pKa and pKb relationship (video) | Khan Academy
Table of Acids with Ka and pKa Values*. CLAS. Acid. HA. A-. Ka. pKa 1, pKa of strong acids are ineffective bases. . (Kb > 1, pKb < 1). Relationship between Ka of a weak acid and Kb for its conjugate base. For a weak acid or base, the equilibrium constant for the ionization reaction quantifies the relative amounts of each species. In this article, we will discuss the relationship between the equilibrium constants. The relationship between pH, pKa, and the relative concentrations of an acid and Make your choice: Table of pKa values for acids or Table of pKb values for.
A pKa may be a small, negative number, such as -3 or It may be a larger, positive number, such as 30 or The lower the pKa of a Bronsted acid, the more easily it gives up its proton. The higher the pKa of a Bronsted acid, the more tightly the proton is held, and the less easily the proton is given up. The pKa scale as an index of proton availability. Low pKa means a proton is not held tightly.
Acids and Bases: pKa and pKb
High pKa means a proton is held tightly. Some Bronsted acidic compounds; these compounds all supply protons relatively easily. For example, nitric acid and hydrochloric acid both give up their protons very easily. Nitric acid in water has a pKa of On the other hand, acetic acid found in vinegar and formic acid the irritant in ant and bee stings will also give up protons, but hold them a little more tightly.
Their pKas are reported as 4. Water can certainly give up a proton, but not very easily; it has a pKa of around Methane is not really an acid at all, and it has an estimated pKa of about Water is very, very weakly acidic; methane is not really acidic at all.
The pKa measures the "strength" of a Bronsted acid. A strong Bronsted acid is a compound that gives up its proton very easily. A weak Bronsted acid is one that gives up its proton with more difficulty. Going to a farther extreme, a compound from which it is very, very difficult to remove a proton is not considered to be an acid at all. When a compound gives up a proton, it retains the electron pair that it formerly shared with the proton. It becomes a conjugate base.
Looked at another way, a strong Bronsted acid gives up a proton easily, becoming a weak Bronsted base. The Bronsted base does not easily form a bond to the proton. It is not good at donating its electron pair to a proton. It does so only weakly.
In a similar way, if a compound gives up a proton and becomes a strong base, the base will readily take the proton back again. Effectively, the strong base competes so well for the proton that the compound remains protonated. The compound remains a Bronsted acid rather than ionizing and becoming the strong conjugate base. It is a weak Bronsted acid. The pKa scale and its effect on conjugate bases. This term is usually used to describe common acids such as sulfuric acid and hydrobromic acid.
This term is often used to describe common acids such as acetic acid and hydrofluoric acid. And so this is equal to the concentration of the products.
It's becoming tedious to keep switching colors. Actually, I'll do it. Because it makes it easier look at, at least for me. HA times the concentration of my hydroxide ions divided by my concentration of my weak base. Remember, this can only be true of a weak base or a weak acid. If we were dealing with a strong acid or is or a strong base, this would not be an equilibrium reaction.
It would only go in one direction. And when it only goes in one direction, writing this type of equilibrium reaction makes no sense-- or equilibrium constant-- because it's not in equilibrium. It only goes in one direction. If A was chlorine, if this was hydrochloric acid, you couldn't do this. You would just say look, if you have a mole of this, you're just dumping a mole of hydrogen protons in that solution and then a bunch of chlorine anions who are not going to do anything.
Even though they are the conjugate base, they wouldn't do anything. So you can only do this, remember, for weak acids and bases. So with that said, let's see if we can find a relationship between Ka and Kb. What do we have here? We have an A minus on both sides of this. We have H over-- OH over A minus. Let's solve for A minus.
If we multiply both sides of this equation by HA over H plus, on the left-hand side we get Ka times the inverse of this.
So you have your HA over H plus is equal to your concentration of your conjugate base. And let's do the same thing here. Solve for A minus. So to solve for A minus here, we might have to do 2 steps. So if we take the inverse of both sides, you get 1 over Kb is equal to A minus over H, the concentration of my conjugate acid times the concentration of hydroxide. Multiply both sides by this.
And I get A minus is equal to my concentration of my conjugate acid times concentration of hydroxide. All of that over my base equilibrium constant. Now, these are the same reactions. In either reaction for given concentrations, I'm going to end up with the same concentration.
This is going to equal that. These are two different ways of writing the exact same reaction. So let's set them equal to each other. So let me copy and paste it, actually.
- The pKa Table Is Your Friend
- How to Use a pKa Table
- Relationship between Ka and Kb
So I'm saying that this thing, copy, is equal to this thing right here. So this is equal to-- let me copy and paste this-- that. That's equal to that. So let's see if we can find a relationship between Ka and Kb. Well, one thing we can do is we can divide both sides by HA. So if we divide both sides by HA.
acid base - Relationship between pKa and pKb - Chemistry Stack Exchange
Actually, I could probably have that earlier on to the whole thing. If we ignore this part right here, this is equal to that. Let me erase all of this.
I'm using the wrong tool. So we could say that they both equal the concentration of A minus. So that's equal to that. We can divide both sides by HA. This over here will cancel with this over here. And we're getting pretty close to a neat relationship. And so we get Ka over our hydrogen proton concentration is equal to our hydroxide concentration divided by Kb. You can just cross-multiply this. So we get Ka, our acidic equilibrium concentration, times Kb is equal to our hydrogen concentration times our hydroxide concentration.
Remember, this is all in an aqueous solution.
What do we know about this? What do we know about our hydrogen times our hydroxide concentration in an aqueous solution? For example, let me review just to make sure I'm jogging your memory properly.
We could have H2O. It can autoionize into H plus. And this has an equilibrium. You just put the products. So the concentration of the hydrogen protons times the concentration of the hydroxide ions. And you don't divide by this because it's the solvent. And we already figured out what this was. If we have just completely neutral water, this is 10 to the minus 7. And this is 10 to the minus 7. So this is equal to 10 to the minus Now, these two things could change.Ranking Acid Base Strength Using Ka pKa Values Leah4sci
I can add more hydrogen, I could add more hydroxide. And everything we've talked about so far, that's what we've been doing. That's what acids and bases do. They either increase this or they increase that. But the fact that this is an equilibrium constant means that, look, I don't care what you do to this. At the end of the day, this will adjust for your new reality of hydrogen protons.
And this will always be a constant. As long as we're in an aqueous solution, a solution of water where water is a solvent at 25 degrees. I mean, in just pure water it's 10 to the minus 7. But no matter what we do to this and this in an aqueous solution, the product is always going to be 10 to the minus 14th power.
So that's the answer to this question. This is always going to be 10 to the minus If you multiply hydrogen concentration times OH concentration. Now they won't each be 10 to the minus 7 anymore, because we're dealing with a weak acid or a weak base. So they're actually going to change these things. But when you multiply them, you're still going to get 10 to the minus And let's just take the minus log of both sides of that.
Let me erase all this stuff I did down here.