Biophysical Chemistry: Water, Buffers, Weak Acids, Ph,

Question 1

Water’s life supporting properties?

at least 5

Answer 1

1. Water is a polar molecule
2. Water is cohesive
3. Excellent Solvent
4. lesser density of ice
5. High heat of vapourization
6. Self ionization of water
7. Acidic and basic properties of water

1. water molecule is bent, non-linear
   * distribution of charge is asymmetric
   * due to high electronegativity of oxygen
   * drawing away -e from H+

2.cohesion-tendency of molecules of a substance to stick together
* Water molecules interact strongly due hydrogen
bonds.
* giving it a high surface tension
Surface Tension: property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules.
* facilitates transport in trees

3.due to its polarity
* ionic compounds and othe polar molecules readily dissolve
* charged particles will form hydrogen bonds with a surrounding layer of water molecules(Sphere of hydration)

4.As water freezes, each molecule forms stable H bonds with neighbouring molecules, holding them at “arm’s
length” and creating a three-dimensional crystal.

6.Smaller percentage of water spontaneously dissociates to produce equal [ ] of H+ and -OH ions i.e 1times 10 power -7
* -OH form H bonds with other water
* H+ attracted to unionized water molecules forming Hydronium ions H3O+

Question 2

*

formulate ionization constant equation, hence define PH

Answer 2

• water dissociates slightly producing equal number of protons (H+) and hydroxide ions (-OH) at concentration of 10^-7 mol/dm3
  H2O ⇋ H+ + -OH
  corresponding equilibrium expression
  Keq= [H+][OH-]/[H2O]
  concentration of pure water at 25 °C [H2O] = 55.5M
  Keq=[H+][OH-]/55.5M
  Keq (55.5M) =[H+][OH-] =Kw (eliminating H2O)
  Kw= [H+][OH-]=10^-14
• concentration more conveniently expressed as PH

PH- is the negative log to the base 10 of the H+ concentration in mol/dm3

ionic product of water remains a constant. if [H+] rises the other must fall to compensates

Question 3

definition of proton hopping

Answer 3

is the process of diffusion of protons (H⁺ ions) through the network of hydrogen-bonded water molecules in the liquid water

hence water acts as both an acid and base (amphoterism)

Question 4

Calculations

1. What is the concetration of H+ in a solution of 0.1M NaOH?
2. What is the concetration of OHin a soulution with an H+ concetration of 1.3 x 10-4 M?

Answer 4

1. (10^-13 M)
2. (7.7 X 10-11 M)

Question 5

pH scale

Answer 5

a measure of the acidity or basicity of a substance(0-14)

each unit is a 10fold change in [H+]

the ionic product of water is the basis for the PH scale

Question 6

Derive the eqution pH+pOH=14 from the ionic product of water?

Answer 6

[H+][OH−] =1.0×10^−14
* On taking logarithmic of this equation :
log [H+] + log [OH−] = log (1.0 × 10−14) = − 14
multiplying by −1: −log [H+]−log [OH−]=14
according to the definitions: pH+pOH=14

Question 7

Describe the Bronsted Lowry Theory of Acids and Bases

Definitions of Acid and Bases

Answer 7

• acid- is a substance that can donate a proton
• Base-is a substance that can accept a proton

proton (H+)

a proton donor(acid) and its corresponding proton acceptor(base) make up a conjugate base pair
HA <> H+ + A-

Question 8

Define Strong and weak acids

Answer 8

• strong acid- a substance that has a greater tendency to lose its proton
  -completely dissociates (or ionizes) in water(HCl and H2SO4)
• weak acid- has a lesser tendency to lose its proton
  -displays a high affinity for its proton
  -does not readily dissociate in water e.g (CH3COOH)

Question 9

give equation for the disociation of HOA acid hence find the dissociation constant equation

Answer 9

HOAc(aq)+H2O(l)<>H3O+(aq)+OAc-(aq)
Ka=[H3O+][OA-]/[HOA]

Strong Acids: Ka>1

Question 10

Calculate the pH and percentage protonation of a 0.20 M aqueous solution of pyridine, C5H5N. The Kb
for C5H5N is 1.8 x 10−9

Answer 10

pH=9.3
percentage protonation=0.0095%

Question 11

Calculations

1. Calculate the pH of a 0.005 M solution of acetic acid CH3COOH? With a Ka of 1.74 x 10 -5. (ICE tables).
2. Calculate the pH of a 0.01 M solution of H2CO3
   carbonic acid, a diprotic acid With Ka1 4.45 x10-7 and Ka2 1x10-4
   .

Answer 11

1. 3.53?

Question 12

buffer solution definition?

Answer 12

• is one that resists a change in pH on the slight addition of acid(H+) or base (OH−), more effectively than an equal volume of water.
  Emphasise on the conjugate acid base pair.

Question 13

Titration of a weak acid and its base

Titration use?

