water – chapter 3. properties of water polar molecule cohesion and adhesion high specific heat...
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Water – Chapter 3
Properties of Water• Polar molecule• Cohesion and adhesion• High specific heat• Density – greatest at 4oC• Universal solvent of life
Polarity of Water• A water molecule forms when two hydrogen
atoms form single polar covalent bonds with one oxygen atom
• Oxygen is more electronegative, the region around oxygen has a partial negative charge.
• The region near the two hydrogen atoms has a partial positive charge.
A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.
δ−
δ+
Polarity of Water
– The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.
– Each water molecule can form hydrogen bonds with up to four neighbors.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.1
HYDROGEN BONDS• Each molecule can form a
maximum of 4 hydrogen bonds
• The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.
• They form, break, and reform with great frequency
HYDROGEN BONDS
• Extraordinary Properties that are a result of hydrogen bonds.–Cohesive behavior–Resists changes in temperature–High heat of vaporization–Expands when it freezes–Versatile solvent
Cohesion and Adhesion
• Cohesion: attraction among substances of the same kind
• Adhesion: attraction amoung molecules of different substances.
Cohesion among water molecules plays a key role in the transport of water against gravity in plants. Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels.
• Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.– Water has a greater surface tension than most
other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.
– Water behaves as if covered by an invisible film.
– Some animals can stand, walk, or run on water without breaking the surface. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.3
is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1oC
Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for:
• Prevention of temperature fluctuations that are outside the range suitable for life.
• Coastal areas having a mild climate
• A stable aquatic environment
Specific Heat
Evaporative Cooling• The cooling of a surface
occurs when the liquid evaporates
• This is responsible for:– Moderating earth’s climate– Stabilizes temperature in
aquatic ecosystems– Preventing organisms from
overheating
Density of Water• Most dense at 4oC• Contracts until 4oC• Expands from 4oC to 0oC
The density of water:
1. Prevents water from freezing from the bottom up.
2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.
– When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.
– As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.
– Ice is about 10% less dense than water at 4oC.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 3.5
Solvent for Life• Solution– Solute: get dissolved– Solvent: Does the dissolving
• Aqueous solution• Hydrophilic– Ionic compounds dissolve in
water– Polar molecules (generally)
are water soluble• Hydrophobic– Nonpolar compounds (oils)
Most biochemical reactions involve solutes dissolved in water.
• There are two important quantitative proprieties of aqueous solutions.–1. Concentration
–2. pH
Concentration of a Solution
• Molecular weight – sum of the weights of all atoms in a molecule (daltons)
• Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular weight in daltons.
• Avogadro’s number – 6.02 X 1023
– A mole of one substance has the same number of molecules as a mole of any other substance.
MolarityThe concentration of a material in solution is called its molarity.
A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water.
Calculate a one molar solution of sucrose, C12H22O16.
C = 12 daltons
H = 1 dalton
O = 16 daltons
12 x12 = 144
1 x 22 = 22
16 x 11 = 176
342For a 2M solution?
For a .05 M solution?
For a .2 M solution?
Dissociation of Water Molecules• Occasionally, a hydrogen atom shared by two
water molecules shifts from one molecule to the other.– The hydrogen atom leaves its electron behind and is
transferred as a single proton - a hydrogen ion (H+).– The water molecule that lost a proton is now a
hydroxide ion (OH-).– The water
molecule with the extra proton is a hydronium ion (H3O+).
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Unnumbered Fig. 3.47
• A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:–H2O <=> H+ + OH-
• This reaction is reversible.• At equilibrium the concentration of water
molecules greatly exceeds that of H+ and OH-.
• In pure water only one water molecule in every 554 million is dissociated.–At equilibrium, the concentration of H+ or
OH- is 10-7M (25°C) .
Acids and Bases• An acid is a substance that
increases the hydrogen ion concentration in a solution.
• Any substance that reduces the hydrogen ion concentration in a solution is a base.– Some bases reduce H+ directly by
accepting hydrogen ions. • Strong acids and bases complete
dissociate in water.• Weak acids and bases dissociate
only partially and reversibly.
pH Scale• The pH scale in any aqueous solution :– [ H+ ] [OH-] = 10-14
• Measures the degree of acidity (0 – 14)• Most biologic fluids are in the pH range
from 6 – 8• Each pH unit represents a tenfold
difference (scale is logarithmic)– A small change in pH actually indicates a
substantial change in H+ and OH- concentrations.
ProblemHow much greater is the [ H+ ] in a solution with pH 2 than in a solution with pH 6?
Answer:
pH of 2 = [ H+ ] of 1.0 x 10-2 = 1/100 M
pH of 6 = [ H+ ] of 1.0 x 10-6 = 1/1,000,000 M
10,000 times greater
Buffers• A substance that eliminates large sudden
changes in pH.• Buffers help organisms maintain the pH of
body fluids within the narrow range necessary for life. –Are combinations of H+ acceptors and
donors forms in a solution of weak acids or bases–Work by accepting H+ from solutions when
they are in excess and by donating H+ when they have been depleted.
Acid Precipitation• Rain, snow or fog with more strongly acidic than pH of
5.6• West Virginia has recorded 1.5• East Tennessee reported 4.2 in 2000• Occurs when sulfur oxides and nitrogen oxides react
with water in the atmosphere– Lowers pH of soil which affects mineral solubility –
decline of forests– Lower pH of lakes and ponds – In the Western
Adirondack Mountains, there are lakes with a pH <5 that have no fish.