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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CHAPTER 2LECTURE
SLIDES
2
Water
• Life is inextricably tied to water
• Single most outstanding chemical property of water is its ability to form hydrogen bonds– Weak chemical associations that form
between the partially negative O atoms and the partially positive H atoms of two water molecules
Water Molecules and Hydrogen Bonding
Water is polar. Hydrogen bonds form between water molecules.
4
Polarity of water
• Within a water molecule, the bonds between oxygen and hydrogen are highly polar– O is much more electronegative than H
• Partial electrical charges develop– Oxygen is partially negative δ+
– Hydrogen is partially positive δ–
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Hydrogen bonds
• Cohesion – polarity of water allows water molecules to be attracted to one another
• Attraction produces hydrogen bonds• Each individual bond is weak and
transitory• Cumulative effects are enormous• Responsible for many of water’s important
physical properties
Cohesion and Adhesion:
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• Cohesion – water molecules stick to other water molecules by hydrogen bonding
• Adhesion – water molecules stick to other polar molecules by hydrogen bonding
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Properties of water
1. Water has a high specific heat
specific heat : The amount of energy required to raise the temperature of 1 gram of a substance by 1 C.
– A large amount of energy is required to change the temperature of water
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Properties of water
2. Water has a high heat of vaporization– The evaporation of water from a surface
causes cooling of that surface
13
Properties of water
3. Solid water is less dense than liquid water
– Bodies of water freeze from the top down
Structure of Water: Liquid and Ice
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4. Water is a good solvent– Water dissolves polar molecules and ions
Properties of water
18
5. Water organizes nonpolar molecules– Hydrophilic “water-loving”– Hydrophobic “water-fearing”– Water causes hydrophobic molecules to
aggregate or assume specific shapes
Properties of water
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6. Water can form ions
H2O OH– + H+
hydroxide ion hydrogen ion
Properties of water
21
Acids and bases
• Pure water– [H+] of 10–7 mol/L– Considered to be neutral– Neither acidic nor basic
• pH is the negative logarithm of hydrogen ion concentration of solution
pH = −log [H+].
• Acid– Any substance that dissociates in water to
increase the [H+] (and lower the pH)– The stronger an acid is, the more hydrogen
ions it produces and the lower its pH
• Base– Substance that combines with H+ dissolved in
water, and thus lowers the [H+]
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Buffers
• Substance that resists changes in pH
• Act by – Releasing hydrogen ions when a base is
added– Absorbing hydrogen ions when acid is added
• Overall effect of keeping [H+] relatively constant
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• Most biological buffers consist of a pair of molecules, one an acid and one a base