Topic 18.3: pH curves Flashcards
Titration definition
Technique used in analytical chemistry to determine the concentration of an unknown acid or base
Titration curve definition
Graph of the pH (vertical axis) versus the amount of reagent progressively added to the original sample
Strong Acid + Strong Base Titration Curve
a) Initial pH = pH of strong acid
b) pH changes only gradually until equivalence
c) Very sharp jump in pH at equivalence (point of inflexion)
d) After equivalence the curve flattens out at the pH of strong base
e) pH at equivalence = 7
Weak Acid + Stronger Base Titration Curve
a) Initial pH = pH of weak acid
b) pH stays relatively constant until equivalence – labelled as buffer region
c) Jump in pH at equivalence, which is not as much of a jump as for a SA – SB titration
d) After equivalence, the curve flattens out at the pH of strong base
e) pH at equivalence > 7
Half-equivalence point
It represents where half of an acid has been neutralized by base and converted into salt, while the other half of the acid in the flask remains unreacted.
What mixture is found in the half-equivalent point?
A buffer, explaining why the pH in this region is relatively resistant to change in pH on the addition of small amounts of base
(pH = pKa)
Strong Acid and Weak Base Titration Curves
a) Initial pH = pH of strong acid
b) pH stays relatively constant through the buffer region to equivalence
c) Jump in pH at equivalence from about pH 3.0 – 7.0
d) After equivalence the curve flattens out at a pH of weak base
e) pH at equivalence is <7
Weak Acid and Weak Base Titration Curves
a) Initial pH = pH of weak acid
b) Addition of base causes the pH to rise steadily
c) Change in pH at the equivalence point is much less sharp than in the other titrations
d) After equivalence the curve flattens at the pH of weak base
Definition of indicator
Weak acid / base where the components of the conjugate acid-base pair have different colors
Chemical formula of indicator
a) Weak acid
b) Weak base
a) HIn ⇌ H+ + In-
b) In- + H2O ⇌ HIn + OH-
What factor influences the change of color of indicators?
pH of the solution
Explanation of the effect of pH on the change of color of indicator
a) HIn ⇌ H+ + In-
a) Increasing [H+]: The equilibrium will shift to the left in favor of HIn
b) Decreasing [H+]: The equilibrium will shift to the right in favor of In-
When do indicators change color?
a) When the pH is equal to their pKa
b) The addition of a very small volume of acid/base will shift the equilibrium and cause the indicator to change color
Indicators used with:
a) Strong acids
b) Strong bases
a) Methyl orange
b) Phenolphthalein
End-point of an indicator
pH at which it changes color (pKa = pH)
Equivalence point
Point where stoichiometrically equal amounts of acid and base have neutralized each other
When is an indicator effective?
When its end-point coincides with the pH at the equivalence point.
How to choose a proper indicator for a titration
a) The equivalence point of the titration occurs within the pH range or pKa of the indicator
b) One drop of the solution being added from the burette should change the indicator from color I to color II
What does the pH range of an indicator imply?
pH values between which the indicator has intermediate colors because comparable amounts of the unionized and ionized forms are present.
Neutralization reaction
Reaction between an acid and base to produce salt (MOH + HA => M+A- + H2O)
What are the conjugate base and acid in the salt produced during a neutralization reaction?
a) Cation is the conjugate acid of the parent base
b) Anion is the conjugate base of the parent acid
Salt hydrolysis
Reaction in which a salt reacts with water to give back the acids and the base
Type of salt produced in a neutralization reaction between:
a) SA + SB
b) WA + SB
c) SA + WB
d) WA + WB
a) Neutral (pH = 7)
b) Basic (pH > 7)
c) Acidic (pH < 7)
d) Depends on the relative Ka and Kb of both cation and anion
Buffer definition
Solutions that can resist pH change upon the addition of an acid or alkali
Composition of a buffer
Acid and base of a weak conjugate pair
How is an acidic buffer produced?
Made by mixing a solution of a weak acid with its salt of a strong alkali
What reacts in an acidic buffer with additional acid?
Conjugate weak base
What reacts in an acidic buffer with additional base?
Weak acid
How is a basic buffer produced?
Made by mixing a solution of a weak base with its salt of a strong acid
What reacts in an basic buffer with additional acid?
Weak base
What reacts in an basic buffer with additional base?
Conjugate weak acid
Methods to produce buffers (2)
a) Mixing a weak acid/base with a solution of a salt containing its conjugate
b) Partially neutralizing a weak acid/base with a strong acid/base
Buffer capacity
Amount of acid/base it can absorb without changing pH
Factor that affect buffer capacity
Dilution
