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Tips for Converting to Kp from Kc - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are some tricks and tips for converting Kp from KC. So if you want to get to Kp from Kc, the equation is this. So you have Kp equals Kc times RT to the delta n.

Kp is the equilibrium constant and pressures. So using our example, Kp would have been equal to the pressure of NO2 gas, and that quantity squared,over the pressure of N2O4 gas. Notice how it's pressure, so this deals with pressures in atmospheres.

Kc just slightly different. So I would use brackets [NO2] and then, since there's the coefficient, squared, over [N2O4] that concentration. And this deals with molarity concentrations, so concentration in molarity.

So we want to see what the relationship is and we can actually calculate the difference. So you have Kp, you have Kp. The R since we're dealing with pressure, would be equal to the value of the universal gas constant 0.08206 Liters times atmosphere over moles times Kelvin, mainly because of these. So the atmosphere or the pressure.

Temperature, just like in all gas laws, needs to be in Kelvin and delta n stands for change in moles of gas. So remember, it's gas only, and so products minus reactants. So other things solids, liquids, they don't count. It's only gas.

So let's take a look at our example here. So we have example is N2O4 gas yields 2NO gas. We are given the Kc and we're also given the temperature. So all we do is plug it in. So Kp=Kc, so that's 4.61 times 10 to -3. Then R is 0.08206L.atm/mol.K and then the times and then the temperature needs to be in Kelvins. So 25 plus 273 is 298Kelvin. Then delta n, how many moles?

So we have 2 moles in the products, one mole of gas in the reactants, so 2 minus 1 is 1. So that's to the first part. So you do the math and you calculate Kp. So in this example, Kp is equal to 0.113, so that's our answer.

So some tips and tricks is that when delta n equals 0, that means that Kp will equal Kc. So that happens when basically delta n is 0. So then basically RT is raised to the zero power so that's equal to one. So Kp=Kc and then if you have RT to the 0 power, this part, basically equals 1. So that's the reason why they're the same.

This is one trick to save you a lot of time rather than calculating R and T and then figuring out that that part is equal to 1. If you know that delta n is equal to 0, then Kp equals Kc. Other than that, make sure you use R; 0.08206, use the temperature in Kelvins. And also delta n, that you'll only use the change in moles of gases only.

So hopefully these tricks and tips will help you in converting Kp from Kc.