MCAT topics → Thermodynamics and Kinetics → Reaction Rates and Rate Laws

Reaction Rates and Rate Laws

MCAT trap: Reads reaction orders directly from stoichiometric coefficients instead of experimental data. Reaction orders must be determined experimentally and are independent of stoichiometric coefficients.

Reaction rates and rate laws show up constantly on the MCAT, and they're tested at every level — from straightforward recall of what a rate law looks like, to applying the method of initial rates to a data table buried in a passage. The core idea is simple: rate = k[A]^m[B]^n, where m and n describe how sensitive the reaction rate is to each reactant's concentration. But the exam consistently exploits two traps: students who read orders off the balanced equation instead of the data, and students who can't systematically extract orders when the passage gives them a kinetics table.

What makes this topic tricky is that it looks easier than it is. The math is accessible, but the conceptual discipline required is high. The MCAT will give you a table with three experiments and ask you to identify the rate law — that's a procedure question dressed up as a data question. You need to isolate variables: find two rows where only one concentration changes, take the ratio of rates, and solve for the exponent. If you don't have that procedure locked in, you'll guess or waste time.

The stoichiometry trap is the other killer. Students who memorize that rate laws 'come from the mechanism' often still default to reading coefficients off the balanced equation under time pressure. The right mental model: the balanced equation tells you nothing about the rate law. Orders are always experimental. Separately, when the exam asks you to relate the disappearance rate of one species to another, you must account for stoichiometric coefficients — if B is consumed twice as fast as A, you divide d[B]/dt by 2 to get the unified reaction rate.

Common misconceptions

Common mistake
Wrong: Reaction orders in the rate law equal the stoichiometric coefficients in the balanced equation.
Right: Reaction orders must be determined experimentally and are independent of stoichiometric coefficients.
The balanced equation tells you the overall stoichiometry of a reaction, not the mechanism. Rate laws reflect the mechanism — specifically the slowest step — which can look completely different from the overall equation. For example, the reaction 2NO2 → 2NO + O2 has a rate law of Rate = k[NO2]^2, which happens to match the coefficient, but that's a coincidence. For most reactions, the orders bear no predictable relationship to stoichiometric coefficients. Always treat orders as unknowns to be solved from experimental data.
Common mistake
Wrong: The rate of disappearance of all reactants is equal regardless of stoichiometry.
Right: Rates of disappearance and appearance must be divided by stoichiometric coefficients to give a single reaction rate (e.g., for A + 2B → C, rate = -d[A]/dt = -½d[B]/dt).
When a reaction consumes species at different rates due to stoichiometry, you can't just set those rates equal. For A + 2B → C, B disappears twice as fast as A — so -d[B]/dt is twice -d[A]/dt. To express one unified 'reaction rate,' you divide each species' rate by its stoichiometric coefficient: rate = -d[A]/dt = -(1/2)d[B]/dt = +d[C]/dt. Think of it like unit conversion — the coefficients are the conversion factors between individual species rates and the overall reaction rate.
Common mistake
Gap: Lacks a systematic procedure for extracting reaction orders from initial-rate data tables
To determine reaction order for a reactant, compare two experiments where only that reactant's concentration changes and calculate the ratio of rates to solve for the exponent.
The method of initial rates is a three-step procedure: (1) Find two experiments where only the reactant you care about changes concentration — everything else stays constant. (2) Write the ratio of the two rate equations; all constant terms cancel, leaving (rate1/rate2) = ([A]1/[A]2)^m. (3) Solve for m algebraically or by inspection. If doubling the concentration doubles the rate, m = 1. If it quadruples the rate, m = 2. If it has no effect, m = 0. Once you have all orders, plug any one experiment's numbers back into the rate law to solve for k.
Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →
Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

  1. Defining reaction rate as the change in concentration over time, and correctly relating the rates of individual reactants and products using stoichiometric coefficients to express a single overall rate.
  2. Writing and interpreting the rate law (Rate = k[A]^m[B]^n), understanding that the exponents m and n are determined experimentally — not from the balanced equation — and knowing what each term represents.
  3. Using the method of initial rates: given a table of experiments with varying concentrations and measured initial rates, isolating one reactant at a time to calculate its order and then solving for the rate constant k.
  4. Extracting reaction orders and rate constants directly from concentration-versus-rate data tables presented in a passage, including cases where orders are zero, fractional, or non-integer.

Can you avoid these mistakes?

A reaction A + 3B → products has these initial rate data: Exp 1: [A]=0.1M, [B]=0.1M, rate=2×10⁻³ M/s; Exp 2: [A]=0.2M, [B]=0.1M, rate=4×10⁻³ M/s; Exp 3: [A]=0.2M, [B]=0.3M, rate=4×10⁻³ M/s. What is the rate law, and what is k?
For the reaction 2H2 + O2 → 2H2O, a student writes Rate = k[H2]²[O2]. What assumption is this student making, and why might it be wrong?
For the reaction N2 + 3H2 → 2NH3, if H2 is disappearing at 0.06 M/s, what is the rate of disappearance of N2 and the rate of appearance of NH3?
A reaction is found to be zero order in reactant B. If you triple [B] while keeping everything else constant, what happens to the reaction rate? What does this tell you about B's role in the rate-determining step?

Related topics

First Law of Thermodynamics and Internal Energy Enthalpy and Hess's Law Calorimetry and Heat Capacity Entropy and the Second Law

See how your Anki deck covers this topic.

Upload your deck for a free audit →