Since our shift is to the right, and it's moving towards all reactants, our reaction is going to favor reactants to get to equilibrium. Terms of our number line is that our concentrations are gonna shift so that Q can get closer to K. So our reaction is gonna try to adjust the concentrations to get to equilibrium. At the concentrations we have up here, we have way more products than we should at equilibrium. So we can see that our Q is larger than K and it's closer to having all products. So Q here is equal to 4,083, which I will place right around here. We're mostly gonna wannaĬompare the relative values of our Q and K. Some intermediate values in here but the actual intermediate values here aren't super important. Then we have a bunch of values in between. So that means at Q equals infinity, we have all products. And then if you have no reactants left and all products, we have zero in the denominator and that gives us a Q value of infinity. So that tells us Q equals zero when you have all reactants and no products. When you have no product your numerator is zero and Q is equal to zero. So Q can have values anywhere from zero to infinity. Possible values of Q on a number line or a Q line.
So the other two possibilities are that Q is greater than K, which is the case here. So, if at any point you're not sure if your concentrations are the equilibrium concentrations, you can calculate Q and check if it's equal to K.
So when Q is equal to K, that tells us we're at equilibrium. So next we're going to talk about what it tells you. So if I plug this into my calculator, I get that Qc with this set of concentrations is 4,083. So if we plug these numbers into our expression for Qc, we get 3.5 molar squared in the numerator and 0.10 squared times 0.3 in the denominator. We have 0.10 molar S02, 0.30 molar O2, and 3.5 molar of our product. At some point in our reaction we have the following concentrations. So let's calculate this for a set of example concentrations. And for the reaction quotient, Q, again everything is in terms of molar concentration but we can calculate it with any concentrations and we don't have to be at equilibrium. So the c means everything is in terms of the molar concentration. The equilibrium constant K you calculate only with the equilibrium concentrations. So you may be wondering at this point, what's the difference? The equation for Qc and Kc will always look exactly the same and the main difference So Qc is equal to the concentration of our product squared, so the concentration of the product raised to the stoichiometric coefficient times the reactant concentrations, also raised to their In that case, when you're not sure it's at equilibrium or really at any point in your reaction or any time, we can calculate the Reaction Quotient, Q. Maybe we just don't know if it's at equilibrium. But what if we're interested in looking at the reaction and it's not at equilibrium yet or So we know at some temperature, if you plug in theĮquilibrium concentrations, Kc is equal to 4.3. And then our reactant concentrations, so S02 squared and the concentration of O2. So, Kc is the product concentration raised to the second power so that's from this stoichiometric coefficient.
And if we plug those concentrations in to this expression, we will get Kc. So, at equilibrium we know the concentrations should be constant because the rate of the forward and backward At equilibrium we can calculate the equilibrium constant, Kc. We're gonna start withĪnd example reaction between sulfur dioxide, S02 gas, which will react with oxygen gas, and this is a reversible reaction that makes sulfur trioxide or SO3. In this video, I'm going to go over, how you calculate Q and how you use it. Are going to be talking about the Reaction Quotient, Q.
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