Topic 5: Seawater Chemistry

Covalent bonds

Share electrons

Ionic bonds

Give/take electrons

2 reasons why water is a polar molecule

1. Difference between size of H and O atoms causes bend in geometry of water molecule

2. Negative and positive charges at either end = polarity

How hydrogen bonds are created in water

Water is a molecule with positive and negative ends, it attracts particles with negative and positive ends – like other water molecules

Cohesion

Water attracts water; surface tension (hydrogen bonds)

Adhesion

Water ‘sticks’ to surfaces (electrostatic bonds)

Why water is the “Universal solvent”

Because it is polar and so dissolves other molecules with negative and positive ends (like salts)

Thermal properties of water (and why they are important)

• Water stores heat energy

• Important for ocean circulation + other processes

3 phases of water

1. Solid (ice)

2. Liquid water

3. Gas (water vapour)

Process of water going from solid to liquid

Melting

Process of water going from liquid to gas

Vapourization

Process of water going from gas to liquid

Condensation

Process of water going from liquid to solid

Freezing

How energy changes with water’s phases

Use energy to create bonds (i.e liquid to solid), remove energy as we break bonds (i.e solid to liquid)

Kinetic energy

Energy an object has because of its motion, with a faster object having more (water in all states has kinetic energy)

Heat

Heat is energy; amount of energy transferred from one body to another due to temperature differences; proportional to average kinetic energy

Temperature

A number, way to measure the average kinetic energy (response to addition/removal of heat)

Calorie

The amount of heat required to raise the temperature of 1 gram of water by 1 degree C

Characteristics of water in solid state (ice)

1. Molecules vibrate

2. Stay in same position

Characteristics of water in liquid state (water)

1. More kinetic energy

2. Bonds starting to break

Characteristics of water in gaseous state (water vapour)

1. Most kinetic energy

2. All bonds broken

Heat capacity definition

Heat required to raise the temperature of 1 gram of a substance by 1 degree C

Heat capacity of water

Pure water has a very high heat capacity (1.oo cal/gram/degree C)

Application of heat capacity to water

Water can absorb (or release) large amounts of heat while changing relatively little in temperature (takes a lot of time for water to heat up or cool down)

Ocean heat content definition

Total amount of heat stored by oceans— the large ocean has a large heat storage capacity meaning the atmosphere has not warmed as rapidly as it could (due to climate change)

% of Earth’s heat stored by ocean

More than 90% percent of the Earth’s extra heat (since 1955, even with 2x uptake since 1993)

Consequences of oceans absorbing so much heat

• Oceans being pushed to the edge of their capacity

• Changes in ocean circulation

• Changes in ocean ecosystems

• Changes in ocean climates

Why sea surface temperature is important

Influences weather patterns, storms

Consequences of increasing ocean temperature

• Fish metabolism

• Nutrient availability

• O2 vs CO2 content

• Ecosystems

• Migration and breeding

• Threaten sensitive marine life

• Heatwaves in major ocean basins

Amount sea surface temp has risen by

5 degrees C

Latent heat

When water undergoes a change of state a large amount of heat is absorbed or released

Phase Changes and ‘Latent Heats’

Thermostatic properties

Properties that moderate temperature changes (i.e high ocean heat capacity)

Global Thermostatic Effects

Transfer of heat from the tropics towards the poles, moderates global temperatures

Density of pure water

1.0 g/cm³

How density of water changes at depth

1. Density increases as heat is removed

2. Maximum density just before freezing (as ice is less dense than water)

Why ice is less dense than water

Phase change occurs from liquid to solid, rapid movement means liquid expands slightly as more hydrogen bonds (rigid framework) form, therefore ice is less dense and expanding

Density of ice

0.917 g/cm³

Maximum density and temp of water

1.0 g/cm³ at 3.98 degrees C

Percentage of water and dissolved solids in seawater

• ~96.5% pure water

• 3.5% dissolved solids

How dissolved solids in seawater change freezing point and latent heat

• Reduce freezing point to -1.91 degrees C

• Lowers latent heat by ~4%

How salt determines freezing point of water

More salt = lower freezing point

3 reasons to be concerned about salinity of ocean water

Presence of ions and dissolved gases that cause salinity important because:

1. Make life in ocean possible – needed for some biological processes

2. Affects ocean circulation

3. Can provide information about palaeo-oceanic conditions

Salinity definition

Total amount of dissolved solids and gases in water

Concentration of salt in seawater

Averages 3.44% (given as parts per thousand – 34.4‰)

