Learn about Senior Chemistry, Equilibrium 7, in this comprehensive video by bannanaiscool.
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Rob Lederer: Le Chatelier's Principle simply states that if you take a reaction in equilibrium and you disturb it then the reaction will shift to counteract the disturbance or disturbance because he was French you know, I think his first name is Le. Now here is the reaction -- at equilibrium, it's the last one we had up. Now the expression for this reaction is simply this concentration times this one divided by the two reacted concentrations. Now if I said, okay you know what, we've got a nice reaction occurring in equilibrium. All sudden somebody dumps in more H2, how does the system respond to that change and get back to equilibrium. So this is what Le Chatelier found out. If you dump in more H2, your concentration here is increased, which means what happens to the K value? Now some of you are going to say, well, if this number gets bigger, these are all the same then this number gets bigger. Yeah, careful except for one thing; K doesn't change. It's a constant. Remember, it's a constant and so therefore, if I say to you, if this number goes up, what happens to K. You're supposed to say, it stays the same. So that means all these concentrations you have to adjust if this one getting bigger, so that can be maintained. So how does that happen? If this gets bigger, well then it's got to get smaller, and so does this, but these have to get bigger in order to compensate to make this same number I did before. Here is what that means. When you add to one side, the reaction shifts to the other side so we lose H2 and lose CO2 in order to make more H2 and CO. So here is what we say. If you increase the concentration of H2 in this reaction, the reaction shifts to the left, decreases that H2, decreases the CO2, this maybe CO2 didn't want to be decreased but it has to and you increase the CO and the H2O concentration. So if somebody said to you, hey, okay, let's change that situation. Let's say, we take away water, which way does the reaction shift? Well, if you take away water, which is a gas here, which is being removed from the expression, then if this number becomes smaller, then what's going to happen to the overall number here? That should become larger. No it is supposed to stay the same and so we have to shift. Which way we're going to shift? What we use up here and -- get back. So, the reaction shifts to the left and we lose this and lose this, so we can gain more of this, get it back, and we gain more of that and the reaction then comes to concentrations which again give us the same K value. So when you change concentrations, the reaction shifts either way to maintain equilibrium. Now what about - that's what we have, that's a constant concentration change. Now what happens if you have a change but not in concentration but in temperature? Let's say the reaction is exothermic. If you add heat to this reaction, add heat, treat it like it was like any kind of chemical. If you add heat, which way does it shift? If you're putting in more heat and you increase the heat, the reaction shifts this way because the heat term is on the right hand side and that means then that the reaction is going to make more of this, it will increase more of this, it will decrease this and decrease this if you add heat. These being decreased and this increases lowers the K value and it actually does because remember, heat is not in the expression but it can change the concentrations and so will changes the K. Is that allowed? Is K allowed to change? Hey, it's a change in temperature. K values are temperature-dependent, remember. So they change with different types of temperatures and so that's what happens here if the reaction is exothermic and you add heat. If you took away heat, the reaction would shift to the right to make more heat, making more of these, less of these, these go up, K value goes up. Now figure out what happens when it is endothermic as well. Okay, now what happens if we change pressure? Alright, now pressure is now it's kind of tric