Equilibrium pressure constant?

[salim271]

I was doing some gen chem BR passages today and they were giving Kp in units of atm^-1, it confused me and made me get a couple more wrong than I would have otherwise, so I was hoping someone could explain to me how and why Kp can have units, and what they mean in terms of the reaction... I looked back in the BR book but I couldnt find anything concrete and solid as an explanation.

Any help would be great...

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[MT Headed]

Kp, like all equilibrium constants including Kc, Ka, Kb, and Ksp is unitless. TBR is wrong to assign units to this number.

(note that the little "k" for reaction rates does have units, but that is not what you are asking)

 

[salim271]

Alright well, might as well ask this here instead of starting another thread:

How much VESPR theory do we have to know for the MCAT? I know the basic molecular geometries (linear, trigonal planar, tetrahedral, trigonal bipyrimedal, octehedral) and but other than bent and trigonal pyrimidal, I dont have the more detailed geometries memorized... do I have to know geometries like seesaw, square planar, or square pyrimidal?

Anyone have a definitive answer? If not, I'll just memorize them all and be done with it, its easy points if it comes up.

 

[whiteshadodw]

Alright well, might as well ask this here instead of starting another thread:

How much VESPR theory do we have to know for the MCAT? I know the basic molecular geometries (linear, trigonal planar, tetrahedral, trigonal bipyrimedal, octehedral) and but other than bent and trigonal pyrimidal, I dont have the more detailed geometries memorized... do I have to know geometries like seesaw, square planar, or square pyrimidal?

Anyone have a definitive answer? If not, I'll just memorize them all and be done with it, its easy points if it comes up.

i'd hate to have to memorize all those. lol. i know linear, bent, and tetrahedral.

 

[axo86]

It's not necessarily wrong for equilibrium constants to have units. It all depends on how you define the constant. In my physical chemistry book, all concentrations in equilibrium constant expression were divided by a "standard concentration" (for ex 1 mol/ L for a Kc expression), rendering the equilibrium constant unitless. But it's also perfectly acceptable to write the equilibrium constant expression without dividing by this quantity, in which case the units will not all cancel, and you will be left with a constant that has units. Different books will follow different conventions on this, but neither are incorrect.

 

[salim271]

It's not necessarily wrong for equilibrium constants to have units. It all depends on how you define the constant. In my physical chemistry book, all concentrations in equilibrium constant expression were divided by a "standard concentration" (for ex 1 mol/ L for a Kc expression), rendering the equilibrium constant unitless. But it's also perfectly acceptable to write the equilibrium constant expression without dividing by this quantity, in which case the units will not all cancel, and you will be left with a constant that has units. Different books will follow different conventions on this, but neither are incorrect.

Makes sense, but what really confused me was because the units were atm^-1, i didnt really know if that meant the reaction was moving forward as written even though the Kp was like = 2.034 x 10^5 atm^-1. Should i just ignore the units for the K and just take it for what it is? A large K constant?

 

[Integrated MCAT Course]

Being given units for a Kp in this could be important because it lets you know that you don't have to convert to pascals or baryes or what-not to solve the problem.

 

[axo86]

Makes sense, but what really confused me was because the units were atm^-1, i didnt really know if that meant the reaction was moving forward as written even though the Kp was like = 2.034 x 10^5 atm^-1. Should i just ignore the units for the K and just take it for what it is? A large K constant?

Salim, you can easily obtain units of atm^-1 if you have an equilibrium constant expression that involves 1 more mole of gas on the bottom than on the top, for example, write the Kp expression for 2H2 + O2 -> 2H2O. You will have atm^2 on the top of the expression and atm^3 on the bottom which leaves you overall with atm^-1.

 

[MD Odyssey]

How much VESPR theory do we have to know for the MCAT? I know the basic molecular geometries (linear, trigonal planar, tetrahedral, trigonal bipyrimedal, octehedral) and but other than bent and trigonal pyrimidal, I dont have the more detailed geometries memorized... do I have to know geometries like seesaw, square planar, or square pyrimidal?

Anyone have a definitive answer? If not, I'll just memorize them all and be done with it, its easy points if it comes up.

I think that memorizing molecular geometries is much less useful than understanding why molecules take on a particular geometry (e.g., H2O is bent, but CO2 is linear). In general, I tend to think that memorization is a bad way to study for an exam like the MCAT that is explicitly geared towards testing concepts instead of memorization.

But, your mileage may vary.