MCAT Chemistry: What You Need to Know

MCAT Chemistry Study Tips to Improve Your Chem/Phys Score

To improve your MCAT chemistry score, focus on mastering high-yield topics, applying core concepts to passage-based questions, and using efficient test-taking strategies. Many strong test-takers rely on a small set of techniques that help them solve chemistry problems faster and avoid common mistakes.

Below are several practical tips that can help you prepare more effectively for the chemistry content tested in the Chem/Phys section. We’ll break down each strategy and show you how to apply it to real MCAT-style questions.

Use the 80/20 Rule to Focus on High-Yield MCAT Chemistry Topics

A relatively small set of foundational topics appears repeatedly across Chem/Phys passages and questions, while more obscure topics appear rarely.

One of our admissions experts, Dr. Jason Gomez, an MD/MBA from Stanford University who scored a 524 on the MCAT, emphasizes this point in our Chem/Phys webinar:

“80% of chemistry and physics questions are going to pull from around 20% of topics — so focus on foundational topics like kinematics, electrochemistry, and thermodynamics,” he says.

Many students waste valuable study time by treating every chemistry topic as equally important. The MCAT does not test chemistry that way. 

Certain topics appear frequently because they explain many of the chemical principles behind biological systems and experimental passages. High-yield MCAT chemistry areas include:

• Acid-base chemistry
• Thermodynamics
• Electrochemistry and redox reactions
• Chemical kinetics
• Stoichiometry and reaction calculations

Mastering these concepts helps you solve a wide range of MCAT questions because many passages rely on these same core ideas.

When you’re building your study schedule, prioritize high-yield topics so you don’t spend unnecessary time on lower-yield content. Our one-to-five-month MCAT study schedule templates can help you get started.

Maximize Your Points Per Minute by Checking Units Before Solving Calculations

Many MCAT chemistry questions involve calculations with multiple unit conversions, such as moving between:

• Moles
• Grams
• Liters
• Molarity
• Equilibrium constants

Students often jump straight into calculations. This can lead to unnecessary work and avoidable mistakes.

Dr. Gomez recommends a faster approach:

“Check units first,” he says. “You’d be surprised how many questions can be solved just by noticing a unit mismatch in the answer choices.”

Before you begin calculating anything:

1. Identify the equation being tested.
2. Identify the variables in the problem.
3. Look at the units required for the answer.

Often, two or three answer choices will have incorrect units. Eliminating those choices immediately improves your odds and saves time.

This strategy also helps prevent errors caused by mixing incompatible units during calculations.

Let’s Apply This Tip to a Real MCAT Chem Question

Suppose a question asks you to calculate a reaction rate constant, and the answer choices include:

A) mol/L

B) L/mol·s

C) J/mol

D) mol²/L²

If the rate law indicates a second-order reaction, the correct units must be L/mol·s.

Without performing any calculation, you can eliminate three answer choices immediately and select B.

Always check units before performing calculations. This simple step often saves time and eliminates incorrect answers instantly.

Derive Rate Laws by Identifying the Slow Step in a Reaction Mechanism

Reaction mechanisms and rate laws are one of the chemistry topics that students often find confusing on the MCAT. Many students memorize formulas for reaction rates but struggle when the exam asks them to determine a rate law from a multi-step reaction mechanism.

The MCAT isn’t testing memorization here. Instead, it’s testing whether you understand how reaction rates arise from the individual steps of a reaction.

The most important rule is: The slow step determines the rate law.

The slow step is also called the rate-determining step because it controls how quickly the overall reaction proceeds.

However, many students make a common mistake. They write the rate law directly from the slow step without checking whether the equation includes an intermediate species.

An intermediate is a molecule that forms during the reaction but does not appear in the overall balanced reaction. Rate laws cannot include intermediates. Instead, intermediates must be replaced using information from earlier steps in the mechanism.

When you see a reaction mechanism on the MCAT, follow this process:

1. Identify the slow step in the mechanism.
2. Write the initial rate law from that step.
3. Check whether the rate law contains an intermediate.
4. Use the fast equilibrium step to substitute the intermediate with the original reactants.
5. Rewrite the final rate law using only the original reactants.

This process ensures the rate law describes the overall reaction, not a temporary intermediate step.

Let’s Apply This Tip to a Real MCAT Chem Question

A reaction occurs through two steps:

Step 1: A₂ → 2A (fast)

Step 2: A + B → AB (slow)

What is the correct rate law?

A) Rate = k[A₂][B]

B) Rate = k[A₂]

C) Rate = k[A₂]²

D) Rate = k[A][B]

Answer: A

Because Step 2 is the slow step, you might initially write:

Rate = k[A][B]

However, A is an intermediate produced in Step 1 and does not appear in the original reactants. From Step 1:

A₂ ⇌ 2A

This relationship allows you to rewrite the rate law using the original reactant A₂. After substituting the intermediate, the correct rate law becomes:

Rate = k[A₂][B]

Always check whether the rate law contains an intermediate. The final rate law has to be written using only the original reactants.

