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Introduction to Equilibrium

A reaction reaches dynamic equilibrium when the forward and reverse reactions run at equal rates. The reaction has not stopped; both directions continue, so the concentrations stay constant even though molecules keep converting. Equilibrium is a balance of rates, not a halt.

UNIT 7 TOPIC 7.1 • INTRODUCTION TO EQUILIBRIUM DYNAMIC BALANCE 1. START: MOSTLY A A A A A A A A A B A A A A B A Forward reaction is faster at first: A turns into B. 2. EQUILIBRIUM: A AND B A B A B A B A A B B A B B A B Both directions still occur, but the forward and reverse rates are equal. 3. DYNAMIC, NOT STOPPED rate time forward reverse Rates become equal and then stay equal — concentrations hold constant. CHEMICAL EQUILIBRIUM N₂O₄(g) ⇌ 2 NO₂(g) Reactants and products are both present; the system may favor either side. PHYSICAL EQUILIBRIUM H₂O(l) ⇌ H₂O(g) In a closed container, evaporation and condensation continue at equal rates. THE TAKEAWAY Equilibrium means the forward rate equals the reverse rate. The system stays dynamic, and concentrations stay constant only after the two rates become equal — constant, not zero, not equal amounts. AP Chemistry · Unit 7 · Equilibrium
At equilibrium a reaction has not stopped — the forward and reverse reactions run at equal rates, so concentrations stay constant. 'Balance' means constant amounts, not equal amounts: the equilibrium mixture can be mostly product, mostly reactant, or anything between.
Dynamic Balance · Open the sandbox →

The two foundational traps are thinking equilibrium means the reaction has stopped (it is dynamic, still running) and thinking 'balance' means equal amounts of reactant and product (it means constant, not equal). The equilibrium mixture can be mostly one side.

The work

3 ways in · any order
Lesson
Dynamic Equilibrium ›

Equilibrium is equal forward and reverse rates — dynamic, not stopped — with constant but not necessarily equal amounts. The lesson builds the dynamic picture, then closes with a ten-scenario check.

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

Ten items spanning the Topic 7.1 misconceptions: equilibrium read as the reaction stopping, and 'balance' read as equal amounts of reactant and product.

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