Look, if you’re staring at a stack of past exams, the 2018 AP Chem FRQ probably feels like just another hurdle. But it’s legendary. Not necessarily because it was the hardest—2014 or 2019 might take that crown depending on who you ask—but because of how it balanced pure math with conceptual "why" questions. It was a wake-up call. Students who memorized formulas got crushed, while those who actually understood molecular behavior walked away with 5s.
Honestly, the 2018 set is a masterclass in College Board’s favorite trick: taking a simple concept and wrapping it in a terrifying, unfamiliar scenario. You’ve got to see through the noise.
The Na2S2O3 Nightmare: Kinetics and Stoichiometry
Question 1 was a beast. It started with sodium thiosulfate ($Na_2S_2O_3$), which sounds intimidating if you haven't seen it much in lab. This wasn't just "plug and chug." You had to navigate the oxidation state of sulfur, which is notoriously tricky in thiosulfate because the two sulfur atoms aren't in the same environment.
Most people tripped up on the part asking about the rate law. You were given a graph of $1/[S_2O_3^{2-}]$ versus time. If that graph is a straight line, it’s second order. Simple, right? But then they hit you with the "explain" part. You couldn't just say "it's linear." You had to link the linearity specifically to the integrated rate law for a second-order reaction.
Why Question 1 Matters Now
Kinetics is the heart of the 2018 AP Chem FRQ. If you can't distinguish between a zero, first, and second-order plot, you're toast. The College Board loves using $Na_2S_2O_3$ because it’s a classic "iodometric titration" reagent. Even if the titration wasn't the focus here, the sheer bulk of the molecule often scares students into overcomplicating the stoichiometry. Just keep it simple. Watch the coefficients. They’ll save your life.
The Logic of Le Châtelier in Question 2
Question 2 moved into the world of equilibrium and thermodynamics. Specifically, it dealt with the decomposition of $FeC_2O_4$, iron(II) oxalate. This is where things got weirdly specific. You had to calculate the pressure of $CO_2$ gas.
A lot of students forgot that solids don't appear in the equilibrium constant expression ($K_p$). It’s a rookie mistake, but under the pressure of the clock, it happens to the best of us. If you included the iron oxalate in your $K_p$ calculation, the whole thing fell apart like a house of cards.
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The Temperature Trap
There was a part about how $K_p$ changes with temperature. It’s not enough to say "it goes up." You had to use the sign of $\Delta H$ to justify it. If the reaction is endothermic, adding heat is like adding a reactant. The system shifts right. $K_p$ increases. If you didn't mention the direction of the shift and the specific impact on the ratio of products to reactants, you lost the point. No partial credit for "vibes."
The Absorbance and Beer's Law Confusion
Question 3 jumped into spectroscopy. I’ve seen so many students get confused by the $CuSO_4$ questions. You were looking at absorbance versus concentration.
The question asked what happens if there are fingerprints on the cuvette. Think about it. Fingerprints scatter light. If the detector sees less light because your oily thumb got in the way, it thinks the solution absorbed that light. The reported absorbance goes up. Consequently, your calculated concentration is higher than it actually is. It’s a practical lab question that tests if you’ve actually held a cuvette or if you’ve just read about them in a textbook.
The "Middle" Questions: Where Points Go to Die
Questions 4 through 7 are shorter, but they’re fast. They’re "sprints." In the 2018 AP Chem FRQ, Question 4 dealt with the structure of $CS_2$ versus $COS$.
You had to draw Lewis structures. If your $COS$ structure didn't show the difference in electronegativity between Oxygen and Sulfur, you probably missed the nuance of the dipole moment. $CS_2$ is linear and nonpolar because the dipoles cancel out. $COS$ is linear but polar because Oxygen pulls harder on those electrons than Sulfur does. It’s basic, but under pressure, people forget that "linear" doesn't always mean "nonpolar."
Chromatography and Polarity
Then came the paper chromatography. This is another area where the 2018 exam tested "lab feel." You had to identify which dye was most polar based on how far it traveled up the paper.
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- Stationary phase: The paper (usually polar).
- Mobile phase: The solvent (you have to check if it's polar or nonpolar).
If the solvent is nonpolar and the dye stays at the bottom, the dye is polar. It "likes" the paper more than the solvent. It’s all about "like dissolves like." If you can’t visualize the intermolecular forces (IMFs) acting between the dye and the solvent, you’re just guessing.
