Dynamic equilibrium. Living dead. Jumbo shrimp. Oxymorons? You bet. The English language bends and twists around on itself. How does one define dynamic equilibrium? Isn't "equilibrium" a state where things don't change? Doesn't "dynamic" mean that things are always changing? Will we ever stop asking rhetorical questions?
"Dynamic equilibrium" describes equilibrium on both a small scale and a large scale. On a large scale, the system doesn't appear to be changing. However, on a small scale, individuals within the system may be changing.
A good example is a saturated salt solution. Salt may be dissolving in one spot, and precipitating out in another. Overall, the amount of salt in solution is constant. Another example would be a typical high school. Seniors graduate, new freshmen come in, but the overall number of students sleeping through first period remains the same.
Perhaps a more accurate example for dynamic equilibrium is running on a treadmill. Obviously, you're not going anywhere, but you are moving. You're simply not getting anywhere because you're running in the opposite direction of the treadmill at the same speed. Same deal with reactions. When a reaction is at dynamic equilibrium, the forward and reverse reaction rates are the same.
To be at equilibrium, the conditions of a reaction must be constant. It's hard for a reaction to be at equilibrium if the surroundings are going ballistic, so a reaction must be performed at a given temperature, concentration, etc. To be at dynamic equilibrium means that a reaction has reached equilibrium in the sense that there is no further change in the reaction. This does NOT mean that the reaction itself isn't changing. How does that make sense? Let's talk through it starting with our most basic, favorite example:
Let's start with a whole bunch of A and just a little bit of B, and then put it all in a closed container. We'll let the reaction stew a bit. After a while, depending on a bunch of fabulous factors such as concentration, pressure, temperature, and equilibrium constants, the reaction will, in fact, be in equilibrium. At that moment in time, A is converting to B at exactly the same rate that B is converting to A.
So in the grand scheme of things, nothing is changing. The amount of A and B is the same, but if you were to miniaturize yourself and look at each A and B molecule you'd see a lot of things going on, and probably be in danger of getting run over by the back and forth traffic of A and B. They drive like maniacs, those As and Bs.
In summary, a dynamic equilibrium occurs when you have a reversible reaction in a closed system. Nothing can be added to the system or taken away from it apart from energy. At equilibrium, the quantities of everything present in the mixture remain constant, although the reactions are still continuing. This is because the rates of the forward and the backward reactions are equal.
That's it for our dynamic explanation. Hopefully your mind is now at ease and in equilibrium.
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