Seawater Chemistry

1
Seawater Chemistry
2
Elements
Pure substances that cannot be broken down into
simpler chemical entities by ordinary chemical
reactions.
Periodic Table 112 known elements
3
(No Transcript)
4
Major Elements Comprising the Biological
Molecules of Living Things

• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Phosphorus
• Sulfur

5
Other Major Elements of Living Things

• Calcium (Ca)
• Potassium (K)
• Chlorine (Cl)
• Magnesium (Mg)

6
Atomic structure

• Atoms are the building blocks of all matter
• Nucleus contains
• Neutrons (no charge)
• Protons ( charge)
• Outer shell(s) contain
• Electrons ( charge)

7
Subatomic Particles

• Protons (p )
• Electrons (e -)
• Neutrons (n o)

8
Electron Shell Configurations of Atoms
proton
neutron
electron
hydrogen atom
helium atom
carbon atom
1p, 0n, 1e-
2p, 2n, 2e-
6p, 6n, 6e-
9
atomic number number of p p e- 2He?2e-
and 2p
He
10
atomic mass (atomic wt.) sum of masses of
pn He? 2p 2n, atomic mass 4 4
2He
p n
e-
11
Carbon Atom
p n e- Atomic number Atomic mass
12
Isotope
atoms that differ in the number of neutrons
16 8 O
18 8 O
17 8 O
pn
p
O16 O17 O18 stable isotopes
13
Molecule
Two or more atoms held together by chemical bonds
Oxygen O2 Nitrogen N2 Ammonia
NH3 Carbon Dioxide CO2 Water H2O M
ethane CH4 Glucose C6H12O6
14
Organic Molecules

• Carbohydrates C6H12O6 (glucose)
• Lipids C3H8O3 (glycerol) 3C16H32O2 (fatty
  acids)
• Proteins COOH-NH2
• Nucleic Acids sugar, PO4, N2 containing base

15
Compound
Binding two or more different kinds of elements
together
NaCl CH4 C6H12O6
16
Ion
An atom that has either gained or lost electrons
such that it exhibits a net charge
Na Cl-
17
Sodium (Na) Atom
11 P 12 No
18
Sodium (Na) Ion
19
Chlorine (Cl) Atom
20
Chloride (Cl-) Ion
21
Some Examples of Ions

• Hydrogen H
• Potassium K
• Fluoride F-
• Calcium Ca2
• Magnesium Mg2
• Hydroxide OH-
• Bicarbonate HCO3-
• Nitrate NO3-
• Phosphate PO4-3

22

• Bond Types
• Ionic
• Covalent
• Hydrogen

23
Ionic Bonds
Transfer of electron
17 P 18 No
24
Covalent Bonding electron sharing
H
H
25
Hydrogen Bonding Between Water Molecules
Hydrogen bond
Covalent bond
26
Properties of Water
Boiling Point 100oC Freezing Point 0oC Density
1g/cm3
solid
gas
liquid
27
The formation of ice

• As water cools to 4C
• Molecules slow
• Water contracts
• Density increases
• Below 4C
• Hydrogen bonds form
• Water expands
• As water freezes
• Expands by 9

28
Ice
Density vs Temp (oC)
29
Properties of Water

• High heat capacity
• High heat of vaporization
• High Surface tension
• Polarity solvent properties

30
The ocean moderates coastal temperatures

• Water has high heat capacity, so it can absorb
  (or release) large quantities of heat without
  changing temperature
• Moderates coastal temperatures

31
The ocean moderates coastal temperatures
Hawaii Average Ocean Water Temperature
32
Interconnections of water molecules

• Polarity causes water molecules to form weak
  (hydrogen) bonds between water molecules
• Water sticks to itself and to other substances
• Allows water to be the universal solvent

33
Water as a solvent

• Water dissolves table salt (NaCl) by attracting
  oppositely charged particles
• Pulls particles out of NaCl structure to dissolve
  it

34
Water has a high surface tension
35
Water in the 3 states of matter

• Latent (hidden) heat energy that is either
  absorbed or released as water changes state

36
Properties of Seawater

• Heat capacity
• Heat capacity with salinity
• Evaporation
• Evaporates more slowly than fw
• Specific gravity
• Pure water density 1.000 g/cm3
• Seawater (2 oC) density 1.028 g/cm3

• Seawaters Boiling Point
• As salinity , the boiling point

37
NaCl
NaCl in water
When NaCl dissolves, each ion becomes hydrated
38

• Seawaters Freezing Point
• As salinity , the freezing point
• Salt is an antifreeze- doesnt freeze until -2oC
  (_at_35 o/oo)

Sea ice (pancake ice)
Lattice structure of an ice crystal
39
Determining Salinity
Salinometer
Refractometer
40
Determining Salinity

• Salinity total amount of solid material
  dissolved in water
• Can be determined by measuring water conductivity
  
• Typically expressed in parts per thousand ()

41
Source of Ocean Salts

• Na - Weathering of crustal rock
• Cl- - from the mantle by way of volcanic vents
  and outgassing from mid-ocean rifts
• Mg - mid ocean rifts

42
Processes affecting seawater salinity

• Processes that decrease seawater salinity
• Precipitation
• Runoff
• Icebergs melting
• Sea ice melting
• Processes that increase seawater salinity
• Sea ice forming
• Evaporation

