Lecture 4: Properties of Water Flashcards
What is an example of how H bonds have a partial ionic character?
H-bonding between water molecules with partial negative and positive charged
How many H bonds can a water molecule form? What is important to note about these? Why?
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
What can be said about the lifetime of each individual H bond? What can affect it?
VERY short
Increasing temperatures = decreasing lifetime of H bonds
What confers cohesion of liquid water?
H-bonding network
What is often the effect of dissolution on enthalpy?
Breaking of enthalpically favorable interactions
What happens to the H+ atoms after water ionizes?
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–
What is water’s concentration under standard conditions?
55.5 M
What is the ion products of water?
Kw = [H+].[OH-] = 10^-14 M^2
What is the equation to calculate pH?
pH = log 1/[H+] = – log [H+]
What does alkaline mean?
Basic
What does a change of pH of 1 unit correspond to in terms of concentration?
10 fold concentration change
What is the pH of arterial blood?
7.41
What is the pH of venous blood?
7.38
What can be said of aqueous solutions at equilibrium?
They maintain electrical neutrality: # of anions = # of cations
What is the difference between strong and weak acids?
Strong acids completely dissociate into their component ions in solution
What is Ka? How to calculate it?
The characteristic equilibrium constant of weak acids
Ka = [H+].[A-]/[HA]
What is the pKa equation?
pKa = - log Ka
How many pKas do polyprotic acids have?
1 for each H released
What does a lower pKa mean?
Stronger tendency to release the H
Why are weak acids good buffers? When are they optimal and why?
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-]
What is the Henderson-Hasselbach equation? What does it mean?
pH = pKa + log [A-]/[HA]
Means that when you have more dissociation than not, pH>pKa
What molecule is the buffer in our blood?
H2CO3
What is the reaction to produce H2CO3 in the blood?
H2O + CO2 H2CO3 H+ + HCO3-
What is carbonic acid’s pKa?
3.77 at 37°C
How can we adjust blood pH quickly?
Breathing!
What will happen to the equilibrium between carbonic acid and
bicarbonate if I add a strong acid to this system?
Reaction will be driven to form more carbonic acid because more H+ will be available
What do we use in clinical medicine as the acid in the HH equation?
CO2
What is bicarbonate?
HCO3-
What is the buffer system of mammalian cells?
Phosphate System: H2PO4– H+ + HPO42–
What do humans do to regulate their blood pH when it drops?
Breathe more heavily to get rid of the CO2 faster
How can [HCO3-] be adjusted by the human body?
Exhalation and renal action
