bt 202 aug 12 2011 ppt1997-2004

BT-202 Biochemistry

Aug 16, 2011

• Acids and Bases: Definitions• Ionization of water• pH Scale• Dissociation of Weak acids

and bases• Buffers

Vinegar and Aspirin

Soaps, toothpaste and cleaning agents.

HF + H2O H3O+ + F-

Learning check!

Label the acid, base, conjugate acid, Label the acid, base, conjugate acid, and conjugate base in each reaction:and conjugate base in each reaction:

• H2SO4 + H2O HSO4- + H3O+

• HCl + OH- Cl- + H2O

• HCl + NH3 Cl- + NH4+

• NH3 + H2O NH4+ + OH-

• Identify the conjugate base in each of the pairs below?

• RCOOH, RCOO-

• RNH2, RNH3+

• H2PO-,H3PO4

• H2CO3, HCO3-

Proton donor

Proton Hopping•Ionization can be measured by its electrical conductivity.•Movement of hydronium ion is very fast due to “proton hopping”. •No single proton moves over a long distance but a series of proton hops between water molecules which are hydrogen bonded.

• Most of the acid base reactions in the aqueous solution are exceptionally very fast.

• Proton hopping also plays a role in biological proton transfer reactions.

• Reversible ionization is very crucial to the role of water in cellular function.

• We need to express the extent of ionization in quantitative terms.

• The position of equilirium for any chemical reaction is given by its EQUILIBRIUM CONSTANT, Keq.

• For any generalized reaction:

Equilibrium constants are dimensionless but biochemists have retained the concentration units (M) or molarity.

• Equilibrium constant for a reversible reaction of water is:

Keq = [H+] [OH-]

[H2O]

In pure water at 25°C, the concentration of water is 55.5 M.

• At this pH the concentration of [H+] and [OH-] is 1 X 10-7.

• Whenever [H+] is greater than 1 X 10-7 [OH-] is less and visa versa.

Learning check!

• What is the concentration of H+ in a solution of 0.1 M NaOH?

• What is the [H+] of human saliva if its [OH-] is 4 x 10-8 M? Is human saliva acidic, basic, or neutral?

• What is the [H+] of a sample of lake water with [OH-] of 4.0 x 10-9 M? Is the lake acidic, basic, or neutral?

Learning check!

• What is the concentration of OH- in a solution with an H+ concentration of 1.3 X 10-4M?

If an acidic solution is defined as a solution water where H3O+ is greater than OH-, what can be said of an acidic solution?

[OH-]>[H3O+]

The concentration of H3O+ must be greater than 10-7.

The concentration of H3O+ is always 10-7 if the solution is made in water.

The solution feels slippery and might taste bitter.

pH of some aqueous fluids.pH scale ranges from 0-14.

Measuring pH:•pH indicator is a substance which changes color around a particular pH Eg. Litmus paper.•Universal indicator is a mixture of indicators such that it changes color over a range of pH (Thymol blue, phenolpthalein, bromophenol blue etc).•pH meter is used in laboratory.

Learning Check!

• If the [H+] of a solution is 0.1M, What is the [OH-]? What is the pH?

• What is the difference in [H+] between solution at pH 4 and pH 7?– 3M– 1000-fold difference in [H+]– 3-fold difference in [H+]– 11M

• pH in biological system:-catalytic activity of enzymes.-blood pH changes can be life-

threatning. pH blood is 7.4.

-acidosis.-alkalosis.

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

• Because strong acids completely ionize, the [H+] of a solution made with a strong acid is easily figured out, since it is equal to the molarity of the solution.

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

Learning Check!

• Write out the acid dissociation reaction for hydrochloric acid?

• Calculate the pH of a solution of 5.0 X 10-4 M HCl?

• Write out the acid dissociation reaction for Sodiaum hydroxide?

• Calculate the pH of a solution of 7.0 X 10-5 M NaOH?

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

• The tendency of any acid to lose proton and form its conjugate base in water is defined by the equilibrium constant (Keq) for reversible reaction:

HA H+ + A-

Keq = [H+] [A-] = Ka

[HA]

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

• The degree of dissociation of a weak acid in water is described by the acid dissociation constant, Ka. It is also called the ionization constant.As seen in the figure.

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

• Also included in the figure is pKa which is analogous to pH

pKa = log 1/Ka = -logKa

eg. Ka of acetic acid is 1.74 x 10–5.

pKa = –log (1.74 x 10–5) = 4.76.

• Experimental determination of amount of acid in a given solution is done by titration. It also reveals the pKa.

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

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

ACIDS AND BASES-ACIDS AND BASES-STRENGTHSTRENGTH

• The stronger the acid, the lower its pKa; the stronger the base the higher its pKa.

• pKa can be determined experimentally ; it is the pH at the midpoint of a titration curve for the acid and base.

Learning Check!

• Which of the following aqueous solutions has the lowest pH: 0.1 M HCl; 0.1 M acetic acid (pKa = 4.86); 0.1 M formic acid (pKa = 3.75)?

BufferBuffer

BUFFER

• Buffers are aqueous systems that tend to resist changes in pH when small amounts of acid (H+) or base (OH-) are added.

• A buffer system consists of weak acid (proton donor) and its conjugate base (proton acceptor).

BufferBuffer

The acetic acid-acetate pair as a buffer system

Buffers

Buffering regionThis is the zone in the titration curve of a weak acid which extends 1 pH unit on either side of the pKa. In this zone additon of small amount of either H+ or OH- has much less effect on pH than region outside this zone.

THE HENDERSON–HASSELBALCH EQUATION

• The extent of ionization of a weak acid (the pKa) influences the final concentration of H+ ions (the pH) of the solution, there must be a relationship between pH and the pKa of a weak acid. This relationship is given by the Henderson–Hasselbalch equation:

pH  =  pKa  +  log [conjugate base][acid]

THE HENDERSON–HASSELBALCH EQUATION

• Calculate pKa, given pH and the molar ratio of proton donor and acceptor;

• Calculate pH, given pKa and the molar ratio of proton donor and acceptor; and

• Calculate the molar ratio of proton donor and acceptor , given pKa and pH.

Learning check!

• Which of the following compounds would be the best buffer at pH 5.0: formic acid (pKa = 3.8), acetic acid (pKa=4.76) or ethylamine (pka=9.0)?

Learning check!

(a) What is the pH of a mixture of 0.042 M NaH2PO4 and 0.058 M Na2HPO4?

(b) If 1.0 ml of 10.0N NaOH is added to a liter of the buffer prepared in (a), how much will the pH change?(c) If 1.0 ml of 10.0 N NaOH is added to a liter of pure water at pH 7.0, what is the final pH? Compare with (c)?

Buffers in cells and tissues

Cytoplasm of cells contain many amino acids with functional groups that are weak acids or weak bases

Buffers in cells and tissues

Blood plasma is buffered inpart by the bicarbonate system.