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Chemistry · Rates and Equilibrium

Dynamic Equilibrium / Le Chatelier's Principle

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What you learned

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Key takeaway

Dynamic equilibrium means ongoing forward and reverse changes with matching rates.
A disturbance matters when the current mixture no longer matches the equilibrium target for the conditions.
Le Chatelier's principle describes the shift toward a new equilibrium after that disturbance.
The best equilibrium cue is the forward-reverse rate match, not equal reactant and product amounts.

Common misconception

Do not treat a flat amount graph as a stopped reaction or assume equilibrium requires equal amounts of reactants and products.

At equilibrium, the key condition is equal forward and reverse rates, not zero reaction activity.

From equilibrium to Le Chatelier shifts

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Reuse the particle-collision picture in a thermodynamics benchThermodynamics

Ideal Gas Law and Kinetic Theory

Connect pressure, volume, temperature, and particle number on one bounded particle box, then read the same pressure changes back as changes in particle speed and wall-collision rate.

Next in Collision theoryRates and Equilibrium

Reaction Rate / Collision Theory

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Temperature and Internal Energy

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Reference and support

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Equation snapshotRate balance snapshot · Equilibrium means equal ratesShow 3 equations

Use the rate gap first, then read the direction of the shift from the current mixture versus the equilibrium target.

Rate balance snapshot
$r_{net} = r_{f} - r_{r}$
The sign and size of r_f - r_r tell you which direction the mixture is still shifting and how far it is from balance.
Equilibrium means equal rates
$r_{f} = r_{r} \Rightarrow dynamic equilibrium$
Dynamic equilibrium is reached when the forward and reverse rates are equal, not when the reaction has stopped.
Shift to a new equilibrium
$new conditions \Rightarrow new equilibrium mix$
When conditions change, the equilibrium composition changes, so the system shifts until the forward and reverse rates match again.

Why it behaves this way

Explanation

Dynamic equilibrium does not mean the reaction has stopped. It means the forward and reverse reactions are still happening while the overall amounts look steady. Keeping the particle view, rate bars, and time graphs together makes that balance visible.

A disturbance changes the current mix or the conditions, so the forward and reverse rates no longer match. The system then shifts until it reaches a new equilibrium. Le Chatelier's principle is easiest to understand here as a visible rebalancing process, not a slogan to memorize.

Key ideas

01At dynamic equilibrium, the forward and reverse rates are equal even though microscopic forward and reverse changes continue.

02Changing the amounts or the product-favor setting disturbs equilibrium because the current mix no longer matches the equilibrium mix for those conditions.

03Le Chatelier's principle describes the shift toward a new equilibrium after a disturbance.

Worked examples

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Frozen walkthrough

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Frozen walkthrough

Use the live bench instead of a detached table. The same controls drive the particle view, the rate graphs, and the equilibrium target shown here.

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Example 1 of 2

Frozen valuesUsing frozen parameters

At the current moment, how do the forward and reverse rates compare?

t

Time

R_{0}

Starting reactant amount

P_{0}

Starting product amount

1. Read the current reactant and product amounts

At t = 0 s, the bench shows about 14 reactant units and 4 product units.

2. Compare the forward and reverse rates

The forward rate is about 3.14/s while the reverse rate is about 0.71/s.

3. Use the rate difference to judge how close the system is to equilibrium

That leaves a rate gap of about 2.42, which tells you how far the mixture still is from a balanced exchange.

Current rate balance

r_{f} \approx 3.14, r_{r} \approx 0.71

Forward change is still winning, so the mixture is moving toward more products.

Common misconception

Use this only when you want to pressure-test a mistaken intuition.

If the amounts stop changing, the reaction has stopped or the reactants and products must now be equal.

At equilibrium, the key condition is equal forward and reverse rates, not zero reaction activity.

Reactant and product amounts do not have to be equal at equilibrium. They only have to stay steady because the two rates match.

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Rates and Equilibrium

Dynamic Equilibrium / Le Chatelier's Principle appears later in this track, so it is cleaner to start from the beginning first.

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