Chapter 2-3 Chemistry / Water 1Unique properties of Water

Chapter 2-3 Chemistry / Water 2The Chemistry of Water:Heat Capacity

Hydrogen bonding gives water “thermal inertia”, the ability to resist temperature change and phase change.

Example:

Chapter 2-3 Chemistry / Water 3Properties of Water:Heat of Vaporization

A high input of energy is required to change the phase of water from liquid to gas!!

High heat of vaporization To raise water from 98 to 99 ºC; ~1 calorie To raise water from 99 to 100 ºC; ~1 calorie However, large numbers of hydrogen bonds must be broken to evaporate water (phase change)

To raise water from 100 to 101 ºC; ~540 calories!This is why sweating (and panting) cools

Evaporative cooling is best when humidity is low because evaporation occurs rapidly

Evaporative cooling works poorest when humidity is high because evaporation occurs slowly

Chapter 2-3 Chemistry / Water 4Properties of Water:Heat of Fusion

High energy input is required to change the phaseof water from a solid to a liquid!!

Heat of fusion (melting)To raise ice from -2 to -1 ºC; ~1 calorieTo raise water from -1 to 0 ºC; ~1 calorieTo raise water from 0 to 1 ºC; ~80 calories!

This is why ice at 0 ºC keeps stuff cold MUCH longer than water at 1 ºC

This is why ice is used for coolingNOT because ice is coldBut because it absorbs so much heat before it will warm by one degree

Chapter 2-3 Chemistry / Water 5State Transitions of Water(solid liquidgas)

High energy input

High energy input

Chapter 2-3 Chemistry / Water 6Properties of Water:Water as a Solvent

Solutions consist of:A solvent (the most abundant part) andA solute (less abundant part) that is dissolved in the solvent

Polar compounds readily dissolve in water hydrophilic = “likes water”

Nonpolar compounds dissolve only slightly in water hydrophobic = “dislike water”

Ionic compounds dissociate in waterNa+

Attracted to negative (O) end of H2O Each Na+ completely surrounded by H2OCl-

Attracted to positive (H2) end of H2O Each Cl- completely surrounded by H2O

Know these terms and how to apply them.

Chapter 2-3 Chemistry / Water 7pH of Water:Properties of Acids

AcidsDissociate in water and release hydrogen ions (H+) pH < 7.0

Hydrochloric acid (stomach acid) in pure form is a gas with symbol HCl In water, it dissociates into H+ and Cl-

Dissociation of HCl is almost total, therefore it is a strong acid!

Acid H2CO3A weak acid.

Chapter 2-3 Chemistry / Water 8pH of Water:Bases

Bases:Either take up hydrogen ions (H+) or release hydroxide ions (OH-)

Bitter to tasteSodium hydroxide (drain cleaner) is a solid with symbol NaOH In water, it dissociates into Na+ and OH-

Dissociation of NaOH is almost total, therefore it is a strong base

Ex. NaOH Na+ + OH –

Chapter 2-3 Chemistry / Water 9

pH Scale

pH scale used to indicate acidity and alkalinity of a solution.Values range from 0-14

0 to <7 = Acidic 7 = Neutral >7 to 14 = Basic (or alkaline)

Logarithmic Scale Each unit change in pH represents a change of 10X

pH of 4 is 10X as acidic as pH of 5 pH of 10 is 100X more basic than pH of 8

Chapter 2-3 Chemistry / Water 10

Buffers and pH

When H+ is added to pure water at pH 7, pH goes down and water becomes acidic

When OH- is added to pure water at pH 7, pH goes up and water becomes alkaline

Buffers are solutes in water that resist change in pHWhen H+ is added, buffer may absorb, or counter by adding OH-

When OH- is added, buffer may absorb, or counter by adding H+

Chapter 2-3 Chemistry / Water 11Homeostasis!Buffers in Biology

Health of organisms requires maintaining pH of body fluids within narrow limitsHuman blood normally 7.4 (slightly alkaline)Many foods and metabolic processes add or subtract H+ or OH- ions Reducing blood pH to 7.0 results in acidosis Increasing blood pH to 7.8 results in alkalosis Both life threatening situations

Bicarbonate ion (-HCO3) in blood buffers pH to 7.4

Chapter 2-3 Chemistry / Water 12Water as a Transport Medium

Later on in this semester we will be looking at a process in plants called evaporative transpiration in which the

special hydrogen bonding properties of water allow it to transport nutrients many tens of meters up into trees against the enormous pull of gravity in the opposite

direction.

For now write in your notes: The polarity of water and its resulting hydrogen bonding allow it to act as an all-important transport medium in plants and animals.

Chapter 2-3 Chemistry / Water 13Properties of Water:Cohesion & Adhesion

Cohesive and Adhesive Cohesion – Hydrogen bonds hold water molecules tightly together

Adhesion – Hydrogen bonds form between water and other polar materials

Allow water be drawn many meters up a tree in a tubular vessel

High Surface Tension Water molecules at surface hold more tightly than below surface

Amounts to an invisible “skin” on water surface Allows small nonpolar objects (like water strider) to sit on top of water

More vocabulary!

Chapter 2-3 Chemistry / Water 14

Surface tension

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Chapter 2-3 Chemistry / Water 15

Review

Chemical Elements Atoms = the basic building block of elements

Isotopes = elements with the same number of protons but ..…………………differing numbers of neutrons

Molecules and Compounds = combinations of different elements. Joined by covalent and or ionic bonds.

Chemical Bonding Ionic and Covalent - donate electrons vs share electrons

Hydrogen - electrostatic bond between partial pos. and neg. charges. Example = water

Properties of Water – polar, H-bonding, specific heat, solvent

Acids and Bases – scale 1-14, 7= neutral, hydrogen ions contribute to acidity, hydroxide ions contribute to alkalinity, logarithmic scale (10X).