Chemical Equilibrium Lab

About this model

Two models of the same reaction. Each system is one reversible reaction written as a forward step and a reverse step. Their rates follow the mass-action rate laws: rate_f = k_f·[reactants]^coefficients and rate_r = k_r·[products]^coefficients. The graphs and readouts integrate these equations (a smooth, continuous model). The molecule view runs the same steps with a fixed number of molecules, choosing each forward or reverse event at random with the right probability (Gillespie’s exact method, 1977). Both use the same rate constants, so their equilibrium amounts agree; the molecule counts wobble a little because there are only about 100–150 molecules. With N molecules per mole and volume V, a rate of r (M/s) is r × N × V events per second.

Temperature. Each rate constant follows the Arrhenius equation, k = A·e^−Eₐ/RT. The two barriers differ by ΔH (Eₐ,forward − Eₐ,reverse = ΔH), so K = k_f/k_r changes with temperature by the van ’t Hoff equation, ln(K₂/K₁) = −ΔH/R·(1/T₂ − 1/T₁). Only temperature changes K. Adding or removing substances, diluting, and squeezing a gas change Q, not K. The vessel warms or cools toward the bath with a 3-second time constant.

A catalyst lowers both barriers by the same amount, so both rates speed up by the same factor: equilibrium comes sooner and nothing else changes.

The clock is slowed down. Real NO₂ molecules pair up in microseconds. Each system’s speed is set so the approach to equilibrium takes seconds on screen; the effects of temperature and catalysts on that speed are physical.

The reactions and values. A ⇌ B is a made-up isomerisation (K = 3.0 at 25 °C, ΔH = −8 kJ/mol; made-up colours). 2NO₂ ⇌ N₂O₄: ΔH = −57.2 kJ/mol (from NIST enthalpies of formation), Kc = 217 M⁻¹ at 25 °C (the inverse of the textbook 4.61 × 10⁻³ M). Cobalt chloride: the forward reaction is endothermic (the classic demonstration: hot → blue, ice → pink); ΔH = +50 kJ/mol and Kc = 2.4 M⁻⁴ are model values chosen so the starting solution is purple. Water is the solvent, a pure liquid, so it is left out of K, but adding water still shifts the equilibrium left by diluting everything. Iron thiocyanate: Kc = 138 M⁻¹ at 25 °C (a common textbook value; measured values depend on ionic strength); FeSCN²⁺ absorbs at 447 nm with ε = 4700 M⁻¹ cm⁻¹. Haber: ΔH = −92.2 kJ/mol, Kc = 0.060 M⁻² at 500 °C (textbook), ideal gases; at 450 °C and 100 atm the model gives about 14 % NH₃ (measured: 16.4 %, Larson & Dodge 1923). Activation energies are model values chosen to show the effects clearly. Industrial ammonia plants run at about 400–450 °C and 150–300 atm with an iron catalyst.

Colours are calculated with Beer’s law in three colour channels from each species’ concentration. The absorptivities are tuned by eye so each species looks right at these concentrations (only FeSCN²⁺ at 447 nm is a literature value). A colour word is always shown with the colour.

Lab 3 simplifications. The FeSCN²⁺ standards are treated as exactly their labelled concentration (in a real lab they are made with a large excess of Fe³⁺ so nearly all SCN⁻ reacts). Colorimeter readings have random noise of about ±0.003 from the seed; the same seed repeats the same readings.

AP Chemistry CED (Unit 7) · NIST Chemistry WebBook · Royal Society of Chemistry

What am I looking at?

How the lab works. Pick Explore or one lab at the top. Each lab has five tabs: Pre-lab, Bench, Data & graph, Claim · Evidence · Reasoning, and Test. On the Bench, the coach lists your steps; they tick themselves as you work. Press What do I do now? when you are stuck.

① Bench. A fume hood with the vessel for the system you chose: a flask, a gas syringe in a water bath, a test tube with droppers (tap a dropper to add it), or an ammonia reactor. The chip under the vessel gives its colour and which way it is shifting: → right (making products), ← left (making reactants) or ⇄ at equilibrium. The round lens shows the molecules inside; tap it to zoom in.

② Molecules. Every forward event flashes gold with →; every reverse event flashes cyan with ← and a dashed ring. The panel on the right counts forward and reverse events per second. At equilibrium both keep happening at the same rate. Tap a molecule to name it. Step jumps to exactly the next event.

Graphs. Top: the concentration of each species against time (each has its own colour, dash and marker). Bottom: the forward rate (solid, ▲) and reverse rate (dashed, ▼). Every stress is marked with a labelled dashed line.

Keys: Space play / pause · → step · R reset · Z / X zoom.