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# Introduction to reaction quotient Qc

## Video transcript

today we're going to be talking about the reaction quotient Q in this video I'm going to go over how you calculate Q and how you use it we're going to start with an example reaction between sulfur dioxide so2 gas which will react with oxygen gas and this is a reversible reaction that makes sulfur trioxide or so3 we should make sure this is a balanced reaction we have two sulfur dioxides reacting with 1:02 to give to so3 at equilibrium we can calculate the equilibrium constant k c so at equilibrium we know the concentrations should be constant because the rate of the forward and backward reactions are the same and if we plug those concentrations in to this expression we will get KC so KC is the product concentration raised to the second power so that's from this stoichiometric coefficient and then our reactant concentrations so so2 squared and the concentration of o2 so we know at some temperature if you plug in the equilibrium concentrations KC is equal to four point three but what if we're interested in looking at the reaction and it's not an equilibrium yet or maybe we just don't know if it's it equilibrium in that case when you're not sure it's at equilibrium or really at any point in your reaction or anytime we can calculate the reaction quotient Q so Q C 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 stoichiometric coefficients so so2 squared and - so you might 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 is when you use them the equilibrium constant K you calculate only with the equilibrium concentrations so the C means everything is in terms of the molar concentration and for a 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 molar concentration so let's calculate this for a set of example concentrations at some point in our reaction we have the following concentrations we have zero point 100 leur so2 zero point three Oh Mulder Oh two and three point five molar of our product so if we plug these numbers into our expression for QC we get three point five molar squared in the numerator and zero point one Oh square times zero point three in the denominator so if I plug this into my calculator I get that QC with this set of concentrations is four thousand and eighty three so now we know how to calculate QC so next we're going to talk about what it tells you so there are three possible scenarios so in Q is equal decay that tells us we're at equilibrium so if at any point you're not sure if your concentrations are there e are the equilibrium concentrations you can calculate Q and check if it's equal to K and in this case it's not so the other two possibilities are that Q is greater than K which is the case here or Q can be less than K so let's go through both of those possibilities we can draw all the possible values of Q on a number line or a Q line so Q can have values anywhere from 0 to infinity when you have no product your numerator is 0 and Q is equal to 0 so that tells us Q equals 0 when you have all reactants and no products and then if you have no reactants left and all products we have 0 in the denominator and that gives us a Q value of infinity so that means at Q equals infinity we have all products and then we have a bunch of values in between and I'm going to just write some intermediate values in here but the actual intermediate values here aren't super important we're mostly going to want to compare the relative values of our Q and K so Q here is equal to 4,000 83 which I will place right around here so that's QC and our K and yellow is 4.3 so we'll place that right around here so we can see that our Q is larger than K and it's closer to having all products at the concentrations we have up here we have way more products and we should at equilibrium so our reaction is going to try to adjust the concentrations to get T equilibrium and what that means in terms of our number line is that our concentrations are going to shift so that Q can get closer to K 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 so when Q is greater than K like here we're going to favor reactants reactance and then the last scenario when Q is less than K our reaction will favor products and we can show that also on our number line if we had a different set of concentrations where Q was less than K which I will show using this color here if we had say a Q value around here then our shift would be to the right towards making more products and therefore that would mean our reaction is going to try to reach equilibrium by favoring the forward reaction so that's how you calculate Q and how you use it to see how the reaction concentrations will shift to get to equilibrium in our next video we'll go over an example problem using Q and trying to figure out how the reactant concentrations will shift for another reaction