Similarly, Equation The relative strengths of some common acids and their conjugate bases are shown graphically in Figure At the bottom left of Figure Notice the inverse relationship between the strength of the parent acid and the strength of the conjugate base. Thus the conjugate base of a strong acid is a very weak base, and the conjugate base of a very weak acid is a strong base.
We can use the relative strengths of acids and bases to predict the direction of an acid—base reaction by following a single rule: an acid—base equilibrium always favors the side with the weaker acid and base, as indicated by these arrows:. Hence the ionization equilibrium lies virtually all the way to the right, as represented by a single arrow:. Acid—base properties of salts. Current timeTotal duration Google Classroom Facebook Twitter.
Let's look at NH4 plus. The ammonium ion would function as an acid and donate a proton to water to form H3O plus. If NH4 plus donates a proton you're left with NH3. The Ka for this reaction is 5. Now let's look at NH3 which we know is a weak base, and it's going to take a proton from water, therefore forming NH4 plus.
If we take a proton from water we're left with OH minus. Since we talked about a base here we're gonna use Kb, and Kb for this reaction is 1. What would happen if we add these two reactions together? We have two water molecules for our reactants, so let me go ahead and write H2O plus H2O here.
What about ammonium? We have ammonium on the left side for reactant, we also have ammonium over here for our product. That cancels out. Same thing happens with ammonia, NH3. We have NH3 on the left. We have NH3 on the right. We have NH3 as a reactant, NH3 as a product.
We can cancel those out too. Our only reactants would be two H2O. This reaction should sound familiar to you. This net reaction is the auto-ionization of water where one water molecule acts as an acid, one water molecule acts as a base.
We get H3O plus and OH minus. The equilibrium constant for the auto-ionization of water you've already seen that Kw is equal to 1. We added these two reactions together and we got this for our net reaction. What would we do with Ka and Kb to get Kw? It turns out that you multiply them, Ka times Kb for a conjugate acid-base pair is equal to Kw. Let's do that math. Ka is 5. Kb is equal to 1. We have 5. We're going to multiply that by 1. We get 1. This is equal to 1.
When you add reactions together to get a net reaction you multiply the equilibrium constants to get the equilibrium constant for the net reaction which in this case is Kw for the auto-ionization of water.
Let's go ahead and go in even more detail here. Ka, that's your products over your reactants. That'd be H3O plus, the concentration of H3O plus times the concentration of ammonia. Let's go ahead and do that. The concentration of H3O plus times the concentration of ammonia, and that's all over the concentration of ammonium. This is Ka. Ka, pKa, Kb, and pKb are most helpful when predicting whether a species will donate or accept protons at a specific pH value.
They describe the degree of ionization of an acid or base and are true indicators of acid or base strength because adding water to a solution will not change the equilibrium constant.
Ka and pKa relate to acids, while Kb and pKb deal with bases. Like pH and pOH , these values also account for hydrogen ion or proton concentration for Ka and pKa or hydroxide ion concentration for Kb and pKb. Ka and Kb are related to each other through the ion constant for water, Kw:.
Ka is the acid dissociation constant. Similarly, Kb is the base dissociation constant, while pKb is the -log of the constant. Acids and bases dissociate according to general equations:.
In the formulas, A stands for acid and B for base. A large Ka value indicates a strong acid because it means the acid is largely dissociated into its ions. A large Ka value also means the formation of products in the reaction is favored. A small Ka value means little of the acid dissociates, so you have a weak acid. The Ka value for most weak acids ranges from 10 -2 to 10 The pKa gives the same information, just in a different way.
The smaller the value of pKa, the stronger the acid. Weak acids have a pKa ranging from Kb is the base dissociation constant. The base dissociation constant is a measure of how completely a base dissociates into its component ions in water. A large Kb value indicates the high level of dissociation of a strong base.
A lower pKb value indicates a stronger base. Another important point is pI. This is the isoelectric point. It is the pH at which a protein or another molecule is electrically neutral has no net electrical charge.
0コメント