Reversible reactions and dynamic equilibrium sim

What the sim shows

A reversible reaction can go both ways. In a closed system the forward and backward reactions both keep going; when their rates are equal the amounts stop changing, which is called dynamic equilibrium. The syringe holds N₂O₄ (colourless) and NO₂ (brown): N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57.2 kJ/mol. Its equilibrium constant comes from data-book values (ΔH = +57.2 kJ/mol, ΔS = +175.8 J/(mol K)): Kp = 0.145 bar at 25 °C, rising with temperature. Pushing the plunger squeezes the gas, so it looks darker at once; then it gets paler as the mixture moves to the side with fewer gas molecules. Heating gives more brown NO₂, because the forward reaction takes in heat. The flask holds yellow chromate: 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l), K ≈ 4.2 × 10^14 dm⁹ mol⁻³. Each drop of 1.0 mol/dm³ acid turns more of it to orange dichromate, and alkali turns it back. In the contact process, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197.8 kJ: a lower temperature gives more SO₃ at equilibrium, but the V₂O₅ catalyst works too slowly when cool, so about 450 °C is the compromise (in this model the yield peaks between about 450 and 480 °C); 1–2 atm is enough because the yield is already about 98 %. Predicting the shift (Le Chatelier's principle) is an extension for IGCSE Supplement and AQA Higher; Singapore O-Level asks only for the idea of a reversible reaction and dynamic equilibrium. Model limits: the equilibrium positions are real (the hydrogen chromate ion HCrO₄⁻ is left out, so the flask shows the share changed from chromate rather than a pH); the rates are slowed so the change can be watched, one stage at a time (the real gases settle in far less than a second); the water bath reaches its new temperature at once; the converter's catalyst rate is a simple Arrhenius estimate.

• Forward and backward rates: equal at dynamic equilibrium
• Colour: brown NO₂, orange dichromate, yellow chromate
• Contact process: yield at equilibrium and yield reached

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