Lecture 4: Properties of Water

Question 1

What is an example of how H bonds have a partial ionic character?

Answer 1

H-bonding between water molecules with partial negative and positive charged

Question 2

How many H bonds can a water molecule form? What is important to note about these? Why?

Answer 2

4 H bonds with 4 different water molecules. No double dipping!
Because of tetrahedral orientation of the H atoms and lone pair about the central O

Question 3

What can be said about the lifetime of each individual H bond? What can affect it?

Answer 3

VERY short

Increasing temperatures = decreasing lifetime of H bonds

Question 4

What confers cohesion of liquid water?

Answer 4

H-bonding network

Question 5

What is often the effect of dissolution on enthalpy?

Answer 5

Breaking of enthalpically favorable interactions

Question 6

What happens to the H+ atoms after water ionizes?

Answer 6

Instantaneously hydrated to form hydronium ions (H30+) and are never really found in aqueous solution. So the real reaction looks more like this:
2H2O H3O+ + OH–

Question 7

What is water’s concentration under standard conditions?

Answer 7

55.5 M

Question 8

What is the ion products of water?

Answer 8

Kw = [H+].[OH-] = 10^-14 M^2

Question 9

What is the equation to calculate pH?

Answer 9

pH = log 1/[H+] = – log [H+]

Question 10

What does alkaline mean?

Answer 10

Basic

Question 11

What does a change of pH of 1 unit correspond to in terms of concentration?

Answer 11

10 fold concentration change

Question 12

What is the pH of arterial blood?

Answer 12

7.41

Question 13

What is the pH of venous blood?

Answer 13

7.38

Question 14

What can be said of aqueous solutions at equilibrium?

Answer 14

They maintain electrical neutrality: # of anions = # of cations

Question 15

What is the difference between strong and weak acids?

Answer 15

Strong acids completely dissociate into their component ions in solution

Question 16

What is Ka? How to calculate it?

Answer 16

The characteristic equilibrium constant of weak acids

Ka = [H+].[A-]/[HA]

Question 17

What is the pKa equation?

Answer 17

pKa = - log Ka

Question 18

How many pKas do polyprotic acids have?

Answer 18

1 for each H released

Question 19

What does a lower pKa mean?

Answer 19

Stronger tendency to release the H

Question 20

Why are weak acids good buffers? When are they optimal and why?

Answer 20

Because they only dissociate partially there is both HA and A- in solution.

• When you add a strong acid, the A- reacts with it to produce water: very small pH change
• When you add a strong base the HA reacts with it to produce water: very small pH change

They are optimal at a pH equal to their pKa (within a 1 pH unit range) because the acid has released 50% of its protons at the pKa: [HA] = [A-]

Question 21

What is the Henderson-Hasselbach equation? What does it mean?

Answer 21

pH = pKa + log [A-]/[HA]

Means that when you have more dissociation than not, pH>pKa

Question 22

What molecule is the buffer in our blood?

Answer 22

H2CO3

Question 23

What is the reaction to produce H2CO3 in the blood?

Answer 23

H2O + CO2 H2CO3 H+ + HCO3-

Question 24

What is carbonic acid’s pKa?

Answer 24

3.77 at 37°C

Question 25

How can we adjust blood pH quickly?

Answer 25

Breathing!

Question 26

What will happen to the equilibrium between carbonic acid and
bicarbonate if I add a strong acid to this system?

Answer 26

Reaction will be driven to form more carbonic acid because more H+ will be available

Question 27

What do we use in clinical medicine as the acid in the HH equation?

Answer 27

CO2

Question 28

What is bicarbonate?

Answer 28

HCO3-

Question 29

What is the buffer system of mammalian cells?

Answer 29

Phosphate System: H2PO4– H+ + HPO42–

Question 30

What do humans do to regulate their blood pH when it drops?

Answer 30

Breathe more heavily to get rid of the CO2 faster

Question 31

How can [HCO3-] be adjusted by the human body?

Answer 31

Exhalation and renal action