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Reversible reactions and equilibrium

Reversible reactions are chemical processes that proceed in the forward and backward directions. When the rate of the forward and backward reactions are equal, the process is in equilibrium. At equilibrium, the reactions do not stop. Instead, the amount of each substance becomes constant. Created by Sal Khan.

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Video transcript

- [Instructor] Let's imagine a reaction where we start with the reactants A and B, and they react to form the products C and D. Now, it turns out that in certain situations, the reaction could go the other way. You could start with C + D, and those could react to end up with A + B. So when you have a reaction like this, that could go in either direction, we call that a reversible reaction. And that's why the title of this video, Reversible Reactions and Equilibrium. And one way to say that it could go in either direction is to write it like this, A + B. And we use these half arrows. So it could go in that direction, or it could go in this direction. You could go to C + D. And what's going to happen is the reaction is going to go in both directions. So A + B is going to react to form C + D, and C + D is going to react to form A + B. And at some point, you're going to be hitting an equilibrium. That's the point at which the forward reaction is happening at the same rate as, I guess you could consider it to be the backward reaction, going from C + D to A + B. Now, once you're at equilibrium, it doesn't mean that the reactions stop. It just means that the rate of going from A + B to C + D is the same as the rate of going from C + D to A + B. This is a really important thing to realize. A common misconception is that people think, at equilibrium, somehow these reactions stop. That is not the case. It's just that the rate of the forward reaction and the backward reaction has now become the same rate. So even though you continue to have A + B forming C + D, you have the same rate at which C + D is now also going and forming A + B. Another misconception here is that the concentrations of A + B and C + D, once you're in equilibrium, they would've stabilized, but it's not necessarily that the concentrations are equal. The point at which we're in equilibrium, you might have a much lower concentration of C + D than A + B, or the other way around. What equilibrium is telling us is that at equilibrium, the forward rate of reaction is equal to the backward rate of reaction.