13.1: Unique Properties of Water

Question 1

water has the __ boiling and melting points

Answer 1

highest

Question 2

increase in boiling point is associated with an increase in…

Answer 2

cohesive energy

Question 3

cohesive energy

Answer 3

how strongly molecules associate w/ one another

Question 4

source of cohesive energy =

Answer 4

dipole moment

Question 5

dipole moment

Answer 5

difference in electronegativity b/w atoms

Question 6

dipole moment depends on…

Answer 6

product of charge and separation distance of molecules (size of atom and charge)

Question 7

hydrogen bonding

Answer 7

strong electrostatic interaction b/w H-atom + electronegative atom

Question 8

which molecules form hydrogen bonds?

Answer 8

N, O, F

Question 9

thermocline

Answer 9

boundary b/w cold deep and warm surface waters (heated by sun) bc of lower density of water:
- Warm water rises
- Cold water sinks

Question 10

Winter ice protects aquatic life from…

Answer 10

cold temps above

Question 11

hydrophilic

Answer 11

water-loving molecules

Question 12

hydrophobic

Answer 12

water-hating molecules

Question 13

why are the melting/boiling points of water different from chemicals w/ similar structure/composition?

Answer 13

Bc of it’s unique ability to donate and accept 2 H-bonds at the same time allows it to form a 3-D network of H-bonds
- other molecules can only donate 1 so they can only be linear

Question 14

What is the source of water’s cohesive energy?

Answer 14

1. High dipole moment (H = highly (+) other atom = highly (-))
2. Ability to form network of H-bonds

Question 15

How does water’s dipole moment influence its cohesive energy?

Answer 15

Bc of dipole, strong bonds can form b/w molecules = increased dipole = increased cohesive energy

Question 16

Describe the role of electronegativity in hydrogen bonding.

Answer 16

N, O, F = increased electronegativity bc of increasing nuclear charge = increase in closeness of valence e- to nucleus

Question 17

are hydrogen bonds strong? why?

Answer 17

yes bc the high electronegativity (-) = ability to donate/accept H-bonds bc they retain lone pairs

Question 18

At what temperature is water most dense? Why?

Answer 18

Most dense @ 4°C
- Ice = < dense than liquid H2O = floats

Question 19

What factors influence the solubility of organic molecules in water?

Answer 19

• ability to form hydrogen bonds w/ water and size

Question 20

smaller molecules are ___ soluble

Answer 20

more

Question 21

why are hydrocarbons neutral?

Answer 21

bc C and H have similar electronegativities

Question 22

what H-bonds are most important in nature?

Answer 22

N and O

Question 23

water has the unique ability to…

Answer 23

donate 2 H-bonds (1 from each H atom) and simultaneously accept 2 H-bonds for each e- (lone pair) from O atom

Question 24

at temps below 4°C water…

Answer 24

contracts

Question 25

at temps above 4°C water…

Answer 25

contracts

Question 26

warm water…

Answer 26

rises (less dense)

Question 27

cold water…

Answer 27

sinks (more dense)

Question 28

which is more dense: cold or warm water?

Answer 28

cold water

Question 29

Clathrates (hydrates)

Answer 29

compounds where water molecules pack around nonpolar molecules = arrays of H-bonds = crystalline structure

Question 30

clathrates are stable at temps __ than the melting point of ice

Answer 30

higher

Question 31

methane hydrates

Answer 31

• Anaerobic microbes (sediments or swamps) decompose CH4 –> organic matter
• Releases methane to surrounding water + low temp + high pressure –> methane hydrate

Question 32

methane hydrates occur…

Answer 32

naturally (in ocean sediments/arctic permafrost)

Question 33

Why do oil and water not mix?

Answer 33

H-bond network around nonpolar molecules
- tight packing tendency = water inhibits mixing