acids, bases, pKa, buffers

Question 1

lewis base

Answer 1

electron donor (nucleophile)

Question 1

Lewis acid

Answer 1

electron acceptor (electrophile)

Question 2

amphiprotic

Answer 2

• substance can both accept and donate a proton (H+)
• all amphoteric substances can amphiprotic
  ( bronstead lowry)

Question 3

amphoteric

Answer 3

• refers to the ability to act as both an acid and a base
• all amphiprotic substances are not amphoteric
  (lewis?)

Question 4

examples amphiprotic

Answer 4

water, amino acids, phosphate progression, sulfate progression.
HSO4-, H2PO4- , HPO42-

Question 5

solving for [H+] using Ka expression gives….

Answer 5

[H+] = Ka ([HA] / [A-])

Question 6

using the relationship pH = -log [H+] gives… (Henderson hasselbalch equation)

Answer 6

the henderson hasselbalch (H-H) equation: pH = -log Ka + log ([A-] / [HA])

Question 7

pKa

Answer 7

pKa = -log Ka, pH at which acid and conjugate base are equal in concentration

Question 8

pH values to the left of the pKa

Answer 8

the acidic form is predominant

Question 9

pH values to the right of the pKa

Answer 9

the basic form in predominant

Question 10

why to be familiar with pKa numbers?

Answer 10

they predict what the acid/base is and the Nuc/E+, and the buffering ability

Question 11

Buffer

Answer 11

want pKa at or near mid-point of the buffer range (capacity) for greatest ability to neutralize small added amounts of acid or base

Question 12

What makes a buffer?

Answer 12

• made up of weak acid and salt of conjugate base or weak base and salt of conjugate acid.

Question 13

zwitterion

Answer 13

dipolar ion with no charge

Question 14

why must buffer use weak acid/ base?

Answer 14

in order to establish equilibrium. Strong acids and base have equilibrium constant far to the right.

Question 15

why is zwitterion pH dependent

Answer 15

must be at pI to have the highest concentration.

Question 16

Why is the pI where the substance has the lowest solubility?

Answer 16

• no net charge, wont dissolve in water

Question 17

Why are NH2 and COOH protonated and COOH-H+ deprotonated at physiological pH?

Answer 17

• blood is around 7 pKa
• amino groups around 9 pKa
• carboxylic acid pKa around 2.
  The pKa of blood is larger than the pKa of carboxylic acid, which means it will exist in its conjugate base form.
  The pKa of blood is smaller than the pKa of the amino groups, which means amino groups will exist in their acidic form.

Question 18

most amino acids pI

Answer 18

5-5.6

Question 19

If the amino acid is acidic…

Answer 19

the pI is less than 5 or if basic, pI is greater than 6.5.

Question 20

why are the main chain (alpha) carboxylic acids deprotonated before side chain COOH in Asp/Glu?

Answer 20

• NOT RESONANCE
• inductive effect… through bond effect not involving resonance…
• side chain carboxylic acid is further away from amino group than main chain
• amino group is acting inductively to withdraw electron density away from the conjugate base of the alpha carboxylic acid… but too far away for side chain

Question 21

Why is the main chain deprotonated for a-N first rather before side chain in Arg/Lys?

Answer 21

• NOT RESONANCE
• inductive effect once again