Answer 13

• to determine the amount of an acid in a given solution

concentration of the acid in the original solution can be calculated from the volume and concentration of NaOH added

Question 14

buffer solution consists of a mixture of ?

with the aid of an equation

Answer 14

weak Brönsted acid and its conjugate base dissociated in acqueous solution.

for example, acetic acid and its conjugate base

CH₃COOH + CH3COONa<> Na+ + H+ + 2CH3COO-

Question 15

explain reactions involved upon addition of either base or acid to an acetic acid buffer?

Answer 15

1. addition of alkali or base
   * the OH- ions from NaOH react with H+ ions from the acid to form water
   * no net change of H+ ions hence ph remains constant
   CH3COOH+CH3COONa+NaOH<>2CH3COONa+H2O
2. addition of acid
   * H+ ions from the acid react with the CH3COO- from the sodium acetate to form a weak acetic acid
   * hence the H+ ions are absorbed and the pH remains constant
   CH3COOH+CH3COONa+HCl<>NaCl+2CH3COOH

either cases, there is no change in H+ concentration. The buffer acts
almost as if it were ‘absorbing’ the added free hydrogen or hydroxyl ions

Question 16

titration curve

Answer 16

• is the graph described by the relation of the pH to the addition of H+ /OH– ions
• reveals the pKa of the weak acid @ the point of inflection of the curve

pKa= -logKa

• weak acids have a higher pKa
• At the midpoint of the titration, the concentration of the acid(proton donor) and
  base(proton acceptor) are equal.
• pH is numerically equal to pKa at this point

Question 17

Differentiate between equivalent point and end point

Answer 17

Equivalent is the point at which the amount of added titrant is stoichiometrically equivalent to the amount of analyte(reaction between the titrant and the analyte is complete/all the analyte has reacted with the titrant) whereas endpoint is point at which a physical or chemical change occurs in the solution being titrated, indicating that the reaction between the titrant and analyte is complete.

equivalent point- the concentration of acid and base is the same

Question 18

Explain the Henderson – Hasselbach Equation?

Answer 18

• gives the quantitative relationship between pH, buffering action of a buffer and the pKa of the
  weak acid.

HA<> H+ +A-
Ka=[H+][A-]/[HA]
logKa=log[H+]+log[A-]/[HA]
-log[H+]=-logKa+log[A-]/[HA]
pH=pKa+log[A-]/[HA]

Question 19

Calculations

1. What is the pH of a mixture of 0.042M NaH2PO4 and 0.058M Na2HPO4? With a Ka of 1.8x10^-5
2. What is the pH of a solution consisting of 0.050 M acetic acid,
   CH3COOH and sodium acetate CH3COONa-? With a Ka of 1.74x10^-5

3.Calculate the pH of a solution consisting of K2PO4 and KH2PO4. With a Ka2 of 7.2.

Question 20

Biological Buffers
1.in extracellular fluids
2.in intracellular fluids
3.erythrocytes

Answer 20

1. Biocarbonate buffer + Protein buffer
2. Phosphate buffer + Protein
3. Haemoglobin buffer

Question 21

Importance of biological buffers in living organisms

Answer 21

1. maintain a specific and constant cytosolic pH
2. keeping biomolecules in their optimal ionic state

Question 22

THE PHOSPHATE BUFFER SYSTEM

1. location
2. the proton donor and proton acceptor?
3. pKa and Ph ranges

Answer 22

1. cytoplasm of all cells
2. H2 PO4- <> H+ + HPO42–
3. 6.86, resist pH changes from ranges 6.4 to 7.4.
   * intracellular pH usually 6 - 6.9

Question 23

THE BICARBONATE BUFER SYSTEM

1. location
2. the proton donor and proton acceptor?
3. pH ranges
4. functions?
5. define acidosis and alkalosis

Answer 23

1. Extracellular in Blood plasma (main buffer system in the blood.
2. H2CO3 <> H+ + HCO3
   (carbonic acid and bicarbonate ion)
3. pH depends on the concentration of H2CO3 and HCO3−
4. removal of the CO2 produced by tissue metabolism
5. change in the ratio [HCO3-]/[H2CO3] in favour of H2CO3 e.g a decrease in [HCO3−] or from an increase in [H2CO3].
   * change in the bicarbonate/carbonic ratio in favor of [HCO3-]

• [H2CO3] depends on the concentration of dissolved CO2, which in turn depends on the concentration or partial pressure of CO2 in
  the gas phase.
• [HCO3-]/[H2CO3] ratio of 20 to 1 is
  required for the pH of blood plasma to remain 7.40, dissolved [CO2] in blood is included in [H2CO3]
• change in the ratio in favour of H2CO3
  , acidosis results

Question 24

1. define metabollic acidosis
2. define respiratory acidosis

Answer 24

1. condition due to a decrease of bicabonate ion leading to excess carbonic acid e.g in uncontrolled diabetis with ketosis or starvation.
2. occurs when there is an obstruction to respiration (euphysema, asthma or pneumonia) or depression of respiration (toxic doses of morphine)

patient can go into coma

Question 25

1. define metabollic alkalosis
2. Respiratory alkalosis

Answer 25

1. occurs when the HCO3−
   fraction increases with little or no concomitant
   change in H2CO3 e.g * Severe vomiting (loss of H+ as HCl) or ingestion of excessive amounts of sodium bicarbonate(bicarbonate of soda)
2. is induced by hyperventilation because an excessive removal of CO2
   from the blood results in a decrease in [H2CO3].

