Lec3 - Water: Life Proceeds in an Aqueous Environment Flashcards
What is the chemical reason for solubility?
Hydrogen bonding between water molecules and polar solutes: the formation of H bonds makes solvation energetically favourable
Describe how solvation of an ionic substance such as NaCl occurs
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
Why can benzene not be dissolved in the same way as NaCl?
The hydration shell of a non-polar molecule like benzene would be energetically unfavourable
What is meant by amphipathic (or amphiphillic) molecules?
Molecules that have distinct groups with OPPOSITE water properties (e.g., a lipid with a hydrophillic head and a hydrophobic tail)
What happens when amphipathic molecules interact with water?
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)
What is Kw, what does it represent, and what does this tell us about the concentration of ions in water?
Kw = [H+][OH-] ; it is the ion product constant of water, derived from the K(eq) equation`
Kw = 10^-14 M^2 ; what does this show about the concentration of ions in water?
Since the concentrations of H+ and OH- are equal in the equation, each of these must be 10^-7 M^2
How are pH and pOH related?
Since Kw = [H+] x [OH-], pH + pOH = -log10(10^14), therefore when pH = 3, pOH = 11 ; when pH = 7, pOH = 7, etc
Is water acidic?
Yes, but it is an extremely weak acid (compared to HCl for example)
In the (HA <-> H+ + A-) equation, what is the difference between strong and weak acids in terms of equilibrium?
For strong acids, the equilibrium is far to the right (almost complete dissociation), whereas for weak acids it is much further to the left
How can you work out the pH of a weak acid?
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
How can you calculate the % dissociation of a weak acid after calculating pH (using the method described in another flashcard)?
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
State the Henderson-Hasselbalch equation
log10([CH3COO-] / [CH3COOH]) + pKa = pH
When is pKa equal to pH?
When there is an equal concentration of conjugate acid and base (as this means log([A-]/[HA]) = log1 = 0
When does buffer function occur, and what does this mean?
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
Briefly describe what happens during the titration of a weak acid
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
Briefly describe the titration curve of a weak base with a strong acid
Mirror image of a weak acid curve; midpoint represents the pKa of the conjugate acid of the weak base in question
Briefly describe the titration curve of a polyprotic acid
Multiple “flat” regions, each midpoint represents 50% dissociation of the first proton, second proton, third proton, etc.
Briefly describe the titration curve of an amphiphilic/amphipathic molecule
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)
