Chapter 2 Water

1. Revised 1/9/2014 Biochemistry I Dr. Loren Williams Chapter 2 Water

3. Colorado River 1999 Colorado River 2013

4. Water, the Liquid of Life (part 1) 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.

5. Water, the Liquid of Life (part 2) Water has all sort of strange and unusual properties. Water: • has a high boiling point and high heat of vaporization. • has high surface tension. • dissolves many salts (like sodium chloride) and polar molecules (like ethanol). • does not dissolve non-polar substances (oil and water don’t mix). • has high heat capacity. • expands when it freezes, so ice floats. The density of water decreases when it freezes. • forms compartments in the presence of amphipaths. • is often found on Earth simultaneously as a solid, liquid and gas.

7. 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.

8. Figure 2-1a

11. 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.

12. Figure 2-14

13. Water as Chemical Reagent Condensation-Dehydration & Hydrolysis

14. [H3O+][OH-] [H2O(l)]2 Water as an Acid and a Base Autoionization of water: 2 H2O (l) H3O+ (aq) + OH- (aq) Kw = [H3O+][OH-] = [H+][OH-] Kw = 1.0 x 10-14(at 25oC) In pure water [H+] = [OH-] Kw = [H3O+][OH-]

15. Figure 2-16

16. HA + H2O H3O+ + A – HA = generic acid • 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.

17. It is easier to remove the first proton in a polyproticacid than the second.That is, Ka1 > Ka2 > Ka3 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. H3PO4H++ H2PO4– Ka1 = 7.5x10-3pKa1 = 2.2 H2PO4–H+ + HPO42- Ka2 = 6.2 x10-8pKa1 = 7.2 HPO42- H++ PO43- Ka3 = 4.8x10-13pKa1 = 12.4

18. H3PO4: phosphoric acid H3PO4H++ H2PO4– Ka1 = 7.5x10-3pKa1 = 2.2 H2PO4–H+ + HPO42- Ka2 = 6.2 x10-8pKa1 = 7.2 HPO42- H++ PO43- Ka3 = 4.8x10-13pKa1 = 12.4

19. H3PO4: phosphoric acid

20. Table 2-3

22. Table 2-4

23. Figure 2-17