Lecture 2

Question 1

Bronsted-Lowry Acids and Bases?

Answer 1

Acids and bases can donate or accept a proton, respectively.
- Acids donate protons (H)
- Bases accept protons (H)

Question 2

Water ionization?

Answer 2

Water can ionize, it is a very weak acid and base. Water can also partially ionize (auto ionization) into H ions (protons) and OH ions - the H existing as hydronium ions.

Question 3

Water ionization reaction?

Answer 3

2H2O <-> H3O + OH

• One H2O dissociates, leaving hydroxide and H, but the H won’t exist on its own in solution - so they immediately associate with the second H2O to form hydronium.
• Essentially, the H proton released by one H2O is cited up by a neighbouring H2O, hence why there are 2H2O in the equation.

Question 4

Hydronium ions?

Answer 4

Hydronium ions are very mobile due to their H bonding network .

Question 5

The ionization constant of water?

Answer 5

H2O <-> H + H3O

Water ionization can be expressed as an equilibrium:

Keq = [H] x [OH] / [H2O] = 1.8 x 10^-16 M
this is products over reactants

Since the [H2O] is always ~ 55.5M
Kw = Key x [H2O] = [H] x [OH] = 10^-14 M

Question 6

Pure water is?

Answer 6

Pure water is neutral: [H] x [OH] = 10^-7 M

If [H] > 10^-7M, solution is acidic
If [H] < 10^-7M, solution is basic

Question 7

pH scale?

Answer 7

[H] and [OH] are more easily expressed as pH and pOH.

pH = -log[H]
Also pH + pOH = 14
pOH = -log[OH] (but pH is preferred)

• As [H] decreases, [OH] increases
• When they are equal their concentration is both 10^-7M

Question 8

Log pH scale?

Answer 8

The pH scale is logarithmic. Therefore, if something is pH 8 it is 10 times less acidic than something pH 7 because the measurements use exponents.

• if something is [H] 2, then the pH is 3 as well.

Question 9

Biochemistry pH?

Answer 9

Highly basic - NaOH
Highly acidic - HCl

We are most interested in more neutral pH, as the body is typically around pH 7, and too high or low pH will be harmful to proteins and such.
- Neutral = human blood, tears, milk, saliva.

Question 10

pKa?

Answer 10

A weak acid’s tendency to ionize is defined by its pKa. Strong acids tend towards complete dissociation, but what about weak acids? Weak acids do not complete dissociate.

Question 11

Weak conjugate acid-base pair acetic acid and acetate?

Answer 11

CH3COOH <-> CH3COO + H
acetic acid acetate ion
intact. dissociated
conj. acid conj. base
donate H accept H

The equilibrium (or acid dissociation) constant (Ka) is defined as:
Ka = [CH3COO] x [H] / [CH3COOH] = 1.74x10^-5M

Like pH, this can be expressed logarithmically:
pKa = -log[Ka] = -log(1.74x10^-5) = 4.76

Question 12

Why is it equilibrium concentration?

Answer 12

Where it’s a weak acid, it doesn’t tend towards complete dissociation and can go either way depending on the conditions it is in.

Question 13

pKa numbers meaning?

Answer 13

pKa is the pH value at which a molecule is half dissociated and half not.

Ex. pKa of 4.76

• Numbers tend to be smaller for weak acids because you get fewer dissociated products and more acid.

Question 14

pKa values are?

Answer 14

pKa values are constants for each acid: they’re useful because they tell us how they might behave in a solution.

Question 15

pKa values meaning?

Answer 15

The lower the pKa, the stronger the acid. The higher the pKa, the weaker the acid (and the stronger its conjugate base)

• Some chemicals have multiple ionizable groups/pKa values (phosphoric acid) where the repeat indicates that multiple protons are being release at different pH values - giving the molecule different properties.

Question 16

Which compound is the most acidic?

A) Acetic acid (pKa = 4.76)
B) Trifluoroacetic acid (pKa = 0.18)
C) Phosphate (pKa = 6.82)
D) Boric acid (pKa = 9.24)
E) Sodium hydroxide

Answer 16

B) Trifluoroacetic acid (pKa = 0.18)

• This molecule very easily gives up its hydrogen ion/proton. It likes to do this.

Question 17

Relating pH of an acid solution to its pKa?

Answer 17

For a typical acid-base equilibrium..

