers. Assume that solutions are aqueous unless
otherwise indicated. Represent substances in solutions as ions if the substances
are extensively ionized. Omit formulas for any ions or molecules that are
unchanged by the reaction.
Example: A strip of magnesium is added to a solution of silver nitrate.
(i) Mg + 2 Ag+ → Mg2+ + 2 Ag
(ii) Which substance is oxidized in the reaction?
Answer: Magnesium (Mg) metal
As shown, the new
directions for question 4 require the students to provide a balanced
chemical equation showing only the reacting substances and ask the student to
answer a question about the reaction. The majority of the credit for a response
will be earned for writing the balanced equation; the remaining credit will be
earned for correctly answering the question that follows each reaction, which
will require the student to focus more on the meaning of the reaction. For
instance, the example given above is an oxidation-reduction reaction. Ask
yourself what your students should know about a redox reaction. Here are some
questions that I hope most students would be able to answer:
·
Which substance is being oxidized?
·
Which substance is being reduced?
·
What is the change in the oxidation number of the magnesium?
·
What would you observe happening to the magnesium metal strip in
this reaction?
Any one of
these questions -- and you may be able to come up with more -- is representative
of what question 4 will include on the 2007 AP Chemistry Exam. Table 2 (below)
lists some sample questions -- suggested by members of the AP Chemistry
Development Committee -- that might be asked about the reactions that appeared
on the 2006 exam. Part (i) asks for the balanced chemical equation, omitting any
ions or molecules that are unchanged in the reaction; part (ii) asks one
question that focuses on such things as the type of reaction (redox,
proton-transfer, Lewis acid-base, and so forth), the stoichiometry of the
reaction, and the physical observations that one might expect as the reaction
occurs.
Table 2: Question 4
Reactions on the 2006 AP Chemistry Exam Recast in the 2007 NEW Format
|
(a) Solid potassium chlorate is strongly heated and decomposes,
resulting in a change in the oxidation numbers of both chlorine and
oxygen.
(i) 2 KClO3 → 2 KCl + 3 O2 |
(ii) What is the oxidation number of chlorine before and
after the reaction?
Answer: Chlorine has an oxidation number of +5 in
KClO3 and -1 in KCl. |
|
(b) Solid silver chloride is added to a solution of
concentrated hydrochloric acid, forming a complex ion.
(i) AgCl + Cl− → [AgCl2]−
|
(ii) Which species acts as a Lewis base in the reaction?
Explain.
Answer: The chloride ion acts as a Lewis base in
the reaction because it donates an electron pair. |
|
(c) A solution of ethanoic (acetic) acid is added to a solution
of barium hydroxide.
(i) HC2H3O2 + OH− →
H2O + C2H3O2−
|
(ii) Explain why a mixture of equal volumes of equimolar
solutions of ethanoic acid and barium hydroxide is basic.
Answer: In the mixture there are initially twice as
many moles of hydroxide ions as molecules of acid; since they react in a
1:1 ratio, there is an excess of hydroxide ions after the reaction is
complete, leading to the basic solution. |
|
(d) Ammonia gas is bubbled into a solution of hydrofluoric
acid.
(i) NH3 + HF → NH4+ + F−
|
(ii) Identify a conjugate acid-base pair in the reaction.
Answer: NH3 (base) and NH4+
(acid)
Or: HF (acid) and F− (base) |
|
(e) Zinc metal is placed in a solution of copper(II) sulfate.
(i) Zn + Cu2+ → Zn2+ + Cu |
(ii) Describe the change in color of the solution that occurs
as the reaction proceeds.
Answer: The blue color of the solution due to the
presence of the hydrated copper(II) ion fades as the copper(II) ion reacts
and the colorless hydrated zinc(II) ion forms. |
|
(f) Hydrogen phosphide (phosphine) gas is added to boron
trichloride gas.
(i) PH3 + BCl3 → H3PBCl3
|
(ii) Which species acts as a Lewis acid in the reaction?
Explain.
Answer: BCl3 acts as a Lewis acid in the
reaction because it accepts the non-bonded pair of electrons of the
phosphorus atom in PH3. |
|
(g) A solution of nickel(II) bromide is added to a solution of
potassium hydroxide.
(i) Ni2+ + 2 OH− → Ni(OH)2
|
(ii) Identify the spectator ions in the reaction mixture.
Answer: The spectator ions are the bromide ion (Br−)
and the potassium ion (K+). |
|
(h) Hexane is combusted in air.
(i) 2 C6H14 + 19 O2 → 12 CO2
+ 14 H2O |
(ii) When one molecule of hexane is completely combusted, how
many molecules of products are formed?
Answer: 1 molecule of hexane produces 13 molecules
of products. |
The examples given in the table represent one kind of question about each
reaction; there are many more such questions that can be asked, but recall that
these questions should not require the use of a calculator, which is not
permitted for Part B of Section II of the exam. If you ask your students to
write and answer a question about each reaction that they study, then they will
be very well prepared to tackle question 4 on the 2007 AP Chemistry Exam!
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