chapter 15
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Chapter 15
Applications of Equilibrium
Common Ions
• An ion that is present in both
– An acid and its conjugate base
• HNO2 and NaNO2
– A base and its conjugate acid
• NH3 and NH4Cl
• This common ion effects the position of the equilibrium and the pH
• How will the pH of a solution containing 0.10M HNO2 compare to a solution containing 0.10M HNO2 and 0.10M NaNO2? Why?
Points To Remember
• In common ion situations you MUST consider the initial concentrations of any common ions in solution before you begin the problem
• The larger Ka or Kb will govern the equilibrium
• Calculate the pH of a solution containing 0.10M HNO2 and 0.10M NaNO2? (Ka = 4.0x10-4)
Homework
• P. 774 #’s 21,23a&d, 34,36
Buffered Solutions
• Solutions that resists changes in pH when an acid or base is added
• Made of– Weak acid and its conjugate base– Weak base and its conjugate acid
• Made by adding both chemicals or reacting
Adding
.1M HF
NaF
Reacting
.1M HF
.1M NaOH
• Determine the pH of a solution made by dissolving NaHCO3 to 0.25M in 0.0233M H2CO3 (Ka = 4.3x10-7)
Henderson-Hasselbalch Equ.
• A way of calculating buffer problems
• ONLY USED 4 BUFFERS!!!!!!
HA H+ + A-
][
]][[
HA
AHKa
][
][log
Acid
BasepKapH
• Determine the pH of a solution made by dissolving NaHCO3 to 0.25M in 0.0233M H2CO3 (Ka = 4.3x10-7) Use H-H Equ.
How Do Buffers Work
• Strong acid or base is not allowed to build up in solution
• Strong acid is replaced by weak acid
• Strong base is replaced by weak base
– The strong acid or base reacts completely!!!
– Then equilibrium kicks in
Acidic Buffers
HF H+ + F-
Add NaOH
Add HCl
Basic Buffer
NH3 + H2O NH4
+ + OH-
Add NaOH
Add HCl
• Determine the pH after 0.20 grams of NaOH is added to 500.mL a solution made by dissolving NaHCO3 to 0.25M in 0.0233M H2CO3 (Ka = 4.3x10-7)
What Makes a Good Buffer?
• The best buffers have acids and bases at the same, large concentration
– Added acid or base has little effect on pH
– When the concentrations are the same pH=pKa
Homework
• P. 774 #’s 23d,31,37ab, 40, 42,46,48,49
Titrations
• The controlled addition of a chemical of known concentration to a chemical of unknown concentration
• Known Chemical – Standard or titrant
• Unknown Chemical - Analyte
• Titrations are carried out to the equivalence point– Point when mole ratio of chemicals is reached– Often an invisible point
• Marked by and indicator
• Indicator is a chemical that changes color in different pH’s
• Indicator color change marks the endpoint– Hopefully endpoint and equivalence point are
the same
Titration Curves
• Plotting of the pH of the titration solution as solution is added
• Used as a way of determining equivalence and Ka or Kb values
• Specific ranges for specific acid/base combinations – Strong/Strong or Strong/Weak
Titration Curve HCl vs NaOH
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
Titration Curve Problems
• Remember the process. Do NOT memorize!
• Treat each step as a separate problem
– For Strong/Strong systems just stoichiometry
• For Strong/Weak First stoichiometry then equilibrium
Strong Acid / Strong Base
• Beginning – Just the pH of the chemical you have
• Before Equivalence – Stoichiometry
• Equivalence – pH is 7.00
• After Equivalence - Stoichiometry
• Strong Acid / Strong Base Titration
#54 page 776
80.0 mL of 0.100M Ba(OH)2 is titrated with 0.400M HCl
0.00 mL
20.0 mL
30.0 mL
40.0 mL
80.0 mL
Weak / Strong Titration
• Beginning – Just the pH of the chemical you have (Might need equilibrium)
• Before Equivalence – Stoichiometry then equilibrium
• Equivalence – pH is NOT 7 – Stoichiometry then equilibrium
• After Equivalence - Stoichiometry
Weak / Strong Combos
• Weak Acid / Strong Base
• Equivalence Point is ABOVE seven– Conjugate base of a weak acid controls pH
• Weak Base / Strong Acid
• Equivalence Point is BELOW seven– Conjugate acid of a weak base controls pH
• At halfway pH = pKa or pOH = pKb!!!!!!!!