• Alkalosis can produce convulsive seizures in children and tetany in adults (characterized by sharp flexion of the wrist and ankle joints, muscle twitchings, and cramps).
• Hyperventilation can result in anxiety, hysteria, prolonged hot baths or lack of O2
  at high altitudes.

Question 26

How is the bicarbonate/carbonic acid buffer ratio maintained

Answer 26

• urinary elimination of the bicarbonate ion
• respiratory elimination of the H2CO3 as CO2 in blood

if the bicarbonate buffer neutralizes an acid in blood this can change the buffer ratio

Question 27

THE PROTEIN BUFFER SYSTEM

1. location
2. the proton donor and proton acceptor?
3. pH ranges
4. functions?

Answer 27

1. blood plasma and intracellular fluid although their [ ] is very low in cerebrospinal fluid, lymph and interstitial fluids
2. proteins exist as anions serving as conjugate bases (Pr−) at the blood
   pH 7.4 and form conjugate acids (HPr) accepting H+
3. pH 7.4
4. have the capacity to buffer some H2CO3 in the blood
   H2CO3+Pr− <—> HCO3– +HPr

Question 28

AMINO ACIDS BUFFER SYSTEM

1. Explain their structure
2. Explain how they are amphoteric
3. explain the addition of an acid or base to an amino acid solution

with aid of equations

Answer 28

1.Amino acids contain in their molecule both an acidic (-COOH) and a basic (-NH2) group (zwitterion)
2. they can donate or accept a H+ ion to form an anion or a cation
+H3N-CH2-COOH ←+H3NCH2-COO− <>H2N-CH2-COO− + H+
Cation form Zwitterion Anion form
3. Base(OH-) when OH− ions are added to the solution of amino acid, they take up H+ from it to form water, and the anion is produced
* Acid(H+) H+ ions added are taken up by the zwitterion to produce the cation form.

• a solution of an amino acid is not neutral but is either predominantly acidic or basic, depending on which form is present in greater quantity.
• different amino acids may be used as buffers for different pH values, and a mixture of
  them possesses a wide buffer range.

Question 29

THE HEMOGLOBIN BUFFER SYSTEM

1.function
2.proton acceptor and proton donor

Answer 29

1. buffering CO2
   inside erythrocytes i.e RBC
2. Hb- and H2CO3 respectively
   HbO2– <> Hb− + O2
   Hb− + H2CO3 <—> HHb + HCO3−

3.

2. Inside the erythrocytes, CO2 combines with H2O to form carbonic acid (H2CO3) under the action of carbonic anhydrase, at the blood ph of 7.4. Carbonic acid dissociates and needs immediate buffering.
   * oxyhemoglobin(HbO2-) loses O2 to form Hb-(deoxyhemoglobin), which remains dissociated by accepting H+ from carbonic acid, thus buffering the acid.

Question 30

THE HEMOGLOBIN BUFFER SYSTEM

What happens to the bicarbonate ions in the hemoglobin buffer

Answer 30

• diffuses out into the blood plasma to maintain balance between intracellular and plasma bicarbonates.
• causing an influx of Cl- ions along the electrical gradient created by the HCO3- outflow (chloride shift)

Question 31

THE HEMOGLOBIN BUFFER SYSTEM

explain the chloride shift effect

Answer 31

this is the influx of Cl- ions in erythrocytes due along an electrical gradient created by the outflow of HCO3- ions from erythrocytes

Question 32

THE HEMOGLOBIN BUFFER SYSTEM

Explain what happens to the HHbO2 produce by the oxygenation of HHb from lungs

Answer 32

• immediately ionizes into H+ and HbO2-
  HHb+O2 <> HHbO2<> HbO2- +H+
• released hydrogen ions (H+) are buffered by HCO3- inside erythrocyte to form H2CO3
  HCO3- + H+<> H2CO3
• H2CO3 is dissociated into H2O and CO2 by carbonic anhydrase
• H2CO3< carbonic anhydrase> H2O+CO2
• CO2 diffuses out of erythrocytes and escapes in alveolar air.
• Some HCO3- return from the plasma to erythrocytes in exchange of Cl− and are changed to CO2
  .
  .

Question 34

BUFFER CAPACITY

What is buffer capacity

Answer 34

a measure of the resistance to changes in pH when strong acids or bases are added

amount of H+ or OH– ions that can be neutralised by the buffer