A reversible reaction can proceed in both the forward and reverse directions. At dynamic equilibrium, the rates of the forward and reverse reactions are equal - but both reactions are still occurring.
Four Criteria for Dynamic Equilibrium
- Closed system: No matter can enter or escape the system (energy transfer is permitted).
- Equal forward & reverse rates: \(\text{Rate}_{\text{forward}} = \text{Rate}_{\text{reverse}} > 0\) (reaction does not stop).
- Constant macroscopic properties: Concentrations, colour intensity, pressure, and density remain constant over time.
- Reversible from either direction: Equilibrium can be reached starting from pure reactants or pure products.
Reaching Dynamic Equilibrium
At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, but the reactions have not stopped.
Open vs Closed Systems: Carbonated Drinks
Inside a sealed bottle, \(\text{CO}_2(\text{aq}) \rightleftharpoons \text{CO}_2(\text{g})\) is at dynamic equilibrium (closed system). When opened, \(\text{CO}_2(\text{g})\) escapes continuously into the atmosphere (open system), driving the forward process until the drink becomes completely flat.
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