Mistake Master
Home› Unit 7 · Equilibrium› 7.1· 7.2· 7.3· 7.4· 7.5· 7.6· 7.7· 7.8· 7.9· 7.10· 7.11· 7.12

Direction of Reversible Reactions

A reversible reaction can run in either direction, and comparing the reaction quotient Q — built from the current concentrations — to the equilibrium constant K tells you which way. If Q < K the system shifts right (toward products); if Q > K it shifts left; if Q = K it is at equilibrium.

UNIT 7 TOPIC 7.2 • DIRECTION OF REVERSIBLE REACTIONS Q VS. K COMPASS REACTION QUOTIENT Q aA + bB ⇌ cC + dD Q = [C]c [D]d [A]a [B]b Q uses current, non-equilibrium values. Compare Q to K to predict the direction of shift. DIRECTION DECISION Q < K ⇌ SHIFT RIGHT make products Q = K ⇌ NO SHIFT at equilibrium Q > K ⇌ SHIFT LEFT make reactants GASES CAN USE Kp Kp uses partial pressures For gases, brackets may be replaced by partial pressures. Pure solids and liquids are still omitted. Example: CaCO₃(s) ⇌ CaO(s) + CO₂(g) Kp = P(CO₂) DO NOT INCLUDE liquid solid Omit pure solids and pure liquids from Q and K expressions — their activity is fixed at 1. TAKEAWAY Compare Q with K. Q < K → products form (shift right). Q > K → reactants form (shift left). Q = K → net rates equal, so the system is at equilibrium (no shift). AP Chemistry · Unit 7 · Equilibrium
Comparing Q (built from current, non-equilibrium concentrations) to K tells you which way a reaction shifts. Q < K shifts right (toward products); Q > K shifts left (toward reactants); Q = K means the system is at equilibrium.
Rate Compass · Open the sandbox →

The traps assume a reaction runs one way only (reversible reactions go both), and pin Q to K (Q changes with current conditions and equals K only at equilibrium). Q is the moving indicator; K is the fixed target.

The work

3 ways in · any order
Lesson
Direction of Reactions ›

Comparing Q to K decides a reversible reaction's direction: QK left, Q=K at equilibrium. The lesson reads the Q/K compass, then closes with a ten-scenario check.

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

Ten items spanning the Topic 7.2 misconceptions: a reaction assumed to run one way only, and Q incorrectly pinned to the value of K.

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