Types of salt in seawater

6 major ions represent 99% of all dissolved solids, Cl- and Na+ dominate

Major gases dissolved in seawater

How gases enter seawater

Common atmospheric gases are easily dissolvable in seawater because the ocean is in contact with the atmosphere

Why nitrogen is important in the ocean

• Upper layers of ocean are saturated with nitrogen

• Required by organisms for protein-

  building

Why oxygen is important in the ocean

• If you have gills – you need this gas for metabolism

• Comes from photosynthetic activity and

  atmospheric diffusion

How carbon dioxide enters the ocean

• Very soluble in seawater

• Seawater is a carbon reservoir

Controls on amount of each gas dissolved in the ocean

Dependent on solubility and saturation of the gas

Solubility

Amount of a dissolved gas that the water can hold under a particular set of conditions (0 degrees C and 1 atm of P)

Saturation

Amount of gas currently dissolved in the water, relative to the maximum possible content

Undersaturated

More gas can dissolve

Saturated/supersaturated

Gas may be released

Why are most atmospheric gases are saturated in the ocean, but O2 and (CO2) are not?

They are rapidly used by living organisms

2 major factors affecting solubility of a gas

1. Gas solubility increases with increasing pressure

2. Also increases with decreased temperature

Why does O2 concentration still decrease in deep ocean?

• No photosynthesis (light cannot penetrate)

• Reduced atmospheric diffusion

Why does CO2 concentration increase in deep ocean?

• CO2 is more soluble with lower temp and higher pressure

• No photosynthesis to absorb CO2

OML/OMZ (Oxygen Minimum Layer/Zone)

Around ~ 200 to 1000 m deep in ocean where O2 saturation is at its lowest

Why OML/OMZ forms

• No input from atmosphere or

  photosynthesis

• Output – respiration and

  decomposition

• No better conditions for O2 dissolution (after this zone, water becomes deeper + colder and more O2 dissolves)

Where salinity varies

• On the surface

• At depth

• Around the globe

Salinometer

Measures salinity of seawater by measuring electrical conductivity

Principle of constant proportions (Forchhammer’s Principle)

Ratio of dissolved solids in the ocean is constant, different salinities but proportions of major ions remains the same (i.e both sites have 55.04% of dissolved solids being Cl- ions)

Chlorinity

Measure of Cl- in seawater (most abundant dissolved solid in seawater)

Salinity/chlorinity equation

3 ways ions/compounds enter seawater

1. River discharge

2. Volcanic eruptions

3. Hydrothermal activity at mid-ocean ridge (seafloor spreading)

Residence time

Average length of time that an ion spends in the ocean (times vary depending on how chemically active an ion is)

Composition of seawater vs freshwater

Why the ocean doesn’t get saltier

Because it is in equilibrium— ion removal rate = addition rate

4 ways ions/compounds are removed from seawater

1. Adsorption and precipitation

2. Sea spray

3. Biological processes

4. Hydrothermal activity at mid-ocean ridge (seafloor spreading)

2 Global salinity trends

1. Tropics of Cancer and Capricorn – high

   evaporation rates (higher salinity)

2. Poles and Equator – freshwater influx (lower salinity)

2 ways salinity can increase

1. Evaporation of seawater

2. Freezing of seawater

4 ways salinity can decrease

1. Precipitation of rain and snow

2. River runoff

3. Melting of ice (glacier melt)

4. Groundwater flow to ocean

How to change salinity

Add or remove water (less water = higher salinity)

‘Layered ocean’

Variation in salinity at depth in the ocean

Halocline

Layer of rapidly changing salinity with depth

How surface salinity differs across latitude

Higher salinity at surface at low latitudes due to more evaporation

Seawater density

~1.022 to 1.030 g/cm³

4 ways seawater density increases

1. Increases with greater depth

2. Increases with greater salinity

3. Increases with decreased temperature

4. Increases with increased pressure

3 density zones (water masses)

1. Surface zone (2% of ocean water)

2. Pycnocline (18% of ocean water)

3. Deep zone (80% of ocean water)

Pycnocline

Layer of rapidly changing density (between surface and deep zones)

Pycnocline vs Thermocline

Same thing! Rapidly changing density = rapidly changing temperature (because temp is greatest influence on density)