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WATER

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Without water there is no life!

Without water there is no life!

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WATER WATER Presentation Transcript

  • Water Without Water There Can Be No Life!
  • The Amazing Properties of Water
    • Are due to its ability to form hydrogen bonds
  • Hydrogen Bonding Ability of Water
    • Is due to two features
      • The polar covalent bonds that bind the hydrogen atoms to the oxygen atom
      • The asymmetric distribution of these polar covalent bonds
  • The Polar Covalent Bonds
    • Bind the hydrogen atoms to the oxygen atom
      • The oxygen is more electronegative than the hydrogens
      • Oxygen pulls on the bond electrons harder
        • Oxygen atom acquires two partial positive charges
        • Each hydrogen atom acquires a partial negative charge
      • These polar covalent bonds are ~1/3 ionic and 2/3 covalent in character
  • Water Is a Covalent Compound
  • Polar Covalent Bonds Hold Atoms Together in Water
  • The Asymmetric Distribution of These Polar Covalent Bonds
    • Creates the V shape of the water molecule
    • Causes the two partial positive charges to be on the hydrogen side and the two partial positive charges on the oxygen side of the molecule
    • Causes water to be a dipole
  • Water Molecules Are V-shaped
    • Dipole-dipole hydrogen bonds are the bonds that water forms with itself and other polar molecules
    • Ion-dipole hydrogen bonds are the bonds that form between water and ions
    Water Forms Two Kinds of Hydrogen Bonds
    • Hydrophilic means “water-loving”
    • Hydrophobic means “water-hating”
    Hydrophilic Versus Hydrophobic Substances
  • Hydrophilic Substances Are Water-soluble
    • Form hydrogen bonds with water
      • Ionic compounds and moieties
      • Polar organic molecules and moieties
    • Hydrophilic substances are lipophobic (“lipid-hating”)
  • Many Inorganic (Ionic) Compounds Dissolve in Water Via Ion-Dipole Hydrogen Bonds
  • All of These Functional Groups Are Polar and Form Hydrogen Bonds
  • Dipole-Dipole Hydrogen Bonds Between Water & Polar Organic Molecules
  • Hydrophobic Substances Are Water-insoluble
    • Do not form hydrogen bonds
    • Hydrocarbon molecules and moieties
    • Hydrophobic substances are lipophilic (“lipid-loving”)
    • Means “both hydrophilic and hydrophobic”
    • Also called amphiphilic
    • Molecules that contain both polar and hydrocarbon moieties (e.g., phospholipids)
    Amphipathic Substances Are Water-soluble and Lipid-soluble
  • Phospholipids Are Amphiphilic
  • Membranes Are Mostly Hydrophobic
  • Membranes Separate Living Cells From Their Nonliving Watery Environment
  • Properties of Water
    • “Universal solvent” for electrolytes and polar molecules
    • High melting point (0 o C)
    • High boiling point (100 o C)
    • Wide range of temperatures in which water is in liquid state (0 o C – 100 o C)
  • Water Ought To Be A Gas!
    • By molecular weight (MW), water ought to be a gas:
    • CO2 (MW=44), O2 (MW=32), CO (MW=28), N2 (MW=28), CH4 (MW=18), and H2 (MW=2) are all gasses at room temperature.
    • Water (MW=20) is a liquid. Why?
    • Because water molecules display cohesion and thus have a much reduced tendency to fly off into the overlying atmosphere than these other listed molecules
  • It Takes A Lot Of Energy To Heat Water
    • High heat of fusion
      • 80 cal/g at 0 o C
    • High heat of vaporization
      • 539 cal/g at 100 o C
      • 580 cal/g at 20 o C
    • High specific heat of liquid water
      • 1 cal/g o C
  • Water Has A High Heat Of Vaporization (Evaporation)
    • 539 cal/g at 100 oC
    • 580 cal/g at 20 oC
        • Because evaporation also involves the breaking of hydrogen bonds, water resists vaporizing (evaporating)
        • Consequently, it takes a lot of heat to evaporate water
        • This high heat of vaporization is also utilized by organisms as a cooling process, e.g., sweating or panting
  • High Specific Heat
    • By definition, a temperature increase is an increase in the motion of the molecules and atoms making up a substance.
        • Because of cohesion, water molecules resist increasing their motion. (This is another way of saying that water molecules resist the net breaking of hydrogen bonds). Consequently, water resists heating; water has a very high specific heat
        • This tendency to not want to change temperature causes resistance to radical temperature swings
        • Causes bodies of water (e.g., a lake) to strongly resist rapid changes in temperature
        • This temperature buffering capacity of water is taken advantage of to a great extent by organisms
  • Surface Tension
    • The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface
    • This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.
    • Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm
    • Equivalently, it can be stated as surface energy in ergs per square centimeter
    • Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.
  • Water Has A High Surface Tension
    • The surface tension of water is 72 dynes/cm at 25°C. It would take a force of 72 dynes to break a surface film of water 1 cm long
    • The surface tension of water decreases significantly with temperature as shown in the graph
    • Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension
    • Soaps and detergents further lower the surface tension
  • Water Molecules Are Cohesive And Adhesive
    • Cohesion
      • The attraction of one water molecule to another resulting from hydrogen bonding
      • By placing a drop of water on a surface you can directly observe cohesion in the resistance that water droplet shows to wetting, i.e., water clumps up in a pile despite being a liquid, rather than spreading out over the surface. (Note that wetting is less likely to occur in the absence of adhesion to a wet surface)
    • Adhesion
      • Adhesion is similar to cohesion except adhesion involves the attraction of a water molecule to a non-water molecule
      • Cohesion is thus a special case of adhesion
  • Capillary Action
    • Capillary action is the result of adhesion and surface tension
    • Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward
    • The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward
  • Water Is Cohesive and Adhesive
  • Water Is Less Dense As Solid Than As Liquid
    • Maximum density at 3.98 oC
    • Ice floats above liquid water because is less dense as solid than as liquid
      • The density of water is actually less than it could otherwise be because hydrogen bonded water is packed slightly less favorably than could be achieved without hydrogen bonding
      • Ice represents a maximal hydrogen bonding of water, indeed the crystallization of water into the structure formed upon hydrogen bonding. Thus, ice occupies a greater volume per unit mass and, consequently, floats on water
  • Ice Floats
  • Ice Crystal
    • Ice represents a maximal hydrogen bonding of water
    • High pressures tend to inhibit the solidification of water, so ice forms at top of liquid water
  • Viscosity
    • Viscosity of a liquid is a measure of its inability to flow
    • Measured in N s m-2 (SI Units) or poise (P) or centipoise (cP)
    • 1 P = 0.1 N s m -2
    • 1 cP = 0.001 N s m -2
    • Viscometers are used to measure viscosity
  • Viscosity of Water Depends on the Temperature
    • Viscosity decreases as temperature increases
    0.282 100 0.355 80 0.467 60 0.653 40 1.002 20 Viscosity (cP) Temperature (°C)
  • Viscosity and Surface Tension of Various Liquids at 293 K - very large Glasses 0.0634 1490 Glycerol - 986 Castor oil - 84 Olive oil 0.436 1.554 Mercury 0.0228 1.200 Ethanol 0.0728 1.002 Water 0.0270 0.969 Carbon tetracholoride 0.0289 0.652 Benzene 0.0271 0.58 Chloroform 0.0728 0.233 Diethyl ether Surface tension N m -1 Viscosity cP Common liquid
  • Water Ionizes into an Equal Number of Positive Hydronium Ions And Negative Hydroxide Ions 2H 2 O(l) -> H 3 O + (aq) + OH - (aq)
  • H2O Can Act As Both A Proton Donor And Acceptor For Itself
    • A proton can be transferred from one water molecule to another
    • Resulting in the formation of one hydroxide ion (OH-) and one hydronium ion (H3O + )
    • 2H 2 O(l)  H 3 O + (aq) + OH - (aq)
    • This is called the autoionization or dissociation of water. This equilibrium can also be expressed as H 2 O(l) -> H + (aq) + OH - (aq)
    • In the above equilibrium, water acts as both an acid and a base
    • The ability of a species to act as either an acid or a base is known as amphoterism
  • The Concentrations Of H3O+ And OH- Produced By The Dissociation Of Water Are Equal
    • The corresponding equilibrium expression for this would be:
    • K C = {[H+][OH-] / [H2O]}
    • In pure water at 25oC, [H2O] = 55.5 M
    • This value is relatively constant in relation to the very low concentration of H+ and OH- (1 x 10-7 M).
    • Therefore, K C = [H+][OH-] / 55.5 M
    • Rearranging gives:
    • K C (55.5 M) = [H+][OH-] = Kw
    • So now we can eliminate [H2O] from the equation and we are left with:
    • K W = [H+][OH-]
    • Where K W designates the product (55.5 M) K C and is called the ion-product constant f or water. At 25oC, K W is equal to 10 -14
  • The Ion-product Constant Always Remains Constant At Equilibrium
    • Consequently, if the concentration of either H+ or OH- rises, then the other must fall to compensate
    • In acidic solutions, [H+] > [OH-]
    • In basic solutions [H+] < [OH-]
  •  
  • Life Thrives on Our Water Planet
  • THE END