Chemistry ★★☆ Medium

⚗️ Chemical Equilibrium

Explore how temperature, enthalpy, entropy and initial concentrations determine whether a reaction “goes”. Watch the Gibbs free energy G(Q) curve find its minimum at equilibrium, animate ICE concentration bars, and verify Le Châtelier’s principle live.

a A  +  b B  ⇌  c C  +  d D

Thermodynamics

Stoichiometry & conc.

Equilibrium results

ΔG° (kJ/mol)—
K (T)—
Q at start—
Extent ξ(eq)—
Δn gas—
K_p / K_c ratio—
Reaction favoured—

Chemical Equilibrium Theory

Gibbs Free Energy & Equilibrium

At constant T and p the reaction quotient Q evolves until ΔrG = ΔrG° + RT ln Q = 0, i.e. Q = K. The G(Q) curve has a minimum exactly at K. For Q < K the reaction proceeds forwards (ΔrG < 0); for Q > K it reverses. K = exp(−ΔG°/RT) quantifies how far a reaction lies to the right at equilibrium.

Le Châtelier’s Principle

If a system at equilibrium is perturbed (add/remove reactant or product, change T or p), it shifts to counteract the change. Increasing T favours the endothermic direction (shifts K). Increasing total pressure favours the side with fewer moles of gas (Δn = c + d − a − b for all-gas reactions). Adding inert gas at constant volume does not shift equilibrium.

van’t Hoff Equation

d ln K / dT = ΔH° / RT². Integrating: ln(K2/K1) = −ΔH°/R · (1/T2 − 1/T1). A plot of ln K vs 1/T is linear with slope −ΔH°/R, allowing enthalpy extraction from temperature-dependent equilibrium data — widely used in enzyme kinetics (pH-stat calorimetry) and industrial reactor design.

Kp and Kc

For gas-phase reactions Kp uses partial pressures (bar) and Kc uses molar concentrations (mol/L). Kp = Kc(RT)Δn where Δn = Σνproducts − Σνreactants. For reactions with equal numbers of gas moles (Δn = 0) the two are identical. The Haber process for NH3 synthesis has Δn = −2, making high pressure favour the product side.

About this simulation

This simulation models a general reversible reaction aA + bB ⇌ cC + dD and lets you watch chemical equilibrium emerge from thermodynamics rather than just being handed a K value. Set ΔH°, ΔS°, temperature and starting concentrations, and see the free-energy-versus-reaction-quotient curve G(Q), the ICE (Initial-Change-Equilibrium) concentration table, a van't Hoff plot, and live Le Chatelier shifts all update together.

🔬 What it shows

ΔG° = ΔH° − TΔS° sets the standard free energy change, which fixes K = exp(−ΔG°/RT). The G(Q) tab plots free energy as a function of the reaction quotient Q, with the minimum sitting exactly at Q = K — the equilibrium point. The van't Hoff tab plots ln K against 1/T, and the ICE table tracks how initial concentrations [A]₀, [B]₀ shift to their equilibrium values via reaction extent ξ.

🎮 How to use

Drag ΔH° and ΔS° to change the reaction's thermodynamics, T to change temperature, and the stoichiometric coefficients a, b, c, d to change the reaction's balanced form. Adjust [A]₀ and [B]₀ to set starting concentrations. Switch between the G(Q) and van't Hoff tabs to see the same equilibrium from two different views.

💡 Did you know?

The equilibrium constant K is not just an empirical number — it's mathematically the exact quotient Q at which the free-energy curve G(Q) reaches its minimum. This is why a reaction always relaxes toward K regardless of which side it starts from: any Q away from K sits on a slope of the G(Q) curve, and the system rolls downhill until Q = K.

Frequently asked questions

What determines whether K is larger or smaller than 1?

K = exp(−ΔG°/RT). A negative ΔG° (spontaneous forward reaction) gives K > 1, favouring products at equilibrium; a positive ΔG° gives K < 1, favouring reactants. Since ΔG° = ΔH° − TΔS°, both enthalpy and entropy — and temperature — jointly determine which side wins.

Why does the G(Q) curve have a minimum, and why does it sit at Q = K?

Free energy as a function of Q combines the fixed ΔG° term with a mixing-entropy term (RT ln Q) that diverges at both Q→0 and Q→∞, guaranteeing an interior minimum. Setting the derivative dG/dQ to zero and solving reproduces exactly ΔG° + RT ln K = 0, i.e. the minimum is the equilibrium point by construction.

How does the van't Hoff plot relate ln K to temperature?

The van't Hoff equation ln K = −ΔH°/(RT) + ΔS°/R predicts a straight line when ln K is plotted against 1/T, with slope −ΔH°/R and intercept ΔS°/R. This is exactly how ΔH° and ΔS° are measured experimentally: run the reaction at several temperatures, plot ln K vs 1/T, and read off the slope and intercept.

What does the ICE table and reaction extent ξ represent?

ICE stands for Initial, Change, Equilibrium — a bookkeeping method where each species' concentration changes by its stoichiometric coefficient times a single reaction extent ξ. Solving for the ξ that satisfies K = Q at equilibrium gives every species' equilibrium concentration in one step, which is exactly what the simulation's ICE table displays live.

How does Le Chatelier's principle show up in this simulation?

Changing [A]₀ or [B]₀ shifts Q away from K, and the simulation immediately recomputes ξ so the system relaxes back toward K — visibly consuming the added species and forming more of the other side, or vice versa. This is Le Chatelier's principle made computable rather than just a qualitative rule of thumb.