Use Thermodynamic Sign Logic to Eliminate Wrong Answers Quickly

Thermodynamics questions on the MCAT often look like complex calculation problems. Students frequently try to plug numbers into equations immediately. That wastes time during the exam.

In many cases, calculations aren’t even necessary. For these questions, the MCAT tests whether you understand how thermodynamic variables determine reaction spontaneity.

The key relationship is the Gibbs free energy equation:

ΔG = ΔH − TΔS

Rather than calculating values, start by analyzing the signs of ΔH and ΔS. These combinations determine whether a reaction is spontaneous:

• ΔH < 0 and ΔS > 0 → spontaneous at all temperatures
• ΔH > 0 and ΔS < 0 → never spontaneous
• ΔH < 0 and ΔS < 0 → spontaneous at low temperatures
• ΔH > 0 and ΔS > 0 → spontaneous at high temperatures

When you see a thermodynamics question, ask three quick questions:

1. Is ΔH positive or negative?
2. Is ΔS positive or negative?
3. Does temperature matter?

Often, this reasoning eliminates multiple answer choices without performing any calculations.

Let’s Apply This Tip to a Real MCAT Chem Question

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

ΔH = −98 kJ/mol

Which statement about ΔG is correct?

A) ΔG < 0 at all temperatures

B) ΔG > 0 at all temperatures

C) ΔG > 0 only at high temperatures

D) ΔG < 0 only at low temperatures

Answer: A

The reaction is exothermic, so ΔH is negative.

Entropy increases because the reaction produces a gas from liquid reactants, meaning ΔS is positive.

Substituting into the Gibbs equation:

ΔG = ΔH − TΔS

A negative minus a positive value remains negative at all temperatures.

Recognize Buffer Conditions Quickly Using the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation frequently appears in MCAT questions involving buffers and acid-base chemistry.

The equation is:

pH = pKa + log([A⁻]/[HA])

While many students use it only for calculations, experienced test-takers use it as a reasoning shortcut.

When the concentrations of the conjugate base and weak acid are equal:

[A⁻] = [HA]

The logarithm term becomes:

log(1) = 0

This means:

pH = pKa

This condition represents the point where the solution has maximum buffering capacity, and it appears frequently in MCAT questions.

Recognizing this pattern allows you to answer many buffer questions immediately without lengthy calculations.

Let’s Apply This Tip to a Real MCAT Chem Question

A solution contains:

0.1 M acetic acid (Ka = 1.8 × 10⁻⁵)

0.1 M sodium acetate

What is the pH?

A) 2.3

B) 3.7

C) 4.7

D) 5.3

Answer: C

Acetic acid is a weak acid, and sodium acetate provides its conjugate base.

Because the concentrations are equal:

pH = pKa

pKa = −log(1.8 × 10⁻⁵) ≈ 4.7

If a buffer contains equal concentrations of a weak acid and its conjugate base, the pH equals the pKa.

To find out how well you’ll do on the chemistry section of the MCAT, take our free MCAT pop quiz.

FAQs: MCAT Chemistry

Is MCAT Chemistry Hard?

MCAT chemistry can feel challenging because the questions often require applying concepts to experimental passages, rather than simply recalling formulas or definitions. For example, you may need to interpret reaction mechanisms, analyze thermodynamics data, or predict how a buffer system affects pH in a biological context.

However, the chemistry content itself comes primarily from introductory general chemistry and organic chemistry courses. Students who understand the core principles, such as acid-base chemistry, thermodynamics, kinetics, equilibrium, and electrochemistry, usually perform well on this section.

How Much Chemistry Do You Need for the MCAT?

The MCAT tests chemistry mainly through the Chem/Phys section, which contains 59 questions and lasts 95 minutes. Chemistry concepts account for a significant portion of this section, alongside physics and some biochemistry.

Most of the chemistry tested comes from general chemistry and organic chemistry topics typically taught in undergraduate courses. Common areas include:

• Atomic structure and periodic trends
• Chemical bonding and intermolecular forces
• Stoichiometry and reaction calculations
• Thermodynamics and chemical equilibrium
• Acid-base chemistry and buffers
• Electrochemistry and redox reactions
• Basic organic chemistry concepts, such as functional groups and stereochemistry

You don’t need advanced chemistry beyond these core subjects. The MCAT emphasizes whether you can use chemistry concepts to interpret scientific passages, experiments, and biological systems.

For example, a passage may describe an enzyme reaction or metabolic process and ask you to apply ideas from thermodynamics, kinetics, or acid-base chemistry.

How Do I Do Well in the MCAT Chemistry Section?

To do well in the MCAT chemistry portion of the Chem/Phys section, master core concepts and practice passage-based questions rather than memorizing isolated facts.

Start by building a strong understanding of high-yield chemistry topics like thermodynamics, acid-base chemistry, electrochemistry, kinetics, and stoichiometry. 

Next, practice applying those concepts in MCAT-style passages. The exam often presents chemical principles within biological or experimental contexts, so learning to analyze graphs, reaction mechanisms, and data tables is essential.

Combining strong conceptual understanding with consistent practice is the most reliable way to improve your MCAT chemistry performance.

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Dr. Jonathan Preminger was the original author of this article. Snippets of his work may remain.