Weak Base / Strong Acid Titration
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
Weak Acid / Strong Base Titration
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
• Strong Base/ Weak Acid Titration
#55 page 776
100.0 mL of 0.200M Acetic Acid is titrated with 0.100M KOH
0.00 mL
50.0 mL
100.0 mL
200.0 mL
250.0 mL
Homework
• P. 775 #’s 52,53,56
Which Indicator to Use?
• Choosing the appropriate indicator is important when doing titrations
• There are so many good ones to use
• So how does a chemist know???????
Which Indicator to Use?
• Indicators are weak acids and change color with varying pH
• Pick the indicator that changes a little above your calculated equivalence point.
• Pick indicator with pKa value close to equivalence point
• Change color +/- 1 from pKa
Example
• Choose an indicator for the titration of HCl with NH3 pH=5.36
Polyprotic Acids
• These acids have multiple equivalence points
• Carbonic acid has two
• Phosphoric acid has three
• Multiple step points on a titration curve
Diprotic Acid Titration
Molar Mass of an Acid
• You can find the molar mass of an acid by titration too
• Titrate a acid of known mass with a standard to determine number of moles
• Divided grams by moles– Molar Mass
Solubility Equilibrium
• Dissolving is an equilibrium process
• Chemicals that we say are insoluble are actually very slightly soluble– Meaning only a small amount of the solute
dissolves – Concentrations are very small
• 10-5M to 10-20M are common
Solubility Equilibrium
• Iron (II) Sulfide is “insoluble” by solubility rules
• However, it still dissolved to a small extent
• Consider the reaction
FeS(s) Fe+2 + S-2
• Since dissolving is an equilibrium process we can write an equilibrium expression
• Keq = [Fe+2] [S-2]
• FeS is omitted because it is a solid
Solubility Product Constant
• The previous equilibriums expression is a “special” type of equilibrium expression
• Ksp – solubility product constant
• Ksp = [Fe+2] [S-2]– Only used for slightly soluble salts– Never includes the reactants– Ksp’s are very small– Check out the table 15.4 page 753
Ksp Values
• Lead (II) Chloride 2.4X10-4
• Strontium Carbonate 3.8X10-9
• Nickel (II) Hydroxide 5.5X10-16
• Copper (II) Hydroxide 2.2X10-20
• Cadmium Sulfide 8.0X10-28
• Silver Sulfide 6.0X10-51
• And my personal favorite• Bismuth Sulfide 1.6x10-72
Ksp Values
• Q) What does a small Ksp value mean?
• A) Low concentration of ions• Small solubility
• Q) Is the compound with the smallest Ksp the least soluble?
• A) Not necessarily – There are different numbers of ions that changes
the expression
Solubility of Salts
• We can easily compare solubility for salts that have the same # of ions
• When there are the same # of ions the salt with the smallest Ksp is least soluble
• Which salt is least soluble, most soluble AgCl, FeS, BaSO4
– FeS 6.0x10-19 Least soluble– AgCl 1.6x10-10 Most soluble
– BaSO4 1.1x10-10 Middle
Calculating Solubility
• We can calculate solubility of salts and the concentration of the ions in solutions from Ksp
• Deal with saturated solutions– The salt has dissolved as much as it can
• Ksp has been reached
• In saturated solutions the concentrations of the ions are related to the mole ratio
Example• Calculate the solubility of silver chloride
Ksp=1.6x10-10
Example• Calculate the solubility of mercury (II) sulfide
and the concentration of each ion in a saturated solution Ksp=1.6x10-54
Example• Calculate the solubility of calcium phosphate
Ksp=1.3x10-32
Example• A saturated solution of Iron (III) Hydroxide
has a concentration of Fe+3 of 1.8x10-8. What is the Ksp of Iron (III) Hydroxide?
Common Ion Revisited
• Anyone ever have a barium shake for a scan?
• Contains barium sulfate.– Barium ions are very toxic to the body– Treated as calcium
• How can you have fewer barium ions in solution?
• Add Sodium Sulfate
• What is the concentration of barium ions in a saturated solution of barium sulfate?– Ksp = 1.1x10-10
• What is the concentration of barium ions in a when 0.10M Sodium Chloride is added to a saturated solution of barium sulfate
Precipitation
• If you combine Silver Nitrate with Sodium Chloride will a precipitate always form?
• Only if Ksp is exceeded
• Will a ppt. form if 2x10-5M Silver Nitrate is mixed with 3x10-3M Sodium Chloride?
• Ksp of AgCl is 1.6x10-10
• No
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