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What Is Dynamic Equilibrium?
Chemical reactions can either go in both directions (forward and reverse) or only in one direction. The ones that go in two directions are known as reversible reactions, and you can identify them by the arrows going in two directions, like the example below.
H2O(l) ⇌ H+(aq) + OH-(aq)
Dynamic equilibrium only occurs in reversible reactions, and it’s when the rate of the forward reaction is equal to the rate of the reverse reaction. These equations are dynamic because the forward and reverse reactions are still occurring, but the two rates are equal and unchanging, so they’re also at equilibrium.
Dynamic equilibrium is an example of a system in a steady state. This means the variables in the equation are unchanging over time (since the rates of reaction are equal). If you look at a reaction in dynamic equilibrium, it’ll look like nothing is happening since the concentrations of each substance stay constant. However, reactions are actually continuously occurring.
Dynamic equilibrium doesn't just occur in chemistry labs though; you've witnessed an dynamic equilibrium example every time you've had a soda. In a sealed bottle of soda, carbon dioxide is present in both the liquid/aqueous phase and the gaseous phase (bubbles). The two phases of carbon dioxide are in dynamic equilibrium inside the sealed soda bottle since the gaseous carbon dioxide is dissolving into the liquid form at the same rate that the liquid form of carbon dioxide is being converted back to its gaseous form.
The equation looks like this: CO2(g) ⇌ CO2(aq).
Changing the temperature, pressure, or concentration of a reaction can shift the equilibrium of an equation and knock it out of dynamic equilibrium. This is why, if you open a soda can and leave it out for a long time, eventually it'll become "flat" and there will be no more bubbles. This is because the soda can is no longer a closed system and the carbon dioxide can interact with the atmosphere. This moves it out of dynamic equilibrium and releases the gaseous form of carbon dioxide until there are no more bubbles.
Dynamic Equilibrium Examples
Any reaction will be in dynamic equilibrium if it’s reversible and the rates of the forward and reverse reactions are equal. For example, say that you prepare a solution that is saturated with an aqueous solution of NaCl. If you then add solid crystals of NaCl, the NaCl will be simultaneously dissolving and recrystallizing within the solution. The reaction, NaCl(s) ⇌ Na+(aq) + Cl-(aq), will be in dynamic equilibrium when the rate of the dissolution of the NaCl equals the rate of recrystallization.
Another example of dynamic equilibrium is NO2(g) + CO(g) ⇌ NO(g) + CO2(g) (again, as long as the two rates are equal). Nitrogen dioxide (NO2) reacts with carbon monoxide (CO) to form nitrogen oxide (NO) and carbon dioxide (CO2), and, in the reverse reaction, nitrogen oxide and carbon dioxide react to form nitrogen dioxide and carbon monoxide.
If you’re observing a reaction, you can tell it’s not at dynamic equilibrium if you can see changes occurring in the amounts of reactants or products. (If you can’t see any changes, that doesn’t guarantee it’s at dynamic equilibrium, since it may be at static equilibrium or the changes may be too small to see with the naked eye.)
An example of an equation that could never be at dynamic equilibrium is: 4 Fe(s) + 6 H2O(l) + 3O2 (g) → 4 Fe(OH)3 (s). This is an equation for the formation of rust. We can see that it’ll never be in dynamic equilibrium because the arrow for the reaction only goes one way (which is why a rusty car won’t become shiny again on its own).