Chemical Equilibrium & Le Chatelier's Principle

What is Chemical Equilibrium & Le Chatelier's Principle?

Chemical equilibrium is the state where the forward and reverse reactions of a reversible process happen at the same rate. The concentrations of reactants and products stop changing — but the reactions don't stop. Both directions keep going at equal speed, like two escalators moving people up and down at the same rate. The position of that balance is captured by the equilibrium constant K, a single number that summarizes the ratio of products to reactants when nothing more is changing on the macroscopic scale. Le Chatelier's principle then predicts what happens when you disturb the balance: the system shifts to partially counteract the change. Add more reactant, products grow. Heat an exothermic reaction, equilibrium shifts toward reactants. Tune temperature, pressure, and concentration in this lab and watch the system rebalance in real time, with the on-screen rates and concentrations tracing the path back to a new equilibrium.

Parameters explained

[Reactant] added(M)

The [Reactant] added slider sets the starting reactant concentration used by the equilibrium calculation. Raising it gives the system more reactant particles to convert, so the live display recalculates the equilibrium amounts of reactant and product for the selected preset. Watch the particle counts, Q, K, and direction readout together: changing concentration changes Q first, then the system is interpreted relative to K. The Add Reactant button is a fast version of the same stress, increasing this slider by a fixed amount. Compare all three presets at the same concentration to see how stoichiometry and K change the final balance.

Temperature(K)

Temperature controls the Kelvin value used in the simulation's van't Hoff-style K adjustment. Because each preset has its own enthalpy sign, the same temperature move can have different meaning: the N₂O₄ ⇌ 2NO₂ preset is endothermic, while the FeSCN²⁺ and Haber presets are exothermic. As you move the slider, focus on whether K grows or shrinks and then use Q vs K to explain the displayed direction. This is the cleanest way to show the AP Chemistry rule that concentration and pressure can move Q, but temperature is the stress that changes the equilibrium constant.

System Pressure(atm)

Pressure sets the displayed system pressure in atmospheres for Le Chatelier discussion. Use it with the gas-phase presets to ask which side has fewer gas particles and predict the qualitative pressure shift before checking the sidebar rules or quiz. In this HTML model, the live K and Q calculation is driven by the selected preset, reactant amount, and temperature, so pressure is best treated as a reasoning prompt rather than a separate equilibrium solver. That distinction is useful: students can separate what the visual model calculates from the chemical rule they must apply on AP-style questions.

Common misconceptions

• MisconceptionAt equilibrium, the forward reaction stops because everything has converted to products.

  CorrectBoth reactions keep happening at equilibrium — they're just happening at the same rate. Concentrations stay constant, but molecules are constantly converting in both directions. That's why we call it dynamic equilibrium.

• MisconceptionAdding more reactant increases the equilibrium constant K.

  CorrectK only changes with temperature. Adding reactant shifts the position of equilibrium (more product forms) but K is the same. The reaction quotient Q is what changes — the system shifts until Q matches K again.

• MisconceptionA higher pressure setting always increases K and creates more products in the simulator.

  CorrectPressure can shift a gas equilibrium position when the two sides have different gas-particle counts, but it does not change K. In this HTML lab the Pressure slider is a discussion variable and readout; use the preset stoichiometry and Le Chatelier rule to predict the qualitative pressure effect.

• MisconceptionThe preset buttons only change colors, so the same K and temperature behavior apply to every reaction.

  CorrectEach preset loads a different reaction model with its own K, stoichiometric coefficients, enthalpy sign, labels, and particle colors. That is why the same slider move can produce different Q, K, and particle-count patterns across N₂O₄, FeSCN²⁺, and Haber cases.

How teachers use this lab

1. Preset comparison warm-up: load N₂O₄, FeSCN²⁺, and Haber in sequence with the same [Reactant] added value, then have students rank the cases by K and product formation using the live particle counts as evidence.
2. Q vs K reasoning drill: move the [Reactant] added slider or press Add Reactant, pause, and ask students to explain the displayed direction from Q and K rather than from memorized 'shift right' language alone.
3. Temperature investigation: keep [Reactant] added constant, move Temperature upward for each preset, and have students infer which reactions are endothermic or exothermic from whether K increases or decreases.
4. Pressure limitation discussion: use the Pressure slider with N₂O₄ and Haber to make qualitative Le Chatelier predictions, then explicitly separate the slider readout from the HTML model's K/Q calculation.
5. AP-style explanation practice: assign each group one preset and one slider change, then require a two-sentence claim that cites K, Q, stoichiometry, or enthalpy sign as the evidence.