Unit 2 - Equilibrium Part 3

Question 1

Three components of a titration

Answer 1

1) determines the quantity of a substance A by adding measured increments of substance B
2) substance B reacts with substance A
3) has some means of indicating the endpoint at which all of A has reacted

Question 2

Equivalence point

Answer 2

moles of titrant added is equal to the moles of titrand

Question 3

Endpoint

Answer 3

an observable change that approximately signals the equivalence point. It shows that the amount of reactant necessary for a complete reaction has been added to a solution

ex. phenolphthalein indicator turns pink

Question 4

Titration curves for strong and weak acids

Answer 4

Strong acids always have a pH of 7.0 at the equivalence point

Weak acids have variable pH at equivalence point
for weak acids, pH=pKa at the half-equivalence point

Question 5

Half-equivalence point

Answer 5

the volume is half the volume to get to the equivalence point

Question 6

Titration of a weak diprotic acid

Answer 6

has two equivalence points and two Ka’s

Question 7

Henderson-Hasselbalch Equation

Answer 7

an equation used to estimate the pH of buffer

Question 8

Buffer

Answer 8

a mixture of a weak acid and its conjugate base that resists changes in pH with the addition of small amounts of another acid or base

Question 9

Guidelines of titration curves for diprotic titration

Answer 9

Initial - pH determined by Ka1

1st buffer region - pH determined by HA-/H2A buffer; try HH eq.

1st equivalence point - pH determined by geometric mean of Ka1 and Ka2

2nd buffer region - pH determined by A2-/HA- buffer; try HH eq.

2nd eq. point - pH determined by Kb1 = Kw/Ka2

Excess base - pH determined by excess OH- from strong base

Question 10

Guidelines for two clear endpoints for the titration of a weak diprotic acid

Answer 10

As (pKa,2–pKa,1) = 4 or less, the first endpoint begins to disappear
As pKa,2≥9, the second endpoint beings to disappear

If Ka1 >/equal to 10^4, we have a clear first endpoint. Titration curves can be simply derived
If pKa2

Question 11

pH indicator dyes

Answer 11

exhibit a colour change as a function of pH. When it’s below a certain pH, the indicator is colourless. When it’s above a certain pH, the indicator is coloured

Question 12

Why should pH indicator be approx. the same pH as the sol. endpoint?

Answer 12

this is so that if the solution endpoint is, for example, at pH = 8, we can see the colour change if pH jumps from 7 to 9, if the indicator pH = 8

In general, you want to make sure the pKa of the indicator is greater than the pKa of the solution, but still relatively close in value