BIOC 503 - Water - Lecture 3

heterolytically

O-H bonds are polar and can dissociate ___ when water is ionized

proton, hydroxide

ionization of water is rapid and REVERSIBLE, producing a ___ (H+) and a ___ ion (OH-)

electrical resistance, left

most water molecules remain unionized thus pure water has a very low ____ and the equilibrium lies strongly to the __ aka low Keq

temperature

the extent of dissociation depends on ___

hydronium

protons do not exist free in solution, they are immediately hydrated/solvated by neighboring water molecules to form ___ ions (H3O+).

A __ ion is a water molecule with an additional H associated with one of the lone pairs on the O

interchangeable

Regarding water and hydronium ions, the covalent and hydrogen bonds are ___, allowing an extremely fast mobility of protons in water via “proton hopping”

proton hopping

• extremely rapid net movement of a proton over a long distance

• As a hydronium ion gives up a proton, a water molecule some distance away acquires one, becoming a hydronium ion.

• ___ is much faster than true diffusion and explains the remarkably high ionic mobility of H⁺ ions compared with other monovalent cations such as Na⁺ and K⁺

1.0×10^-14

Keq for dissociation of water AKA Kw

Kw = [H+]*[OH-] = ___ M²

-log[H+]

pH = ___

pH

the negative logarithm of the hydrogen ion concentration

• typical scale from 0 to 14

• can be negative

• can be >14

7

a neutral solution has a pH of __ due to the fact that [H+] = [OH-]

strong

__ electrolytes (acids/bases) dissociate fully in water

weak

__ electrolytes dissociate partially in water and have a pKa as a result

dissociation constant

extent of dissociation for weak electrolytes is determined by the ___ (Ka) which is [H+][CB]/[HA]

weak electrolyte assumption

IF the amount of dissociated species is MUCH LESS than the amount of undissociated acid, then the initial acid concentration DOES NOT CHANGE through reaction.

Example: 0.1M instead of 0.1-x

-log(Ka)

pKa measures acidity

pKa = __

buffers

mixture of weak acids and their conjugate bases that resist change in pH

50/50

at pH = pKa there is a ___ mixture of acids and their conjugate bases in the buffer

pH = pKa

buffering capacity is the greatest at ___

1

buffering capacity is lost when the pH differs from the pKa by more than __ pH unit

plateau

The ___ of a titration curve is usually the point where pH = pKa

Handerson-Hasselback equation

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

Derivation:

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

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

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

optimal pH

enzyme catalyzed rxns often have ___ based on the organism or cell they happen in

phosphate, bicarbonate, histidine

buffer system IN VIVO are mainly based on ___, ___, ___