Chapter 2Water

Revised 1/9/2014

Biochemistry IDr. Loren Williams

Colorado River 1999 Colorado River 2013

Water is polar. The oxygen atom in a water is partially negative (it carries a partial negative charge). Each hydrogen atom is partially positive. This phenomenon is called polarity. Water is a polar molecule.

Oxygen is electronegative. It does not share electrons equally with hydrogen atoms. The hydrogen atoms, with no inner shell electrons, are stripped of much of their electron density and carry a partial positive charge, the protons are exposed.

In liquid water and especially in solid water, the molecules interact strongly with each other, with preferred orientations.

Water, the Liquid of Life (part 1)

Water has all sort of strange and unusual properties.

Water: o has a high boiling point and high heat of vaporization.o has high surface tension.o dissolves many salts (like sodium chloride) and polar

molecules (like ethanol).o does not dissolve non-polar substances (oil and water

don’t mix).o has high heat capacity. o expands when it freezes, so ice floats. The density of

water decreases when it freezes. o forms compartments in the presence of amphipaths.o is often found on Earth simultaneously as a solid, liquid

and gas.

Water, the Liquid of Life (part 2)

Capillary action is the tendency of a liquid to flow into narrow spaces in opposition to external forces. Water is lifted by surface tension (caused by cohesion between water molecules) and adhesive forces between the water and the surface.

Figure 2-1a

Osmosis is the spontaneous net movement of molecules into regions of lower concentration. The net movement of molecules is in directions that tend to equalize concentrations.

Osmotic pressure is the pressure required to halt the next flux of water across the semipermeable membrane.

Figure 2-14

Condensation-Dehydration & HydrolysisWater as Chemical Reagent

Water as an Acid and a Base

Kw = [H3O+][OH-] = [H+][OH-] [H3O+][OH-]

[H2O(l)]2

Autoionization of water:

2 H2O (l) H3O+ (aq) + OH- (aq)

Kw = 1.0 x 10-14 (at 25oC)

In pure water [H+] = [OH-]

Kw = [H3O+][OH-]

Figure 2-16

• There is competition for the proton between two bases, H2O and A–

• If H2O is a stronger base than A– the equilibrium lies to the right.

• If A– is a stronger base than H2O the equilibrium lies to the left.

HA + H2O H3O+ + A –

HA = generic acid

Polyprotic Acids

Note: Phosphoric acid is a strong acid in its first dissociation, and a weaker acid in its second step and an even weaker acid in its third step.

H3PO4 H+ + H2PO4– Ka1 = 7.5x10-3 pKa1 = 2.2

H2PO4– H+ + HPO4

2- Ka2 = 6.2 x10-8 pKa1 = 7.2

It is easier to remove the first proton in a polyprotic acid than the second. That is, Ka1 > Ka2 > Ka3

HPO42- H+ + PO4

3- Ka3 = 4.8x10-13 pKa1 = 12.4

H3PO4: phosphoric acid

H3PO4 H+ + H2PO4– Ka1 = 7.5x10-3 pKa1 = 2.2

H2PO4– H+ + HPO4

2- Ka2 = 6.2 x10-8 pKa1 = 7.2

HPO42- H+ + PO4

3- Ka3 = 4.8x10-13 pKa1 = 12.4

H3PO4: phosphoric acid

Table 2-3

Table 2-4

Figure 2-17