Basics Free-Response Questions for AP Chemistry
Review the following concepts if necessary:
- Units and Measurements for AP Chemistry
- The Structure of the Atom for AP Chemistry
- Periodic Table for AP Chemistry
- Oxidation Numbers for AP Chemistry
- Compounds Nomenclature for AP Chemistry
- Writing Formulas and Transition Metals Nomenclature for AP Chemistry
- The radii of the iron cations are less than that of an iron atom, and Fe3+ is smaller than Fe2+.
- 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.
- The electron affinity of F is higher than the electron affinity of O.
- 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 of the solids adopt the same crystal structure.
Answers and Explanations
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 bulk of material. The graders look for certain keywords or phrases. The answers should not contain statements that contradict each other.
- The observed trend of radii is: Fe > Fe2+ > Fe3+. There is an increase in the effective nuclear charge in this series. As electrons are removed, the repulsion between the electrons decreases. The larger the effective nuclear charge, the more the electrons are pulled towards the nucleus and the smaller the atom or ion becomes. Give yourself 1 point for "effective nuclear charge," and 1 point for explaining the effective nuclear charge discussion.
- When moving across a period on the periodic table, the value of the effective nuclear charge increases with atomic number. This causes a general increase from Li to Be to B. This effective nuclear charge argument is worth 1 point.
- The effective nuclear charge in F is greater than the effective nuclear charge in O. This causes a greater attraction of the electrons. You get 1 point for this answer.
- Because all the solids adopt the same structure, the structure is irrelevant. The sizes of the anions are similar; thus, anion size arguments are not important.
The even higher value of Be (greater than B) is due to the increased stability of the electron configuration of Be. Beryllium has a filled s-subshell. Filled subshells have an increased stability, and additional energy is required to pull an electron away. Give yourself 1 point for the filled subshell discussion.
Two factors, other than structure, are important here. The two compounds with the highest lattice energies contain divalent ions (+2 or –2) while NaF contains univalent ions (+1 or –1). The higher the charge is, the greater the attraction between the ions is. The lattice energy increases as the attraction increases. You get 1 point for correctly discussing the charges.
The difference between the CaO and BaO values is because the larger the ion is, the lower the attraction is (greater separation). The lower attraction leads to a lower lattice energy. This size argument will get you 1 point.
Total your points. The maximum is 7.
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