SPRING 2015 Chemical Principles: Water and Polarity.
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Transcript of SPRING 2015 Chemical Principles: Water and Polarity.
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SPRING 2015
Chemical Principles:
Water and Polarity
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Overview: A Chemical Connection to Biology(Chapter 2 and Chapter 3)
• Living organisms are subject to basic laws of physics and chemistry
• To understand living organisms you have to understand their physics and chemistry
• “Reductionist” approach
• Life is an “emergent property” of atoms and molecules
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• In a nonpolar covalent bond, the atoms share the electron equally
• In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally
• Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule
All covalent bonds are not the same
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Water is an excellent example of a compound with polar covalent bonds
–
+ +H H
O
H2O
The oxygen end is slightly negative and the hydrogen ends are slightly positive
Oxygen’s electron distribution causes the bent shape
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Methane is an example of a compound with non-polar covalent bonds
• Roughly equal sharing of electrons between C and H
• Electron distribution is even
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Weak Chemical Bonds
• Strong chemical bonds such as covalent bonds are not the only important bonds in living systems.
• Weak chemical bonds are also important
• Weak chemical bonds reinforce shapes of large molecules, help molecules attach to each other and provide on/off switches
• Living systems are smart! They don’t waste energy making things stronger than they need to be.
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Hydrogen Bonds a key type of weak bond
• A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom
• The hydrogen atom is shared between two more electronegative atoms
• In living cells, the electronegative partners are usually oxygen or nitrogen atoms
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Hydrogen bonding between water and ammonia-a simple and non-biological example
+
+
+
+
+
Water (H2O)
Ammonia (NH3)
Hydrogen bond
H-bonds are
weak, short-range,temporary,directional
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Van der Waals Interactions-a less important weak bond
• If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative charge
• Van der Waals interactions are attractions between molecules that are very close together as a result of these charges
• Individually very weak but collectively strong
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The polarity of water molecules results in hydrogen bonding in water
• The H bonds make water molecules stick together
• They will also stick to other polar molecules (but not nonpolar ones)
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Five emergent properties of water result from H bonding and contribute to
water’s fitness for life
–Cohesion/adhesion/surface tension–Ability to moderate temperature•High specific heat•Evaporative cooling
–Expansion upon freezing–Properties as a solvent–Dissociation
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Cohesion, Adhesion, Surface Tension
• Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion
• Adhesion is an attraction between different substances, for example, between water and plant cell walls
• Both help water flow inside organisms for example the transport of water against gravity in plants
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Water-conductingcells
Adhesion
Cohesion
Directionof watermovement
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• Cohesion of water molecules leads to a property called surface tension
• Surface tension is a measure of how hard it is to break the surface of a liquid
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Ability to Moderate Temperature-Important
• Water can absorb or release a large amount of heat with only a slight change in its own temperature-it has a high specific heat.
• Water absorbs heat from warmer air and releases stored heat to cooler air-it moderates temperature changes
• It protects against large changes in temperature
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Water’s High Specific Heat
• The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC
• The specific heat of water is 1 cal/g/ºC
• Water resists changing its temperature because of its high specific heat
• Note: The “calories” on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal
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• Water’s high specific heat can be traced to hydrogen bonding
– Heat is absorbed when hydrogen bonds break
– Heat is released when hydrogen bonds form
• Added heat breaks hydrogen bonds in water without raising water’s temperature very much.
• high specific heat of water minimizes temperature fluctuations to within limits that permit life
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Water acts as a temperature moderator-global example
• The Gulf Stream is an ocean current that carries heat from the equator to the northern latitudes of Europe
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It always heats up faster than water. Why?
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Efficient evaporative cooling by water is also a consequence of hydrogen bonding
•Evaporation is transformation of a substance from liquid to gas•Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas•As a liquid evaporates, its remaining surface cools, a process called evaporative cooling •Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water
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Evaporative cooling in action
• This is the most effective way for many animals to maintain body temperature because turning water into steam gets rid of so much excess heat.
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Expansion Upon Freezing-Insulation of Bodies of Water by Floating Ice• When water solidifies, each molecule is H-
bonded to four others in a rigid array: ice
• All the bonding holds the molecules relatively motionless at a certain distance apart
• In liquid water molecules move more freely and get closer together
• Water reaches its greatest density as a liquid at 4°C not as a solid at 0°C
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H Bonds in Ice and Liquid Water
Hydrogenbond
Hydrogen bonds break and re-form
Molecules can approach more closely
Hydrogen bonds are stable
Molecules do not move easily
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Solvent properties-The Solvent of Life
• Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily with many solutes.
• When a polar or ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell or sphere of hydration.
• Non-ionic or non-polar substances cannot form a hydration shell with water.
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• Water can dissolve compounds made of polar or ionic molecules: like dissolves like.
• Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions
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Hydrophilic and Hydrophobic Substances
• A hydrophilic substance is one that has an affinity for water (water-loving)
• A hydrophobic substance is one that does not have an affinity for water (water hating)
• Oil molecules are hydrophobic because they have relatively nonpolar bonds; carbohydrates and proteins tend to be hydrophilic because of polar covalent bonds.
IMPORTANT!!!!!
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Water dissociates or ionizes
• A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other:
– The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)
– The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+
– The molecule that lost the proton is now a hydroxide ion (OH–)
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• This reaction is called the dissociation of water.
• Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed
• But even pure water has some hydronium and hydroxide in it.
Hydroniumion (H3O+)
Hydroxideion (OH–)
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• Though statistically rare the dissociation of water molecules has a great effect on organisms
• Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell
• In pure water hydronium=hydroxide
• Some substances shift the balance so there is more hydronium = acids.
• Others shift the balance so there is more hydroxide = bases.
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• Biologists use something called the pH scale to describe whether a solution is acidic or basic
• Pure water has a pH of 7.0
• Acidic solutions have pH values less than 7
• Basic solutions have pH values greater than 7
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Most liologicalfluids have pH values in the range of 6 to 8
Life unctionsbest near neutral pH Neutral
solution
Acidic solution
Basic solution
OH–
OH–
OH–
OH–
OH–OH–
OH–
H+
H+
H+
OH–
H+ H+
H+ H+
OH–
OH–
OH–OH–
H+
OH–
H+
H+
H+
H+
H+
H+
H+
OH–
Neutral [H+] = [OH–]
Incr
easi
ng
ly A
cid
ic [
H+]
> [
OH
–]
Incr
easi
ng
ly B
asic
[H
+]
< [
OH
–]
pH Scale0
1
2
3
4
5
6
7
8
Battery acid
Gastric juice,lemon juice
Vinegar, beer,wine, cola
Tomato juice
Black coffee
Rainwater
Urine
SalivaPure water
Human blood, tears
Seawater
9
10
Milk of magnesia
Household ammonia
Householdbleach
Oven cleaner
11
12
13
14
Therefore living
systems must maintain their
internal pH close to pH 7
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Buffers
• Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution
• Living systems have to contain buffers
• Most buffers consist of an acid-base pair that reversibly combines with H+