Equilibrium Sandbox

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How to use this lab

Start at equilibrium, predict a shift, then change one condition and watch the system respond.

Disturb the equilibrium and use the live concentrations to explain the shift.

  1. Disturb the reaction. Add or remove N₂O₄ or NO₂. Watch the concentrations jump and then settle to a new equilibrium.
  2. Change the volume. Compress or expand the vessel. Predict which direction is favored by the number of gas molecules.
  3. Change the temperature. The forward reaction is endothermic. Compare the new Kc with the value at 298 K.
  4. Explain the evidence. Compare Qc with Kc and use the live graph. Pause to inspect a change; Reset experiment starts fresh.
Try a compression

Reduce the volume from 5 L to 2 L. Both concentrations rise immediately, then the reaction shifts toward N₂O₄, the side with fewer gas molecules.

Download your data as CSV or print the lab. This idealized model uses accelerated time for learning.

N₂O₄(g) ⇌ 2 NO₂(g) ΔH° = +57.2 kJ/mol

02 / OBSERVE THE SYSTEM

The reaction vessel

VARIABLE-VOLUME REACTORIDEAL GAS MODEL

Blue atoms: N · amber atoms: O   /   Schematic molecule counts

Dynamic equilibrium

Equal rates do not require equal concentrations. Bars compare the two directions at this instant and are scaled to the faster rate. Use the numbers to compare across times.

Forward →
← Reverse

Both reactions continue at equilibrium. Rate indicators use an illustrative reversible rate law.

03 / FOLLOW THE RESPONSE

Concentration vs. time

━ N₂O₄   ┅ NO₂ · vertical dotted lines mark disturbances. Concentration (mol/L); time in simulated seconds.

About the model

Kc = [NO₂]²/[N₂O₄]. Qc uses the current concentrations. Qc < Kc favors products; Qc > Kc favors reactants. Equilibrium means equal forward and reverse rates, not equal concentrations.

Ideal-gas, reversible mass-action teaching model. Kc = 0.150 at 298 K; van ’t Hoff temperature dependence uses constant ΔH°. The illustrative rates are rforward = 0.25 Kc[N₂O₄] and rreverse = 0.25[NO₂]²; they are not measured kinetics. The rate law models one reversible elementary step, not a rule for all reactions. Simulated seconds are accelerated. Pressure uses P = nRT/V.

About this activity

Stress a reaction at equilibrium by changing concentration, volume or temperature. Watch the live concentration graph and compare Qc with Kc to predict which way the reaction shifts.

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