Unit 8: Topic 4 - Acid-Base Reactions and Buffers Flashcards
Explain what happens when a strong acid and strong base mix in an aqueous solution.
A strong acid in aqueous solution will dissociate almost completely into H+ ions and a conjugate base with no effect on pH. A strong base in aqueous solution will dissociate into OH- ions and a conjugate acid with no effect on pH. The only components with an affect on pH are the H+ ions from the acid and the OH- ions from the base. Therefore, the pH depends on the excess reactant of the reaction H+(aq) + OH-(aq) –> H2O.
0.10M of HCl mixes with 0.20M of NaOH. What is the pH of the mixture after the reaction?
If we assume 1L of water, then there are 0.10 moles of HCl and 0.20 moles of NaOH. Since the reaction is HCl + NaOH –> H2O + NaCl, the HCl is limiting and completely mixes to form products with no effect on pH, and there are 0.10 moles of NaOH remaining. NaOH is strong, so the molarity of NaOH equals that of OH-. Therefore, 0.10M of OH- ions remain, with a pH of 13.
A weak acid is placed in an aqueous solution. Explain qualitatively what happens.
When a weak acid is placed in aqueous solution, the acid will dissociate into H+ ions, but the process does not proceed to completion: instead, there will be an equilibrium mixture of the acid HA, the H+ ions, and the conjugate base A-. The rate of dissolution depends on the Ka of the acid. This solution will be acidic, but not nearly as acidic as if a strong acid were used in place of a weak acid. For example, the pH of a 0.10M solution of a strong acid (HCl) is 1, but the pH of a 0.10M solution of HF (Ka = 6.8 * 10^-4) is approximately 4.167.
A weak acid is placed in an aqueous solution and is reacted with OH-. However, the weak acid is still in excess. Explain what occurs.
A weak acid is placed in aqueous solution and is reacted with equal parts of strong base. Explain what occurs.
The reaction is HA + OH- –> H2O + A-. If there are equal parts of HA and OH-, then they fully react and create equal parts of H2O and A-. Of these, only A- will affect the pH: since HA is a weak acid, A- is a weak conjugate base and dissociates in water. This will create a slightly basic solution dependent on the Kb of the weak base.
A weak acid is placed in an aqueous solution, and excess hydroxide ion is added. Explain what occurs.
OH- is a strong base, whereas A- is only a weak base. Therefore, the effect on pH is dominated by the OH-. We can calculate the amount of excess OH- after the reaction proceeds and use this to calculate the pH of the solution.
A weak base is placed in an aqueous solution. Explain what happens:
1. before any strong acid is added
2. when strong acid is added but is still limiting
3. when equal parts of strong acid and weak base are added
4. when excess strong acid is added.
This is very similar to adding strong base to weak acid; the only difference is that the pH slowly decreases here as the solution becomes more acidic.
1. The weak base will partially dissociate in water. The resulting solution is slightly basic.
2. The strong acid will completely react to produce some conjugate acid and create a buffer solution. The resulting solution is relatively basic and comparable to the pKb of the base.
3. The strong acid and weak base completely react to produce the conjugate acid, which will dissociate and produce a slightly acidic solution.
4. The strong acid dominates the conjugate acid, and the pH depends on the concentration of the excess strong acid.
What happens when a weak base and weak acid are mixed in aqueous solution?
A weak base and weak acid will mix with the reaction HA(aq) + B(aq) ⇌ A-(aq) + HB+(aq). This will reach equilibrium, but equilibrium concentrations are relatively hard to evaluate because there is a Ka for the acid and Kb for the base.
Note: this isn’t on the exam.