HA <-> H + A and Ka = [H][A]/[HA]

Rearranged Ka to solve for [H]:
[H] = Ka ([HA/[A])

Taking negative log of both sides:
-log[H] = -log[Ka] - log([HA]/[A])

This gives the Henderson Hasselbalch equation, which is used to determine the pH of a weak acid:
pH = pKa + log([A]/[HA])

Question 18

Henderson-Hasselbalch equation?

Answer 18

pH = pKa + log([A]/[HA])

Using the H-H equation, we can determine the pH of a solution of a weak acid when a strong acid or base is added to it.

Question 19

What do buffer systems do?

Answer 19

Buffer systems resist pH changes.

Question 20

Le Chatelier’s principle?

Answer 20

When you perturb a system in a certain way, it will respond in a way that counteracts the perturbation - compensating for changes.
- We can predict what will happen!

Question 21

Buffer system and Le Chatelier’s principle?

Ka = [H][PO4]/[HPO4]

Answer 21

pK = 6.82

H2PO4 <-> H + HPO4

• When a strong acid is added (more H), buffer shifts to take up H <-
  Production of H2PO4 favoured
• When a strong base is added (less H), buffer shifts to release H. ->
  Production of H is favoured - compensating for lost H.

Question 22

Buffers…

Answer 22

Buffers compensate for the addition or loss of H ions

Question 23

Acetate buffer system example?

Answer 23

If not too much acid or base is added, and the pH is at the right level, the buffer will compensate for any alteration.

CH3COOH + H2O <-> H3O + CH3COO
Added H3O is a shift to the left
Added OH is a shift to the right

• In this example, the equation shows acetic acid and water on the left and acetate and a hydronium ion (protonated water) on the right. These are in equilibrium and can shift to the right if base is added and shift to the left if acid is added.
• At a specific pH, there will be equal concentrations of acetic acid and acetate ion. This is the pKa for this buffer (4.76) where at 4.76 you will have equal amounts of conjugate acid and conjugate base.

Question 24

Henderson - Hasselbalch titration curve for acetate?

Answer 24

pKa = 4.76

Fast curve when below 3.76 and above 5.76. Effective buffering is within that range, where the curve slows down - meaning the buffer is more capable of resisting change.

[A]/[HA] = 0.1 means pH = pKa -1
[A]/[HA] = 10 means pH = pKa + 1

Question 25

Buffer region?

Answer 25

The pKa plus or minus one unit. Has a shallow slope and resists pH change. Buffers work best when they have at least 10% of the buffer in both forms.

Question 26

Buffers and homeostasis?

Answer 26

Buffers resist pH changes and are critical to maintain homeostasis

Question 27

Buffer scenario #1

Answer 27

ADD PHOTO

Question 28

Buffer scenario #2

Answer 28

ADD PHOTO

Question 29

Buffer scenario #3

Answer 29

ADD PHOTO

Question 30

Buffer scenario #4

Answer 30

ADD TWO PHOTOS

Question 31

Phosphate?

Answer 31

Phosphate is an intracellular buffer - carefully regulating the pH inside our cells. Phosphate is an important intracellular buffer due to its physiological pKa of 7.2 and relatively high concentration of ~15mM.

Question 32

Bicarbonate?

Answer 32

Bicarbonate is the major extracellular buffer in blood, plasma, lymphatic fluid. ADD PHOTO

Question 33

Homeostasis?

Answer 33

The body can compensate to maintain homeostasis. H + HCO3 <-> H2CO3 <-> H20 +C02 - H2O and HCO3 are regulated by kidneys - CO2 is regulated by breathing Equilibrium shifts right due to excess acid (metabolic acidosis) or low CO2 (hyperventilation) Equilibrium shifts left due to insufficient HCO3 (renal problems) or high CO2 (poor lung function) - Breathing (fast) and kidney function (slow) can be used to regulate the pH of the body, and compensate for different disorders (acidosis)

Question 34

pH regulation is critical?

Answer 34

Life is only tolerable within a very narrow range of blood pH (7.2-7.6)

Question 35

Proteins at high and low pH?

Answer 35

At lower values of pH, proteins carry a more positive charge. At higher values of pH, proteins carry a more negative charge. - These charge changes will alter the functions of any charged molecules or proteins.

Question 36

Ionizable groups?

Answer 36

Biomolecules have many ionizable groups (NH3, COO) and can change properties with pH.

Question 37

Isoelectric point?

Answer 37

In the example, the isoelectric point (pl) is 6.8. This is the point at which the molecule is far less soluble and will not dissociate.