Chapter 17 - Additional Aspects of Aqueous Equilibria Flashcards
17.1
Explain the common ion effect.
When a weak acid, such as HC2H3O2, and a strong electrolyte with the same ion, such as NaC2H3O2, are combined into a solution, the strong electrolyte causes the H+ to ionize less than the weak acid were by itself. This is because, with the additional amount of ion, equilibrium shifts to the non-dissociated side, reducing the amount of H+ as well. The same thing happens with bases and OH-.
17.1 - Give It Some Thought p. 723
A mixture 0.10 mol of NH4Cl and 0.12 mol of NH3 is added to enough H2O to make 1.0 L of solution. (a) What’re the initial concentrations of he major species in the solution? (b) Which of the ions in this solution is a spectator ion in any acid-base chemistry occurring in the solution? (c) What equilibrium reaction determines [OH-] and therefore the pH of the solution?
(a) .10 M NH4+, .10 M Cl- .12 M NH3
(b) Cl-
(c) NH3 + H2O NH4 + OH-
17.2
Define “buffered solutions (buffers)”.
Substances which contain a weak conjugate acid-base pair that can resist drastic changes in pH upon the addition of small amounts of strong acid or strong base.
17.2
Describe the requirements needed to be a buffer.
- Contains an acid to neutralize OH- & a base to neutralize H+
- Acid & base can’t consume each other through a neutralization reaction
- Fulfilled by a weak acid-base conjugate pair
- -Ex. CH3COOH-CH3COO- or NH4+-NH3
- Often prepared by mixing a weak acid or base with a salt of that acid or base.
- -Ex. CH3COOH-CH3COO- by adding CH3COONa to CH3COOH.
- -Ex. NH4+-NH3 by adding NH4Cl to NH3.
17.2 - Give It Some Thought p. 723
Which of the following conjugate acid-base pairs will not function as a buffer: C2H5COOH and C2H5COO- ; HCO3- and CO3 2- ; HNO3 and NO3-? Explain.
HNO3 and NO3-. HNO3 is a strong acid, so its conjugate base, NO3-, is an negligible base, and will not affect the pH of anything.
17.2
What is [H+] determined by?
The value of Ka for the weak-acid component of the buffer & the concentration ratio of the conjugate-acid base pair.
HX = weak acid buffer is composed of. MX = salt plus ion
HX (aq) H+ + X-
Ka = [H+][X-]/[HX] [H+] = Ka (HX/X-)
17.2
What happens if OH- ions are added to the buffered solution?
They react with the acid component of the buffer to produce H2O and X-.
HX + OH- —-> H2O + X- , causing HX to decrease and X- to increase.Only changes a pH a small amount, assuming that the amounts of HX and X- in the buffer are large compared to the amount of OH- added.
17.2
What happens if H+ ions are added to the buffered solution?
They react with the base component of the buffer.
H+ + X- —> HX ; H3O+ + X- —> HX + H2O, decreasing X- and increasing HX, causing the pH to change slightly.
- 2 - Give It Some Thought p. 724
(a) What happens when NaOH is added to a buffer composed of CH3COOH and CH3COO-? (b) What happens when HCl is added to this buffer?
(a) The OH- reacts with the acid member of the buffer, CH3COOH, thus decreasing the [CH3COOH] and increasing the [C2H3O2-].
(b) The H+ reacts with the base member of the buffer, [C2H3O2-], decreasing the ion’s concentration and increasing [HC2H3O3].
