Reversible reactions and equilibrium
About this video
A reversible reaction can go both ways: A + B form C + D, and C + D form A + B. Equilibrium is reached when the forward and backward reactions happen at the same rate. The video stresses two misconceptions: reactions don't stop at equilibrium, and the concentrations of reactants and products need not be equal.
You'll be able to
- describe a reversible reaction and how it is written with half arrows.
- define equilibrium as equal forward and backward reaction rates.
- explain why reactions don't stop at equilibrium and why concentrations need not be equal.
Key words
- Reversible reaction
- A reaction that can go in either direction, from reactants to products and back.
- Equilibrium
- The point where the forward reaction happens at the same rate as the backward reaction.
- Forward reaction
- The reaction from A + B to C + D.
- Backward reaction
- The reaction from C + D back to A + B.
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.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.