or basic. Learning the characteristics allows you to compare and
contrast multiple species, predict reactivity, and balance reac-tions… while memorizing the concept does not.
Testing cumulative knowledge
Many chemical concepts and chemistry problem-solving skills
are cumulative. Core skills — such as writing balanced equations,
using the unit-factor method, and converting chemical names to
chemical formulas — are often required for every test, not just
the first. In fact, you could be penalized for failing to grasp skills
or concepts that were either taught early in the course or are
expected to be remembered from previous courses.
For instance, to predict likely intermolecular forces for a molecule, you need to understand covalent bonding, Lewis structures, and polarity. The intermolecular forces are used to predict
physical properties such as boiling and melting points. Many
disciplines have cumulative concepts, so this is not unique to
Math- and logic-intensive
Chemistry requires you to integrate concrete math skills with
very abstract chemical concepts. Even in organic chemistry and
other courses that never require you to use a calculator, you need
to use reasoning that is similar to that found in mathematics and
computer science. While it’s generally true that students who
enjoy math will have an easier time in math-intensive courses,
such as general or physical chemistry, it is even more likely that
students who enjoy working through thought problems with a
progression of logical arguments will enjoy greater success on
chemistry exams than those who don’t.
Many chemical concepts and problems are multistep in nature.
An error early in the process will quickly lead to an incorrect
answer. Drawing the Lewis structure for a polyatomic ion is a
great example. You might have to first convert the name of the
ion into a formula, then determine or know the charge of the
ion, count the number of valence electrons, arrange the skeletal
structure with the correct atom in the center, subtract the correct
number of electrons for the skeletal structure, add the remaining
electrons to the outer and central atoms, and then draw multiple
bonds if needed. Then you check the structure for an expanded
octet and draw resonance forms if necessary.
If you skip any of the steps or perform them incorrectly, the
entire process fails to yield a correct structure. Drawing an accurate Lewis structure is the cornerstone of valence bond theory
and allows for prediction of the most likely geometry, bond
angles, and hybridization in molecules and ions. Failing to learn
how to perform each step in a multistep process could have far-reaching consequences in general chemistry and throughout your
Chemistry professors tend to write exams similar to the ones
they took as undergraduates and as graduate students. This
results in tests that require simultaneous mastery of both meticulous detail and broad basic concepts.
You might encounter a test on ionic bonding that requires you
to produce a Born–Haber cycle for a given ionic compound. To
solve the problem, you must understand the concept of a Born–
Haber cycle in enough detail to balance each equation in the
cycle, include correct states on each reactant and product, and
provide the correct name for each step’s enthalpy change. Attention to detail is required to write accurate, blanaced equations
and for the correct manipulation of enthalpy changes to calculate
an unknown. And yes, significant figures count.
A similar problem involving a Born–Haber cycle for a very
simple compound like NaCl was probably shown in a lecture, or
assigned as a homework problem. However, on a test the student
is often expected to extend the concept to a more difficult compound like Al2O3 (three ionization energies required, not just one,
etc.). Not only will a professor choose a more difficult compound;
they might also ask a student to solve for an enthalpy change in
a different part of the cycle. Examples in lecture may have shown
the solution for the lattice enthalpy, whereas the test gives lattice enthalpy and asks the student to solve for ionization energy
or electron affinity. For full credit, you must immediately extend
what was learned in lecture and apply chemical knowledge to a
novel problem, all while paying careful attention to details.
You’re also expected to understand the big picture behind
the details. To expand on the last example, lattice enthalphy can
be used as an objective way to compare ionic bond strength in a
variety of ionic compounds. You might be required to go beyond
simply solving a mathematical problem— perhaps applying the
results of a Born-Haber cycle calculation to compare the strength
of bonding in a series of compounds or simply understanding
that lattice enthalpy is greater in ionic compounds with higher
charge-to-size ratios because of the steps involved in the cycle.
Your professor might expect you to know why lattice enthalphy
is a better measure of ionic bonding strength than another criterion, such as enthalpy of formation. Notice that a professor can
test one concept on a variety of levels, none of which requires
Preparation, practice, payoff
Although chemistry examinations might be somewhat different
from examinations in other subjects, they are not impossible to
master. Prepare well ahead of time, work plenty of practice problems, vary your study methods, and follow up when the test is
returned. You will not only master the concepts that will help you
in the short term on your next test, but you will also master
important concepts and skills that will be required in subsequent
Carl “Burt” Hollandsworth is an assistant professor of
chemistry at Harding University in Searcy, AR.