How to find keq from ka and kb relationship

Equilibrium Constants for Reverse Reactions Chemistry Tutorial

how to find keq from ka and kb relationship

Is it possible to convert the acid dissociation constant (Ka) into the equilibrium constant (Keq)? for example, if I was given the Ka could I work I am just missing the Keq and am confused of how to find it. Thanks!. is known as Ka. To work out Ka when you know pKa, use a calculator to find the antilog. They exist together according to the equilibrium equation AH ⇌ A- + H+. How to Calculate the pH of Ammonia Water Using KB. Learn what pH, pKa, Ka, pKb, and Kb mean for acids and bases, plus understand the differences between If you know an equilibrium constant, you can calculate the others. pKa and pKb are related by the simple relation.

So we're just gonna write our standard reduction potential as positive. Next, we need to look at our balanced equation, all right.

  • Is there a relation between Keq and ph?
  • Calculating a Ka Value from a Known pH
  • Is there a relation between Keq and ph?

We need to make the number of electrons equal for our half reactions. So for this first half reaction, I'm just gonna draw a line through, I'm just gonna draw a line through this half reaction so we don't get ourselves confused. For our first half reaction here, we have two electrons. Then over here, right, for our oxidation half reaction, we have three electrons. We need to have the same number of electrons. So we need to have six electrons for both half reactions, because remember the electrons that are lost are the same electrons that are gained.

So we need to multiply our first half reaction by three. All right, if you multiply our first half reaction by three, we'll end up with six electrons. And our second half reaction, we would need to multiply the oxidation half reaction by two, in order to end up with six electrons.

So let's rewrite our half reactions. So, first we'll do the reduction half reaction, so we have, let me change colors again here. And let's do this color.

So we have three times, since we have three I two now, We have three I two, and three times two gives us six electrons. So three I two plus six electrons, and then three times two, all right, gives us, three times two gives us six I minus.

All right, so we multiplied our half reaction by three, but remember, we don't multiply the voltage by three, 'cause voltage is an intense of property. So the standard reduction potential is still positive. So we have positive.

Next, we need to multiply our oxidation half reaction by two. So we have two Al, so this is our oxidation half reaction, so two Al, so two aluminum, and then we have two Al three plus, so two Al three plus, and then two times three gives us six electrons. So now we have our six electrons and once again, we do not multiply our standard oxidation potential by two, so we leave that, so the standard oxidation potential is still positive 1.

how to find keq from ka and kb relationship

Next we add our two half reactions together. And if we did everything right, we should get back our overall equation.

how to find keq from ka and kb relationship

So our overall equation here. We have six electrons on the reactant side, six electrons on the product side, so the electrons cancel out. And so we have for our reactants three I two, so we have three I two, plus two Al, and for our products right here, we have six I minus, so six I minus, plus two Al three plus, plus two Al three plus. So this should be our overall reaction, all right, this should be the overall reaction that we were given in our problem. Let's double check that real fast.

So three I two plus two Al, all right, so right up here, so three I two plus two Al, should give us six I minus plus two Al three plus. So six I minus plus two Al three plus. So we got back our original reaction. Remember our goal was, our goal was to find the standard cell potential E zero, because from E zero we can calculate the equilibrium constant K.

So we know how to do that, again, from an earlier video.

How can we predict the pH of a buffer?

To find the standard cell potential, all right, so to find the standard cell potential, all we have to do is add our standard reduction potential and our standard oxidation potential.

So if we add our standard reduction potential and our standard oxidation potential, we'll get the standard cell potential. So that would be positive.

how to find keq from ka and kb relationship

So the standard potential for the cell, so E zero cell is equal to. All right, now that we've found the standard cell potential, we can calculate the equilibrium constant.

So we can use one of the equations we talked about in the last video that relates the standard cell potential to the equilibrium constant. So I'm gonna choose, I'm gonna choose one of those forms, so E zero is equal to 0. So again, this is from the previous video.

So, the standard cell potential is 2. So we need to know the pKa of the acid on the left. So we already know that acetic acid is the acid on the left side here, and acetic acid has a pKa, this proton right here has a pKa of approximately five. On the right what's functioning as an acid? That's of course water. So what is the pKa of this proton right here on water? So let's plug that in to our equation. So the pKeq for the forward reaction is equal to the pKa of the acid on the left, which would be approximately five, minus the pKa of the acid on the right, which is approximately So five minus 16 gives us a pKeq equal to negative So how do we go from the pKeq to the Keq?

Well we can do that because we know from general chemistry pKeq is equal to the negative log of Keq.

how to find keq from ka and kb relationship

So to solve for Keq first we need to put the negative sign on the left, so we have negative pKeq is equal to the log of Keq. And how do I get rid of that log? I have to take 10 to both sides. If I take 10 to both sides that gets rid of our log, so we know that Keq is equal to 10 to the negative pKeq. So we need to take our answer here for pKeq and we just need to plug that in to our equation here.

So we get that the Keq, let me go ahead and put that right here, Keq is equal to 10 to the negative. So that negative sign that I just wrote here is this negative sign, 10 to the negative pKeq, which was negative 11, so 10 to the negative negative Which of course is 10 to the eleventh. So the equilibrium constant for the forward reaction is equal to 10 to the eleventh. And we know what that means from general chemistry. We know that when K is much greater than one like this, at equilibrium we have way more products than we do reactants.

So the equilibrium lies to the right, the equilibrium lies to the right, and we have a large amount of our products compared to our reactants.

Equilibrium constant - Wikipedia

If you wanted to do that a faster way, if you just wanted to figure out which direction the equilibrium lies, look at your pKa values, let's go back up here, and you can see on the left it's five and on the right it's Now we figured out that the equilibrium lies to the right, so therefore the equilibrium lies to the side that has the acid with the higher pKa value.

So the equilibrium favors the weaker acid. So that's the short way of figuring out the position of equilibrium using pKa values. Here's another organic acid based mechanism that we've seen before. Acetone on the left functions as a base and takes a proton from H3O plus, which is hydronium, leaving these electrons behind on the oxygen.

how to find keq from ka and kb relationship

So hydronium functions as an acid. If you protonate acetone you would get this, so this must be the conjugate acid to acetone. And if you take away an H plus from hydronium you are left with water, so water must be the conjugate base to H3O plus.

So for the reverse reaction if water functions as a base, water's going to take this proton leaving these electrons behind on the oxygen, giving us back acetone and forming hydronium, H3O plus. To use our pKa values to predict the position of equilibrium we need to find the pKa for the acid on the left and from that we subtract the pKa for the acid on the right.

The acid on the left is hydronium and hydronium has a pKA of approximately negative two.