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Introduction to equilibrium

At equilibrium a reaction looks frozen, but underneath, molecules are converting as fast in one direction as the other. Nothing has stopped — the rates have simply matched.

§1

A balance of rates, not a halt.

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A reaction reaches dynamic equilibrium when the forward and reverse reactions run at equal rates. Molecules keep converting both ways, so the reaction has not stopped — it only appears still because the two rates cancel.

Because forward and reverse balance, the concentrations stay constant over time. That constancy is the visible sign of equilibrium.

Constant does not mean equal. The equilibrium mixture can be mostly product, mostly reactant, or anything between — 'balance' refers to the rates, not to equal amounts of the two sides.

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
Fig. 7.1.1 At equilibrium a reaction has not stopped — forward and reverse reactions run at equal rates, so concentrations stay constant. 'Balance' means constant amounts, not equal amounts.
§2

Recognizing equilibrium.

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Look for equal rates and constant (not equal) amounts.

  1. Check that both directions run. Equilibrium is dynamic: the forward and reverse reactions both continue.
  2. Compare the rates. Equilibrium is reached when the forward rate equals the reverse rate.
  3. Note constant concentrations. At equilibrium, concentrations stay constant over time.
  4. Do not expect equal amounts. Constant amounts can be lopsided — mostly product or mostly reactant.
§3

The pieces you'll meet.

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A few ideas anchor equilibrium.

dynamic
Dynamic equilibrium
Forward and reverse reactions run at equal rates.
not stopped
Not stopped
Both reactions continue; only the rates cancel.
constant
Constant amounts
Concentrations stay fixed at equilibrium.
not equal
Not equal
Constant does not mean equal amounts of the two sides.
forward
Forward rate
Equals the reverse rate at equilibrium.
reverse
Reverse rate
Equals the forward rate at equilibrium.
§4

Worked example: is this equilibrium?

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Observation. A sealed flask of A and B shows unchanging concentrations, with lots of B and little A.

Dynamic? Concentrations are constant, which is consistent with equilibrium — but only if both reactions are still running (A → B and B → A), which they are.

Equal amounts? No. There is much more B than A, yet the system is at equilibrium: 'balance' means the rates are equal, not the amounts.

Conclusion. This is a dynamic equilibrium with constant, unequal amounts — the forward and reverse rates match even though B dominates.

§5

Mistakes that cost real points.

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Pitfall · 01

"At equilibrium, the reaction has stopped."

Equilibrium is dynamic: the forward and reverse reactions both keep running, at equal rates. Nothing stops; concentrations are constant because conversion in one direction is exactly offset by conversion in the other.

Fix. Picture equilibrium as two reactions running at matched rates, not as a halted reaction.

Pitfall · 02

"Balance means equal amounts of reactants and products."

Balance refers to equal forward and reverse rates, not equal amounts. An equilibrium mixture can be overwhelmingly product or overwhelmingly reactant, depending on the reaction. Constant is not the same as equal.

Fix. Read 'equilibrium' as constant concentrations from matched rates; the two sides need not be equal.

Pitfall · 03

"If concentrations are constant, no molecules are converting."

Constant concentrations at equilibrium come from conversion happening equally in both directions, not from conversion ceasing. Molecules are continually turning into products and back into reactants.

Fix. Attribute constant concentrations to equal, ongoing forward and reverse conversion, not to a lack of reaction.

§6

Skill Check.

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