The Acid-Base Gauntlet
Question 5 was about $HF$ and $F^-$. Weak acids are the bread and butter of the FRQ section. They asked about the $pH$ of a buffer solution.
The most common error? Forgetting the Henderson-Hasselbalch equation:
$$pH = pK_a + \log \left( \frac{[base]}{[acid]} \right)$$
But wait. The 2018 exam asked what happens when you add $NaOH$. You’re neutralizing some of the $HF$ and creating more $F^-$. You have to do the "before and after" stoichiometry before you touch the equilibrium math. If you just plugged the initial concentrations into the $K_a$ expression after adding base, you got the wrong answer. You have to account for the reaction that goes to completion first.
Misconceptions That Still Trip People Up
Looking back at the data from 2018, there were three big areas where students consistently lost points.
- Confusing Intermolecular Forces with Intramolecular Bonds. When something boils, you aren't breaking the bonds between atoms. You're breaking the forces between molecules. In Question 4, many students suggested that $S-C$ bonds were breaking. No. Just no.
- Significant Figures. The College Board is picky. If the data has three sig figs, your answer better have three (or maybe two or four, they usually give a $\pm 1$ tolerance). But if you write down eight digits from your calculator, you’re throwing away easy points.
- Vague Explanations. Writing "the molecules are closer together" isn't enough. You need to say "the London Dispersion Forces are stronger due to a more polarizable electron cloud." Specificity is the difference between a 3 and a 5.
Strategies for Conquering These Types of Questions
If you’re practicing with the 2018 AP Chem FRQ, don't just check the answer key. Read the "Scoring Guidelines." The College Board publishes exactly what phrases they were looking for.
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The "Claim, Evidence, Reasoning" Method
For every "explain" or "justify" prompt, use the CER method.
- Claim: Answer the question directly (e.g., "The boiling point will increase").
- Evidence: State a fact from the data provided (e.g., "Compound X has a larger molar mass and more electrons than Compound Y").
- Reasoning: Link the evidence to the claim using a chemical principle (e.g., "A larger electron cloud is more polarizable, leading to stronger London Dispersion Forces, which require more energy to overcome").
Handling the Math
The math in the 2018 exam wasn't actually that hard. The "heavy" math—like the Nernst equation or complex titration curves—was mostly absent or simplified. The struggle was in the setup.
When you see a multi-part question, the answer to part (a) is almost always used in part (b). If you get stuck on (a), make up a reasonable number and use it for (b). The graders often use "error carried forward" rules. They won't penalize you twice for the same mistake if your logic in the second part is correct based on your (wrong) first number.
Why 2018 Was a Turning Point
Before 2014, the AP Chem exam was very "calculator heavy." It was about how fast you could crunch numbers. Since the redesign, and especially in the 2017-2019 era, the focus shifted toward "particulate-level representations."
You'll notice in the 2018 AP Chem FRQ that there are several places where you have to think about what the atoms are doing. Whether it's drawing particles in a box or explaining why a certain ion is larger than another, the exam wants to see if you have a "mental movie" of the chemistry happening.
Actionable Next Steps for Mastery
Don't just read this and move on. To actually benefit from the 2018 exam's lessons, do this:
- Timed Practice: Sit down and do the 2018 FRQ 1-3 in 45 minutes. No distractions. No phone. Use the periodic table and formula sheet provided by the College Board—not your textbook's version.
- The "Blank Paper" Retest: If you miss a question, don't just read the solution. Wait two days, then try to solve that exact same question on a blank sheet of paper. If you can’t do it from scratch, you didn't learn it; you just recognized the answer.
- Identify Your Weak Link: Did you struggle with the $Na_2S_2O_3$ kinetics? Go back to Unit 5. Was it the $HF$ buffer? That’s Unit 8. Use the 2018 exam as a diagnostic tool to find the holes in your knowledge.
- Check the Chief Reader Report: This is a "secret" document the College Board releases. It's literally the person in charge of grading telling you what thousands of students got wrong. For the 2018 exam, the Chief Reader noted that students really struggled with explaining the physical basis for periodic trends. They could name the trend, but not explain the "Coulombic attraction" behind it.
The 2018 AP Chem FRQ isn't just a ghost of exams past. It's a roadmap. It shows you exactly how the College Board tries to trip you up with lab scenarios and "simple" concepts. Master these questions, and the actual exam will feel like a victory lap.