17 Chapter Additional Aspects of Aqueous Equilibria
Only 17.1, 17.2, and 17.3 sections will be covered in Chem 122.
The Common Ion Effect
The solubility of a partially soluble salt is decreased when a common ion is added
The Common Ion Effect
If acetic acid, CH3COOH, is added to water,
(At equilibrium H+ and C2H3O2- are constantly moving into and out of solution, but the concentrations of ions are constant)
If we then add C2H3O2-, which is a common ion
(e.g. adding the sodium salt)
- then, [C2H3O2-] will increase - so, [H+] must decrease to reestablish equilibrium
Buffered Solutions
A buffer consists of a mixture of a weak acid (HX) and its conjugate base (X-)
HX(aq)
H+(aq) + X-(aq)
A buffer resists a change in pH when a small amount of OH- or H+ is added!
Buffered Solutions
HX(aq)
The Ka expression is
H+(aq) + X-(aq)
[H + ][X - ] [HX] [HX] [H + ] = K a [X - ] Ka =
[H + ] = K a
[HX ] [X - ]
Buffered Solutions
When OH- is added to the buffer, the OH- reacts with HX to produce X- and water - But, the [HX]/[X-] ratio remains ~constant, - so the pH is ~constant Or, when: H+ is added to the buffer, X- is consumed to produce HX. - Once again, the [HX]/[X-] ratio remains ~constant - so the pH is ~constant
Buffer Capacity
Buffer capacity is the amount of acid or base neutralized by the buffer before there is a significant change in pH. - Buffer capacity depends on the composition of the buffer
i.e. The greater the amounts of conjugate acid-base pairs, the greater the buffer capacity
Buffered Solutions
Buffer Capacity and pH
- log[H + ] = - log K a - log pH = pK a + log [X- ] [HX] [HX] [X- ]
Addition of Strong Acids or Bases to Buffers We break the calculation into two parts: - Stoichiometric, and - Equilibrium.
Buffered Solutions
Addition of Strong Acids or Bases to Buffers The amount of strong acid or base added results in a neutralization reaction: X- + H3O+ HX + H2O HX + OH- X- + H2O. By knowing how much H3O+ or OH- was added (stoichiometry) we know how much HX or X- is formed. With the concentrations of HX and X- (note the change in volume of solution) we can calculate (estimate) the pH from the Henderson-Hasselbalch equation (using Ka).
Buffered Solutions
Addition of Strong Acids or Bases to Buffers
= pH ~ pK a + log
conjugate base acid
(note that this equation may be an estimate
since the equation assumes that only one equilibrium equation is important Ka.)
Strong Acid-Strong Base Titrations
Titration curve = A plot of pH versus of volume acid (or base) added to the solution
Acid-Base Titrations AcidStrong Base-Strong Acid Titrations
The equivalence point in a titration is the point at which the acid and base are present in stoichiometric quantities. The end point in a titration is the observed point. The difference between equivalence point and end point is called the titration error.
Strong Acid-Strong Base Titrations
If adding a strong base (e.g. NaOH) to a solution of a strong acid (e.g. HCl):
1. Before any base is added, the pH is dominated by the strong acid solution; & pH < 7 2. When base is added, before the equivalence point, the pH is given by the amount of strong acid still in excess; pH < 7 3. At the equivalence point, the amount of base added is stoichiometrically equivalent to the amount of acid originally present. pH is determined by the salt solution, and pH = 7 4. After the equivalence point, base dominates & pH > 7
Weak Acid -
S trong Base Titrations
Consider the titration of acetic acid, HC2H3O2 & NaOH 1. Before any base is added, the solution contains only weak acid
- pH is given by the equilibrium calculation
2. As strong base is added, the strong base consumes a stoichiometric quantity of weak acid:
HC2H3O2(aq) + NaOH(aq) C2H3O2-(aq) + H2O(l)
There is an excess of HA (the acid) before the equivalence point
(we have a mixture of weak acid and its conjugate base) a. First the amount of C2H3O2- generated is calculated, as well as the amount of HC2H3O2 consumed (Stoichiometry) b. Then the pH is calculated (Henderson-Hasselbalch) using equilibrium conditions
Weak Acid S trong Base Titrations
Weak Acid -
S trong Base Titrations
3. At the equivalence point (which is determined by Ka), - all the acetic acid has been consumed, - and all the NaOH has been consumed - however, C2H3O2- has been generated
Therefore, the pH is given by the C2H3O2- solution. This means pH > 7
More importantly, pH 7 for a weak acid - strong base titration
4. After the equivalence point - the pH is given by the strong base in excess
pH
*Opposite for weak base being titrated with a strong acid
Titrations of Polyprotic Acids
In polyprotic acids - each ionizable proton dissociates in steps - Therefore, in a titration there are n equivalence points corresponding to each ionizable proton For example: In the titration of H3PO3 with NaOH
The first proton dissociates to form H2PO3 Then the second proton dissociates to form HPO32-
End of Chemistry 122 Topics!