Mistake Master
Home› Unit 5 · Kinetics› 5.1· 5.2· 5.3· 5.4· 5.5· 5.6· 5.7· 5.8· 5.9· 5.10· 5.11

Introduction to Rate Law

▶︎  Watch it animatedinteractive step-through · ~3 min · optional

A rate law expresses the reaction rate as rate = k[A]m[B]n, where the orders m and n tell how the rate depends on each concentration and k is the rate constant. Crucially, the orders are found experimentally — by seeing how the rate changes when a concentration changes — not by copying the balanced equation's coefficients.

UNIT 5 TOPIC 5.2 • INTRODUCTION TO RATE LAW RATE LAW FINDER THE RATE LAW rate = k[A]m[B]n ★ Orders m and n are found EXPERIMENTALLY (from initial-rate data) — not from the balanced coefficients. Trial [A] (M) [B] (M) rate (M·s⁻¹) 1 0.10 0.10 2.0×10⁻³ 2 0.20 0.10 8.0×10⁻³ 3 0.20 0.20 1.6×10⁻² READING THE ORDERS OFF THE DATA Trial 1→2: [A] ×2, [B] fixed → rate ×4 2m = 4 → m = 2 Trial 2→3: [B] ×2, [A] fixed → rate ×2 2n = 2 → n = 1 rate = k[A]²[B] Overall order = m + n = 2 + 1 = 3 WHAT k IS k is the rate constant of the reaction. Its units depend on the overall order. Its value changes with temperature. ORDER SUMMARY [A] doubles → rate ×4 ⇒ m = 2 (2nd order in A) [B] doubles → rate ×2 ⇒ n = 1 (1st order in B) rate carries units M·s⁻¹ (set by overall order) CED ANCHOR Reaction orders (m, n) must be determined EXPERIMENTALLY from initial-rate data — they cannot be read off the coefficients of the balanced equation. AP Chemistry · Unit 5 · Kinetics
A rate law, rate = k[A]^m[B]^n, is found from experiment. Compare trials to see how the rate responds when a concentration changes: that response gives each reactant's order. Orders come from data, never from the balanced coefficients.
Rate Law Finder · Open the sandbox →

The two central traps: reading the orders off the coefficients (they must come from data), and misjudging how the rate responds to a concentration change (doubling [A] in a second-order term quadruples the rate). The experiment, not the equation, sets the rate law.

The work

3 ways in · any order
Lesson
Rate Law ›

A rate law's orders come from how rate responds to concentration in experiments, not from coefficients. The lesson finds orders from data, then closes with a ten-scenario check.

Skill check · 10 scenarios
Open lesson →
Diagnostic
10-item topic check ›

Ten items spanning the Topic 5.2 misconceptions: orders copied from coefficients, and misreading how the rate responds to a change in concentration.

Not started · 10 items · ~15 min
Start diagnostic →
Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first