Chapter 6- Aqueous Ionic Equilibrium Flashcards
What is a buffer?
What are two things it has?
It resists pH change by neutralizing added acid or base
It has either:
- Significant amounts of a weak acid and it’s conjugate base
- Significant amounts of a weak base and its conjugate acid
What is an example of a buffer?
Dissolving acetic acid (HC2H3O2) and sodium acetate (NaC2H3O2) in water
Then if you add anything like strong base NaOH, the acetic acid will neutralize it
*as long as the amount of NaOH is significantly less than HC2H3O2
What is the Henderson-hasselbalch equation?
pH= pKa + log [base]/[acid]
Calculates pH of a buffer solution from the initial concentrations of buffer components
Only can be used if x is small is valid
This is provided but good to know
What are the two circumstances the x is small approximation works?
- The initial concentrations of acids (and/or bases) are not too dilute
- The equilibrium constant is fairly small
What are the two steps in calculating the pH change in a buffer solution?
- The stoichiometric calculation- how the addition changes the relative amounts of acid and conjugate base
- The equilibrium calculation- calculate the pH based on the new amounts of acid and conjugate base
Does stoichiometry affect the before addition and after addition table?
Yes
What is the buffer equation when you add a strong acid to a reaction?
(H+)+ (A-) -> HA
A- mol decreases
HA mol increases
What is the buffer equation when you add a strong base to the reaction?
OH- + HA -> H2O + A-
HA decreases
A- increases
When is a buffer most effective for relative amounts of acid and conjugate base?
When the concentrations of the acid and conjugate base are equal
The relative concentrations of acid and conjugate base should not differ by more than a factor of 10 in order for the buffer to be reasonably effective
When is a buffer most effective for absolute concentrations of acid and conjugate base?
When the concentrations of acid and conjugate base are highest
Ex: 0.50 mol HA and A- is better than 0.050 mil of HA and A-
What is the effective range for a buffering system? (Buffer range)
One pH unit on either side of pKa
Ex: if pKa= 5.0
Range is 4.0-6.0
Lowest pH is when base is 1/10 concentrated as acid
Highest pH is when base is 10 times as concentrated as the acid
What is the buffer capacity?
The amount of acid or base we can add to a buffer without destroying its effectiveness
Buffer capacity increases with increasing absolute concentrations of the buffer components
Also increases as relative concentrations of buffer components become closer to eachother
What is the equivalence point of an acid base titration?
The point where the # of moles of base is stoichiometrically equal to the # of moles of acid
On graph it is the middle point where the graph spikes up quick (inflection point)
What is the titrant and the analyte?
Titrant- acid or base being added by burette
Analyte- acid or base in Erlenmeyer flask
What is the equivalence point pH when the salt is neutral?
Acidic?
Basic?
Neutral- pH=7
Acidic- pH<7
Basic- pH>7
How do you find how much acid or base must be added to reach the equivalence point when given the initial volume of the solution in the flashes and the molarities of each?
Example on page 320-323
Find moles of solution in flask, then use stoich to find the moles of the added base or acid you need, then use molarity to convert that to volume
Does the volume of the equivalence point of an acid base titration change if it’s strong acids or bases or a weak acid or base?
No, the point is the same
How do we find the initial pH when titrating a weak acid with a strong base?
We use an ice table since it’s weak
HA + H2O -> H3O+ + A-
Use x is small
How do you calculate the pH at given volumes for a titration (before equivalence point) of a weak acid and with a strong base?
Use the before after addition table then use Henderson hasslebakch equation
OH- + HA -> H2O + A-
HA decreases
A- increases
What the equivalence point of buffers what does pH always equal?
pH=pKa
How do you find the initial concentration of A- for the equilibrium problem after the equivalence point in a titration of a weak acid with a strong base?
Mols of A-/ (initial volume) + (added volume at equivalence point)
What is a solution that is titrated with a weak acid and a strong base between initial pH and the equivalence point?
It is a buffer
So we can use the Henderson hasselbach equation to find pH
What is the difference between the titration of a weak acid with a strong base and the titration of a weak base with a strong acid?
No difference besides on the graphs
Titration of weak acid with strong base will have pH starting low and going up
Titration of weak base with strong acid will start high pH and go down
How are the differences in volume between the first equivalence point and the second equivalence point in a polypeptide acid related?
They are the same distance apart as the first equivalence point is to zero
What is an indicator?
A weak organic acid that is a different colour than its conjugate base
When will a solution with an indicator have an intermediate colour?
When the pH of the solution equals the pKa of the indicator
What is the solubility product constant (Ksp)?
The equilibrium expression for a chemical equation representing the dissolution of an ionic compound
CaF2 (s) Ca (aq) + 2F (aq)
Ksp= [Ca][F]^2
How do you find the molar solubility from Ksp?
Use the equilibrium approach (ICE table)
Set up ice table with both products starting at zero then put (+S) in the change column
Ksp= [A][B]
Under which only circumstance can we compare Ksp and molar solubility values between two reactions?
When the dissociation stoichiometry is the same
EX: 1 mol compound= 3 mol ions
The solubility of an ionic compound is _________ in a solution containing a common ion than in pure water
Lower
How does pH affect the solubility of a compound?
Higher pH, lower solubility for compounds with OH-
What is a precipitation reaction?
Occurs upon the mixture of two solutions containing ionic compounds and one of the combos of cation and anion of the two produces an insoluble solution
How does the relationship between Q and Ksp show whether a solution is unsaturated, saturated, or supersaturated?
QKsp = supersaturated solution (will precipitate)
What is always the pH at the equivalence point of the titration of a strong base and acid?
7
