ch 17 Flashcards
common ion effect
the extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte.
To calculate the pH when a common ion is involved:4
- Identify the major species in solution, and consider the acidity or basicity.
- Identify the important equilibrium that is the source of H+ and therefore determines pH
- Tabulate the concentrations of ions involved in equilibrium. The initial concentration of the common ion is the initial concentration of the strong acid
- Use equilibrium-constant expression to calculate [H+] and then pH
buffer solution
Buffered solutions: solutions that contain weak conjugate acid-base pair that can resist drastic changes in pH upon addition of small amounts of strong acid/strong base.
h h equation
Another way to calculate pH of buffer solution: Henderson-Hasselbalch equation: pH=pKa+log[base]/[acid]where [acid] and [base] are the eq. Conc of the conjugate acid base pair. Good for before equilibrium. At midway point: PH=pka
buffer capacity
Buffer capacity: amount of acid or base the buffer can neutralize before the pH begins to change an appreciable degree. Depends on the amount of acid and base from which the buffer is made. Greater concentrations of both conjugate acid and base, the more resistant the ratio of the concentrations and pH is to change and greater buffer capacity.
pH range
pH range: range over which the buffer acts effectively. When the concentrations of the conj acid and base are the same, the buffer most effectively resists a change in both directions. To maximize range, choose acid whose pKa is close to desired pH. When the conc of one component is more than 10 times conc of other, buffering action is poor.
when strong bases/acids added to buffers3
- Assume strong acids/bases are completely consumed by reactions with buffer (unless buffering capacity is exceeded)
- To calculate pH: 1) Stoichiometry calculation: Consider acid-base neutralization reaction and determine its effect on [HX] and [X-] (strong acid increases HX, strong base increase X). Calculate new conc. Of components
- 2)equilibrium calculation: use Ka and new concentrations to calculate [H+] using Henderson-hasselbalch equation.
pH titration curve
a pH meter can monitor the progress of a acid-base titration and plot a graph as a function of volume added of titrant vs pH.
strong acid with strong base titration: s curve6
Initial pH: determined by initial concentration of strong acid. Should be low.
Between initial pH and equivalence point: as strong base is added, pH increases slowly, then rapidly in vicinity of eqiv point. pH before equiv point is determined by conc of acid that has not been neutralized yet
At equiv point: equal moles of acid and base reacted, leaving only a solution of their salt. pH=7.00 because cation of base and anion of acid do not hydrolyze and don’t affect pH
After equiv point: pH determined by conc of excess base in solution
End point: point in titration where indicator changes color. Closely approximates by not equal to equiv point.
Titrating strong base with strong acid would yield a reflected s curve (pH would be high initially)
weak acid by strong base: s curve
Initial pH: pH of weak acid
Between initial pH and equiv point: solutioin contains mixture of weak acid and anion of acid (being neutralized by OH). To find pH: determine conc of weak acid and anion, then use procedure for buffer reactions to find pH
At equiv point: The acid and base are reacted completely forming their salt. The cation has no effect on pH, but the anion is a weak base so the pH at the equiv point is greater than 7. To calculate pH: determine moles of weak acid initially =moles of acid anion at equiv point, then divide by volume and calculate pH using Kb.
After equiv point: the [OH] from the reaction of anion with water is negligible compared to [OH] from excess strong base. pH is determined by conc of OH from excess strong base (just like strong acid-strong base titration)
How pH titration curves for weak acid-strong base differ from strong acid-strong base3
- Solution of weak acid has higher initial pH than strong acid of same conc.
- pH change at the rapid portion near equiv point is smaller for weak acid than strong acid. (choice of indicator is more crucial for weak acid)
- pH at equiv point is above 7.00 for weak acid
ksp
Solubility-product constant Ksp is the equilibrium constant and indicates how soluble the solid is in water. Ksp=[cation]n[anion]m . do not include solids, liquids and solvents in Ksp. When Ksp=small, only small amount will dissolve in water
Solubility is expressed as g solute/L soln. Molar solubility is moles solute/L soln. Ksp is unitless
factors that affect solubility
Factors that affect solubility: temp, presence of common ions, pH of soln, and presence of complexing agents.
common ion affects solubility
the presence of a common ion (one of the ions in the equilibrium equation) in the solution reduces solubility of ionic compound, shifting equilibrium to the left. The solubility of a slightly soluble salt is decreased by presence of a second solute that furnishes a common ion.
pH affects solubility
The solubility of a substance whose anion is basic will be affected to some extent by pH of the solution. Solubility of Mg(OH2) (OH- is basic) greatly increases as the acidity of the solution increases. Rule: the solubility of slightly soluble salts containing basic anions (CO32-, PO43-, CN-, S2-) increases as[H+] increases and pH is lowered. More basic anion=more solubility is influenced by pH. Anions of strong acids have negligible basicity and are unaffected by pH changes.
