Chapt02 water

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CHAPTER 2LECTURE

SLIDES

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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.

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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

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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

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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

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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