43
Major Solutes in Seawater
Salt Ion Ions in sw (0/00) Cl- 18.980 Na
10.556 SO42- 2.649 Mg2 1.272 Ca2
0.400 K 0.380 HCO3- 0.140 Br-
0.065 H3BO3 0.026 Sr2 0.013 F-
0.001 Total 34.38
44
Ditmars Law of Relative Proportion
Regardless of absolute concentration, the
relative proportion of various salts dissolved in
seawater is constant.
Ex. _at_35o/oo Na 10.8 g/kg Cl- 19.3 g/kg
NaCl ratio 10.8/19.3 0.56 _at_20o/oo
Na 6.2 g/kg Cl- 11.1 g/kg NaCl ratio
6.2/11.1 0.56
45
Ditmars Law of Relative Proportion
Salinity (NaCl, MgSO4)is 35 o/oo Chlorinity
(fluorine, chlorine, bromine, iodine) is 19.2 o/oo
Salinity, g/kg Cx1.80655 Chlorinity, g/kg
S/1.80655
46
Surface Salinity Variation

• Pattern of surface salinity
• Lowest in high latitudes
• around Antarctica, Arctic Ocean
• S.E. Asia
• Western coasts of N. America and central America
• Highest in the tropics
• Mid latitudes in the N. Atlantic, S. Atlantic, S.
  Pacific, Indian Oceans
• Red Sea
• Arabian Sea
• Mediterranean
• Dips at the Equator
• Surface processes help explain pattern

47
Surface Salinity Variation

• High latitudes have low surface salinity
• High precipitation and runoff
• Low evaporation
• Tropics have high surface salinity
• High evaporation
• Low precipitation
• Equator has a dip in surface salinity
• High precipitation partially offsets high
  evaporation

48
Global surface salinity
49
Salinity variation with depth

• Curves for high and low latitudes begin at
  different surface salinities
• Halocline layer of rapidly changing salinity
• At depth, salinity is uniform

50
Salinity variations
51
Thermohaline Circulation
Global ocean circulation that is driven by
differences in the density of the sea water which
is controlled by temperature and salinity.
52
Thermohaline Circulation
White sections represent warm surface currents.
Purple sections represent deep cold currents
53
Thermohaline Circulation
54
Thermohaline Circulation
in Antarctica
55
Thermohaline Circulation
in Antarctica
56
Thermohaline Circulation
in Antarctica
57
Thermohaline Circulation
What effect does global warming play in
thermohaline circulation?
58
Thermohaline Circulation
1
2
3
4
Atmospheric and ocean temp
Subtropical evaporation
High latitude precipitation runoff
CO2 fossil fuel combustion
North Atlantic regional cooling
Deep water formation thermohaline circulation
Nordic seas salinity deep convection
Potential feedback of increased tropical
salinity
6
5
Global climate interconnections
59
Desalination
The production of drinkable water from seawater

• Techniques
• Distillation- water vapor and condensation
• Freezing- ice crystals form leaving salt behind
• Reverse osmosis- sw is forced through a
  semipermeable membrane only water molecules pass
  through
• Electrodialysis- e- charged, semipermeable
  membranes draw salt ions out of sw
• Salt absorption- chemically active resins or
  charcoals are used to draw off the dissolved salt
  ions ? fw

60
Distillation of seawater
plastic
Widely used technique
61
Reverse Osmosis
62
Acids
Proton donor, i.e., they donate H ions HCl is a
strong acid with a pH 1-2
HCl ? H Cl-
63
Bases
Proton acceptor, i.e., they take up H ions NaOH
is a strong base pH 12
Na OH- ? NaOH NH3 H ? NH4 OH - H ?
H2O HCO3- H ? H2CO3
64
Neutralization Buffers
Neutralization- HCl NaOH? H2O NaCl Buffer-
resists dramatic changes in pH ex. tums,
rolaidsbuffers stomach acid
65
pH Scale
0-14
Type of Solution
pH Value
Acidic 0-6 Neutral 7 Basic (alkaline) 8-14
66
pH Scale
Logarithmic scale
67
O2 and CO2 vs ocean depth
0 500 1000 1500 2000
Compensation depth
Depth (m)
CO2
O2
O2 CO2
1.5 3.0 4.5 6.0 7.5
90 94 98 102 106 110
Concentration of dissolved gas (ppm)
68
Carbon Dioxide System in the Ocean
CO2 gas
Air
Water
Photosynthesis
6CO2 6H2O C6H12O6 6O2
Respiration
C6H12O6 6O2 6CO2 6H2O
69
The effects of CO2 in an ocean system
By-product of respiration
CO2 H2O ? H2CO3 ? HCO3- H ? CO32- 2H
carbonic acid
bicarbonate
carbonate
The addition of CO2 makes water acidic
70
Bicarbonate buffer
Seawater too basic H2CO3 HCO3- H pH
drops Seawater too acidic HCO3- H
H2CO3 pH rises
71
Ocean Acidity
72
Global Ocean Acidity
73
Consequences of Ocean Acidity

• Animals with CaCO3 skeletons affected
• Plankton
• Corals
• Mollusks
• Fish

Fisheries
http//news.bbc.co.uk/2/hi/science/nature/7933589.
stm
74
Consequences of Ocean Acidity
In a high CO2 world, the ocean will be

• More acidic
• More stratified
• More oligotrophic, but better light conditions
• Less oxygenated

75
Acid Rain in Marine Environment

• reduces ability of marine organisms to utilize
  calcium carbonate

• Coral calcification rate reduced 15-20
• Skeletal density decreased, branches thinner

76
Inquiry

• How many neutrons in 7 N?
• Why do all the oceans have relatively the same
  proportion of salinity?
• At what temperature is fw most dense?
• Of the following pHs which is most acidic?
• 3 7 6 2
• 5. Why are there no plants at the compensation
  depth?
• 6. Why can a water strider walk on water?
• 7. Besides temperature and salinity, what
  physical factor effects thermohaline circulation?
• 8. What is the oceans most dense sea water
  called?

14