Water: Chemistry and Behavior

1. SGN 3 Water: Chemistry and Behavior

2. Life’s biochemistry is shaped by the aqueous environment in which it works Water is essential for life because it is the medium of biochemistry Because it shapes biochemistry, a fundamental understanding of the chemistry and properties of water is critical to an understanding of biology

3. Water’s Chemistry H2O Angle of the covalent bonds Small compound Covalent bonds are polar Electrons shared unequally Oxygen is very electronegative (measure of an atom’s attraction for electrons) so electrons spend more time near the oxygen atom This gives a water molecule dipoles, or ends with opposite partial charge

4. This means that each of a water molecule’s three atoms can form hydrogen bonds or mixed bonds with other compounds Cohesion – attraction of water molecules to other water molecules Adhesion – attraction of water molecule for another type of compound

5. Water’s Behaviors The ability to form many cohesive or adhesive hydrogen bonds, as well as the typically high concentration of water that exists in the medium of interest (solution, cell) allows water to behave in several different ways that are important for life • Water is a good solvent for many things but not all things • Water changes state on earth and translocates to land • Water resists temperature change • Solid water is less dense than liquid water • Surface tension and capillary action

6. 1. Water is a good solvent for many things but not all things Water is able to adhere to charged and polar moieties and dissolve hydrophilic substances; in their dissolved state solutes can interact and react But water cannot adhere to neutral moieties so some substances will not dissolve in water and can form basically insoluble substances such as cell membranes or cell walls

7. 2. Water changes state on earth and translocates to land, allowing for plentiful terrestrial life

8. 3. Water resists temperature change Water is able to absorb a lot of heat energy and release a lot of heat energy with relatively little change in temperature As water goes from solid to liquid to gas a great deal of energy is absorbed, which is needed to break the many stabilizing hydrogen bonds between the molecules (resists change of state – requires energy input) As water goes from gas to liquid to solid a great deal of energy is released, as stable hydrogen bonds form between the molecules

9. Water has a high specific heat and high heat of vaporization

11. Water helps absorb and distribute heat, stabilizing temperatures within organisms (for example, evaporative cooling), and in ecosystems (locally and globally)

12. Water stabilizes land temperatures Heat is absorbed when air is hot and released when air is cold

13. 4. Solid water is less dense than liquid water Why??

14. Capillary action – ability of water to “climb” up small tubes composed of polar molecules, like glass or cellulose Surface tension – stronger cohesion of water molecules at surface where water interfaces with air 5. Capillary action and surface tension

15. Solute concentration in aqueous solutions Students should understand the different ways to express the concentration of a solution • moles and molarity • % • weight/volume

16. Acidic and Basic Aqueous Solutions In liquid water a small number of H2O molecules disassociate to form hydronium (H3O+, often considered simply as a hydrogen ion/H+) and hydroxide/OH- ions In a pure aqueous solution these two ions always equal each other in concentrationIn any aqueous solution [H3O+] x [OH-] =10-14

17. The pH scale measures the degree to which a solution is acidic or basic (relative concentration of hydrogen and hydroxide ions) Blood, other bodily fluids and cellular cytosolic pH is approximately 7.4

18. In any aqueous solution [H3O+] x [OH-] = 10-14 pH is the negative logarithm of the hydrogen ion concentration pH = -log10 [H3O+] pH + pOH = 14 Each pH unit (on a scale of 0 to 14) represents a 10-fold change in H3O+ concentration Find the pH of a 0.0025 M HCl solution.  The HCl is a strong acid and is 100% ionized in water.  The hydronium ion concentration is 0.0025 M.  pH  =  - log (0.0025) = - ( - 2.60) = 2.60

19. 1. What is the pH of a solution with a [H+] = 10-8 M? 2. What is the pH of a solution with a [OH-] of 10-2 M? 3. Acetic acid has a functional group (collection of atoms) called a carboxyl group, which is symbolized with R–COOH. When acetic acid dissolves in water the hydrogen completely disassociates from the carboxyl group to form R–COO- and H+. You dilute acetic acid in water to a concentration of 10-4 M. What is the pH of this solution? 4. What is the hydronium ion concentration in a solution that has a pH of 8.34? 5. The pH of a solution is 6.5. What are the [H+] and [OH-] of the solution?

20. What is the pH of a solution with a [H+] = 10-8 M? pH = -log10 [H3O+] pH = -log [10-8] pH = 8 What is the pH of a solution with a [OH-] of 10-2 M? pOH = -log10 [OH-] pH + pOH = 14 pOH = -log [10-2] p0H = 2 14 – 2 = 12

21. You dilute acetic acid in water to a concentration of 10-4 M. What is the pH of this solution? pH = -log [10-4] pH = 4 What is the hydronium ion concentration in a solution that has a pH of 8.34? 8.34= -log10 [H3O+] H3O+ = 10-8.34 H3O+ = 4.57 x 10-9 M

22. The pH of a solution is 6.5. What are the [H+] and [OH-] of the solution? pH = 6.5 6.5 = -log10 [H3O+] [H+] = 10-6.5 [H+] = 3.16 x 10-7 or 0.000000316M [H3O+] x [OH-] = 10-14 3.16 x 10-7 x [OH-] = [OH-] = 10-14/3.15 x 10-7 [OH] = 3.18 x 10-8

23. Chemical reactions necessary for life can only occur properly within a narrow pH range. Small pH changes within a cell, body or ecosystem can be destabilizing. Biological systems (cells, multicellular organisms) must have buffering capacity to maintain homeostasis in regard to pH.

24. Example Mammalian blood buffering system Buffers are typically weak acid-base pairs that combine reversibly to remove or donate H+, and thus minimize changes in the system’s pH carbon dioxide + water <-> carbonic acid < - > bicarbonate ion + hydrogen ion CO2 + H20 < -- > H2CO3 < -- > HCO3- + H+ Mammalian blood is typically around 7.4 If blood becomes acidic CO2 is breathed out, shifting the reaction to the left

25. Disruption of the body’s pH can lead to medical acidosis or alkalosis If CO2 cannot be expelled (respiratory acidosis) or acid builds up due to other reasons (metabolic acidosis) blood will become too acidic Caused by… • Lung disease blocking gas diffusion (emphysema, pneumonia, bronchitis, asthma) • Depression of respiratory system by drugs • Diseases such as diabetes or renal failure • Cardiopulmonary arrest • Stroke Alkalosis is much less common (severe vomiting, shock, anxiety attack leading to hyperventilating and expelling CO2 too rapidly)

26. Ocean Acidification 30% increase in ocean acidity since 1800 CO2 + H20  H2CO3 HCO3- + H+ CO3-2 (carbonate ion) + calcium = necessary for invertebrate shells, coral, etc. But in acidic conditions 2H++ CO3-2 H2CO3  HCO3- + H+ Excess H+ reduces availability of carbonate ion and so impedes necessary biological processes in many organisms

29. Describe the molecular and chemical properties of a water molecule. Describe water’s important behaviors, and how these behaviors occur because of water’s chemical properties. Describe how the mammalian blood buffering system works. Write the chemical equation for the MBBS and describe the equation in words, explaining how the reactions buffer the blood. Describe the problem of ocean acidification. Write the chemical equation for ocean acidification and describe the equation in words, explaining how the reactions lead to acidification, and how these chemical conditions endanger sea life.