Most Common Mistakes Seen by AP Readers on the AP Chemistry Exam

Most common mistakes seen by AP Readers on the AP Chemistry Exam

1. By far, the #1 mistake deals with the Gibbs-Helmholtz equation, specifically, making sure that the units of energy are the same. In practically every thermodynamics table, entropy is in units of joules while G and H are in kilojoules! If unit conversions are not made, not only will this produce an incorrect answer if the student is solving for G, but if there are subsequent questions dealing with a change of temperature, like asking about spontaneity (whether or not it is “thermodynamically stable”), the student could get this problem wrong as well.

2.Although this problem should have been corrected in any first-year chemistry course, too many students still forget that - when doing gas laws problems - temperature must be recorded in Kelvin.

3.Like the gas laws problem listed above, another problem in thermodynamics is when students forget that temperature must be in Kelvin.

4.While the current use of the phrase “thermodynamically more (or less) stable” seems to be helping, students continue to have difficulty understanding the significance of the sign notation for enthalpy, entropy, and Gibb’s free energy, especially entropy.

5.It may be a moot point now because of the change in the chemistry curriculum, but if a student is given a buffer solution and asked to calculate the new pH if some acid (or base) is added, when using the Henderson-Hasselbalch equation, most students know to change the amount of one variable (acid/base conjugate pair) but forget to change the other. For example, given 1.0 Liter of a buffer solution containing 1.0 mole of HF and KF. If 0.40 moles of KOH are added, most students know to decrease the moles of the acid by 0.40 moles, but they forget that they have also generated 0.40 moles of the conjugate base.

6.Also in acid-base equilibrium, when using the “I/C/E box” set-up, it is scary how many students can solve for “x” in the equation, but if it’s a basic solution, they forget that “x” is the [OH1-], so they need to solve for the pOH before finding the pH.

7.Same goes for salt hydrolysis… For example, if they are given a 0.20-molar solution of KF they should know that this produces a basic solution and they should be able to write the net ionic equation to show that a basic solution is produced, but when they solve for “x”, they forget that they have solved for [OH1-] and not for [H3O1+].

8.For limited solubility, the relationship between Ksp and molar solubility

(x2, 4x3, 27x4,256x5, and 108x5) can only be used when the students are dealing with a

saturated solution; ie. if it is a “common-ion effect” problem, there are really an infinite number of combinations of the cation concentration as compared to the anion concentration.

9.Buffers are some of the most difficult problems – even the theory. Too many students don’t get the idea that adding some water to a buffered solution doesn’t change the pH…or at least they can’t properly describe why it doesn’t change the pH. Same is true if the volume changed; ex. cut in half.

10.Electrochemistry – students absolutely have to know what happens in aqueous solutions at both the anode and the cathode. Specifically, they need to know that water can be reduced and/or oxidized,depending on what other species are present. As possible lab questions, they need to be able to explain how to ID the species formed.

11.Electrochemistry – voltaic cells. Make sure they know EXACTLY what a salt bridge does.

[Hint: it does NOT allow the electrons to flow completely through the circuit!]

12.Even though there are only about six complexation reactions, these reactions give students a fit. Trying to figure out how adding an aqueous solution of NaOH to a saturated solution of aluminum hydroxide can increase the solubility is beyond some of them. Or even more difficult, how adding some acid to the above reaction cause the equilibrium to shift to the right.

13.Different values for “R” While “R” is given both in terms of atmospheres, kilopascals, (and joules), too often, for equilibrium and for thermodynamics, the unit should be in kilojoules.

14.After two years, how do they STILL not know which elements exist at standard conditions in their diatomic form?! And how do they not know their polyatomic ions either?!

15.Even though the new format doesn’t have a specific question for the prediction of reactions, the students need to be able to predict the products of a chemical reaction…and how to test for the identity of those products. That also means that knowing the few solubility rules would be a good idea. Showing students the relationship of the lattice energy equation to the solubility rules is a worthwhile exercise.

16.Aqueous ammonia – really causes problems because it can be used for hydroxide ion, ammonia gas, ammonia as a ligand, or the ammonium ion (conjugate acid).

17. Under the category of reaction prediction, 1) forgetting to separate a compound into its ions if it is a strong electrolyte; 2) forgetting to remove spectator ions.

18.In the equilibrium expression, adding liquid water as part of the expression (or any pure solid or liquid, for that matter!)

19.Using the term “bonds” or “bonding” when dealing with IMF – they’re “forces” – no electrons are being transferred and/or shared!

20.Using a “mass” argument (vs. number of electrons) when trying to explain why molecular iodine is a solid while molecular bromine is only a liquid.

21.In the VSEPR theory, the electron pairs (lone or bonded) are the cause of the shape – not the atoms themselves.

22.In electrochemistry, not knowing that voltage (emf) is an intensive property, which means it is not affected (ie. multiplied by coefficients) when reacting a metal that loses 3 electrons while another metal gains 2 electrons (as an example). Also, for the above example, saying that 5 electrons are involved in the redox reaction as part of the Nernst equation.

23.How can students not know the most common oxidation states of common elements / ions??

24.Resonance – as an example, for the carbonate ion, an X-ray of this ion would show that the C-O bond length is exactly the same at all 3 sites, even though students insist that there is a double bond and two single bonds. Students need to explain that the delocalized pi bond strengthens all three bond sites equally.