Lec3 - Water: Life Proceeds in an Aqueous Environment

Question 1

What is the chemical reason for solubility?

Answer 1

Hydrogen bonding between water molecules and polar solutes: the formation of H bonds makes solvation energetically favourable

Question 2

Describe how solvation of an ionic substance such as NaCl occurs

Answer 2

Water molecules disrupt the ionic lattice by forming “hydration shells” around the ions, which is enertgetically favourable, as electrostatic forces are weaker in water than most mediums

Question 3

Why can benzene not be dissolved in the same way as NaCl?

Answer 3

The hydration shell of a non-polar molecule like benzene would be energetically unfavourable

Question 4

What is meant by amphipathic (or amphiphillic) molecules?

Answer 4

Molecules that have distinct groups with OPPOSITE water properties (e.g., a lipid with a hydrophillic head and a hydrophobic tail)

Question 5

What happens when amphipathic molecules interact with water?

Answer 5

The hydrophobic tail imposes an unfavourable arrangement on water molecules, meaning hydrophobic tails tend to arrange themselves together to minimise this effect (this causes micelles and membranes to form)

Question 6

What is Kw, what does it represent, and what does this tell us about the concentration of ions in water?

Answer 6

Kw = [H+][OH-] ; it is the ion product constant of water, derived from the K(eq) equation`

Question 7

Kw = 10^-14 M^2 ; what does this show about the concentration of ions in water?

Answer 7

Since the concentrations of H+ and OH- are equal in the equation, each of these must be 10^-7 M^2

Question 8

How are pH and pOH related?

Answer 8

Since Kw = [H+] x [OH-], pH + pOH = -log10(10^14), therefore when pH = 3, pOH = 11 ; when pH = 7, pOH = 7, etc

Question 9

Is water acidic?

Answer 9

Yes, but it is an extremely weak acid (compared to HCl for example)

Question 10

In the (HA <-> H+ + A-) equation, what is the difference between strong and weak acids in terms of equilibrium?

Answer 10

For strong acids, the equilibrium is far to the right (almost complete dissociation), whereas for weak acids it is much further to the left

Question 11

How can you work out the pH of a weak acid?

Answer 11

In the Ka equation, represent both [H+] and [A-] as X (since they must be equal).
Represent [HA] as (Concentration of acid in water - X)
This gives a quadratic equation that can be solved for [x]
Take -log(10) of [x] to get pH

Question 12

How can you calculate the % dissociation of a weak acid after calculating pH (using the method described in another flashcard)?

Answer 12

Take [x] as [H+]; this must be equal to [CH3COO-]; subtracting this from the total acid concentration of (for example) 10 mM gives 9.59 mM of undissociated acid
Do [x] / [undissociated] x 100 to get a percentage

Question 13

State the Henderson-Hasselbalch equation

Answer 13

log10([CH3COO-] / [CH3COOH]) + pKa = pH

Question 14

When is pKa equal to pH?

Answer 14

When there is an equal concentration of conjugate acid and base (as this means log([A-]/[HA]) = log1 = 0

Question 15

When does buffer function occur, and what does this mean?

Answer 15

When the pH of a solution is near the pKa value, adding a small amount of acid or base has very little effect on pH

Question 16

Briefly describe what happens during the titration of a weak acid

Answer 16

The protonated form HAc is dominant at low pH;
Near the pKa, [HAc] and [Ac-] are approximately equal (and within ~1pH either side of pKa, acid buffers effectively);
At pH above pKa, the DEprotonated form dominates, the concentration of conjugated base is much higher than that of undissociated acid

Question 17

Briefly describe the titration curve of a weak base with a strong acid

Answer 17

Mirror image of a weak acid curve; midpoint represents the pKa of the conjugate acid of the weak base in question

Question 18

Briefly describe the titration curve of a polyprotic acid

Answer 18

Multiple “flat” regions, each midpoint represents 50% dissociation of the first proton, second proton, third proton, etc.

Question 19

Briefly describe the titration curve of an amphiphilic/amphipathic molecule

Answer 19

Example; Glycine has 2 pKa values; one in the acidic range (due to the carboxyl group acting as a proton donor) and one in the alkaline range (due to the amino group acting as a proton acceptor)