so far all of the keq expressions that we've created and that you may have practiced with have used concentrations so instead of just listing this with a keq i will now in as many cases as possible list this is a kc to indicate that we're using concentrations if you have something that is all gases though we could use pressures so you'll see a kp listed for anything where partial pressures can be used and i mean obviously let's do an example so two so3 gas in equilibrium with two so2 gas and o2 gas we could as we have always done write this with concentrations and that would be the concentration of so2 squared times the concentration of o2 divided by the concentration of so3 squared but this uses all concentrations and we have pressures so it would be nice if we could use pressure values the format of the equilibrium constant doesn't change when you do this so it's still the pressure of so2 squared times the pressure of o2 divided by the pressure of so3 squared and you'll see that i have parentheses around these and you should put parentheses around the p and then the compound or the element so that we can tell exactly what's being squared cubed etc and what we're doing the pressure of now what's super important is that these are not numerically equal to each other so they're expressing the same thing but we're putting in very different types of values with different units so we need a conversion um assuming that everything behaves as an ideal gas which is is the goal um given that we need an equation to do some calculations here it'd be nice if there were ideal gases to make this a little bit easier um if that is the case then kp is equal to kc times in parentheses so rt is in parentheses and rt and rt only is to the delta n power where delta n is the change in the number of moles of gas so this really means add up the coefficients for the products and subtract the sum of the coefficients for the reactants so in the case of this reaction that we're looking at my product um c plus d is two plus one and i had two moles of so2 and one mole of o2 and i'm gonna subtract two because i only had one reactant with a coefficient of two so my delta n is equal to 1. okay now if my delta n is equal to 1 i will have a numerical difference but can you think of a case where my kp and my kc would be equal to each other okay now we have been putting things now with different units into our keq expression but we haven't given a unit to keq and it doesn't have any so we should talk about why keq has no units this applies to kc and kp so the concentrations and the pressures that go into this expression they're expressed relative to a standard so a one molar standard or a one atmosphere standard and that means that the units are canceled out before we even do the calculation so as an example if our concentration of no2 in a reaction was 1.5 molar in the kc expression what we're really putting in for the concentration of no2 is 1.5 molar referenced against one molar so the units cancel out so we're canceling the units before we put the values in so what we get is something unitless when it is an equilibrium constant we also have not dealt with solids or pure liquids the concentrations of solids and pure liquids well pure solids pure liquids really anything that has an s or an l in your equation they're not included now this is because their concentrations are essentially constant and the way that you will see this presented in a lot of textbooks is that you do initially include everything from a reaction but what we're really plotting in this equilibrium constant expression is the activity of the components of the reaction and because the concentrations of pure solids and pure liquids are essentially constant the activity of those things is one so they they fall out of the equation they don't contribute to the equilibrium constant so the way that we will deal with this in our equilibrium constant expressions is by just leaving them out so of course we should look at an example or two if calcium carbonate is in equilibrium with calcium oxide and o2 where the calcium carbonate and calcium oxide are both solids my equilibrium constant expression is just the pressure of o2 that's it there's there is nothing else in products over reactants the product pressure of o2 coefficient is one so it's to the first power the other product is a solid it doesn't go in and then in the denominator the reactant doesn't go in so this would all just be over one and we could leave it as is another example fe2o3 solid plus three co gas in equilibrium with fe solid and three co2 gas the products over reactants i have solid on both sides but i also have gas on both sides i could express the same pressures or i could express it in concentrations i have the concentrations here so on the product side it's only the co2 so it's the concentration of co2 to the third power and on the reactant side it's only the concentration of carbon monoxide so it's co to the third power