(^66) › STEP 4. Review the Knowledge You Need to Score High
❯ Free-Response Question
Both authors have been AP free-response graders for years. Here is a free-response question for
practice.
You have 10 minutes to answer the following question. You may use a calculator and
the tables in the back of the book.
Question
Use the periodic table and other information concerning bonding and electronic structure
to explain the following observations.
(a) The radii of iron cations are less than those of iron atoms, and Fe^3 + is smaller than Fe^2 +.
(b) Write the electron configurations of Fe, Fe^2 +, and Fe^3 +.
(c) When moving across the periodic table from Li to Be to B, the first ionization energy
increases from Li to Be, then drops for B. The first ionization energy of B is greater
than that of Li.
(d) The electron affinity of F is higher than the electron affinity of O.
(e) The following observations have been made about the lattice energy and ionic radii of
the compounds listed below. Compare NaF to CaO, and then compare CaO to BaO.
All solids adopt the same crystal structure.
Compound
Ionic radius of
cation (pm)
Ionic radius of
anion (pm)
Lattice energy
(kJ/mole)
NaF 116 119 911
CaO 114 126 3,566
BaO 149 126 3,202
❯ Answer and Explanation
Notice that all the answers are very short. Do not try to fill all the space provided on the
exam. You score points by saying specific things, not by the amount of material. The grad-
ers look for certain keywords or phrases. The answers should not contain statements that
contradict each other; otherwise, there may be a penalty. Contradictions most commonly
occur when the student tries to say too much. On the AP exam, the different parts of the
free-response questions tend to be more diverse than this one, as this question focuses only
on this chapter, whereas the AP free-response questions focus on the entire course.
(a) The observed trend of radii is Fe > Fe^2 + > Fe^3 +. There is an increase in the effective
nuclear charge in this series. As electrons are removed, the repulsion between the
remaining electrons decreases. The decrease in electron repulsion leads to an increase in
the effective nuclear charge. The greater the effective nuclear charge, the greater the
attraction of the electrons toward the nucleus and the smaller the atom or ion becomes.
- A—Separating materials in solution is normally
not simple; therefore, removing the solvent
through evaporation is the best choice. - A—Fluoride, F-, yields hydrofluoric acid, HF. B
refers to NO 3 - and HNO 3. C refers to SO 32 - and
H 2 SO 3. D refers to S^2 - and H 2 S.
26. C—Weight depends upon the mass of an object
and gravitational attraction. Weight varies as
gravity varies. Mass, on the other hand, does not
depend on gravity. Mass is fixed, which is why
there is a Law of Conservation of Mass but not
a Law of Conservation of Weight.