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Weak acid–weak base reactions

When a weak acid and a weak base are mixed, they come to an equilibrium state represented by the following equation: HA(aq) + B(aq) ⇌ A⁻(aq) + HB⁺(aq). The equilibrium position lies on the side of the weaker acid and the weaker base, and we can determine whether the resulting solution is acidic, basic, or neutral by comparing the Ka and Kb values of these species. Created by Jay.

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  • blobby green style avatar for user s.s.kazemi.ta
    Excuse me, if we use the same approach that was used at the very end of the video ( comparing the Kb of F- and Ka of NH4+), for the reactant side, we can say that HF reacts less than NH3. So, somehow, just as a limiting reactant would do, it affects how much NH3 can react. I mean, with a 1:1 mole ratio, the [NH3] reacting should equal the [HF] reacting.
    Can this affect the concentration of H3O+ and OH-? (because each reactant can also react with water and produce some OH- or H3O+)
    (4 votes)
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Video transcript

- [Instructor] Let's say the HA represents a generic weak acid and B represents a generic weak base. If our weak acid donates a proton to our weak base, that would form A minus and HB plus. To identify conjugate acid base pairs remember there's only one proton or one H plus difference between an acid and it's conjugate base. So when HA donates its proton and turns into A minus, A minus must be the conjugate base to HA. So there's one H plus difference between HA and A minus. They are a conjugate acid base pair. When B accepts a proton, it turns into HB plus therefore HB plus must be the conjugate acid to B. Since there's one H plus difference between B and HB plus, they are a conjugate acid base pair. Let's look at the situation where the equilibrium constant for this reaction is greater than one. If K is greater than one, that means that at equilibrium, there are more products than reactants therefore, the equilibrium favors the formation of the products. For acid-base reactions the equilibrium always favors the side with the weaker acid and the weaker base. Therefore, since the equilibrium favors the products, the weaker acid and the weaker base, are on the product side and the stronger acid and the stronger base are on the reactant side. So when comparing our two acids, HA and HB plus, HA is the stronger acid of the two. And when comparing our two bases B and A minus, B is the stronger base of the two. When the equilibrium constant is less than one that means at equilibrium, there are more reactants than there are products. Therefore, this time, the equilibrium favors the formation of the reactants. And the equilibrium always favors the side with the weaker acid and the weaker base. Since the equilibrium favors the reactants, the weaker acid and the weaker base, are on the reactant side and the stronger acid and the stronger base are on the product side. So comparing our two acids, HA and HB plus this time, HB plus is the stronger acid of the two and for our bases B and A minus this time, A minus is the stronger base of the two. Let's look at an example of a weak acid, weak base reaction. The Hydrogen Sulfate anion we'll react with the Carbonate anion to form the Sulfate anion and the Hydrogen Carbonate anion. Looking at the reaction, the Hydrogen Sulfate anion is donating its proton so that must be the acid and the Carbonate anion is accepting a proton so that must be the base. After the Hydrogen Sulfate anion donates it's proton it turns into the Sulfate anion, SO four, two minus. Therefore the Sulfate anion must be the conjugate base to HSO four minus. So one conjugate acid base pair is HSO four minus and SO four, two minus. And when the Carbonate anion and accepts the proton, it turns into the Hydrogen Carbonate anion. Therefore the Hydrogen Carbonate anion, must be the conjugate acid to the Carbonate anion. So our other conjugate acid-base pair consists of the Carbonate anion, and the Hydrogen Carbonate anion. The equilibrium constant for this reaction at 25 degrees Celsius is greater than one, therefore the equilibrium favors the formation of the products. And because the equilibrium favors the formation of the weaker acid and the weaker base, we know that the weaker acid and the weaker base must be on the product side and the stronger acid and the stronger base are on the reactant side. Therefore the Hydrogen Sulfate anion is a stronger acid than the Hydrogen Carbonate anion, and the Carbonate anion in as a stronger base than the Sulfate anion. Let's look at another example of a weak acid, weak base reaction. In this case, Hydrofluoric acid reacts with Ammonia to form the Ammonium ion an NH four plus, and the Fluoride anion, F minus. Because Hydrofluoric acid donates a proton, it's an acid and because Ammonia accepts a proton Ammonia functions as a base. And when HF donates a proton, it turns into F minus the Fluoride anion, therefore the Fluoride anion is the conjugate base to HF. And when Ammonia accepts a proton, it turns into NH four Plus the Ammonium ion, therefore the Ammonium ion is the conjugate acid to NH three. Let's say that we mix equal moles of our weak acid HF with our weak base NH three and our goal is to figure out if the resulting solution is acidic, basic or neutral. First, we need to determine if the reactants or products are favored at equilibrium. And to help us determine that here we have the Ka and Kb values for our acids and bases. So here's the Ka value for Hydrofluoric acid at 25 degrees Celsius, so all of these are at 25 degrees Celsius. Here's the Kb value for Ammonia, the Ka value for the Ammonium ion and the Kb value for the Fluoride anion. So looking at our two acids that was Hydrofluoric acid and the Ammonium cation, the Ka for Hydrofluoric acid is a higher value than the Ka for the Ammonium cation. Therefore Hydrofluoric acid is the stronger of the two, the stronger acid of the two. And for the two bases, we have Ammonia and we also have the Fluoride anion, looking at the Kb values, for Ammonia the Kb value is higher than the Kb value for the Fluoride anion, therefore Ammonia is the stronger base of the two. So we have the stronger acid and the stronger base on the reactant side and the weaker acid and the weaker base on the product side. We know the equilibrium favors the side with the weaker acid and the weaker base. Therefore the equilibrium favors the formation of the products. So we could go ahead and write the equilibrium constant for this reaction is greater than one. And at equilibrium we're going to have more of our products than we do of our reactants. So to determine at the solution as acidic, basic or neutral, we have to think about the Ammonium cation and the Fluoride anion in aqueous solution. Since the Ka value for the Ammonium cation is greater than the Kb value for the Fluoride anion, the Ammonium cation is better at producing Hydronium ions in aqueous solution than the Fluoride anion is at producing Hydroxide anions in aqueous solution. So the concentration of Hydronium ions will be greater than the concentration of Hydroxide anions and therefore the resulting solution will be acidic.