oceanography chapter 5

Chapter 5 Lecture - Water and Seawater

Atom and Element Definition

  • Atom: The smallest particle that can contain the chemical properties of an element.
  • Element: A substance composed of atoms that cannot be broken down into smaller, simpler components.

Structure of the Atom

  • An atom is composed of:
    • Protons: Positively charged particles found in the nucleus.
    • Neutrons: Neutral particles also found in the nucleus.
    • Electrons: Negatively charged particles that orbit around the nucleus.

General Chemistry Overview

  • Elements in Pure Form: Rare in nature; typically combine to form compounds.
  • Compound: Consists of two or more elements.
  • Chemical Properties: Determined by the electron arrangement. Atoms with incomplete shells interact with others to gain, lose, or share electrons.

Bonding of Atoms

  • Atoms bond to form molecules and compounds (e.g., Water - H2O and Carbon Dioxide - CO2).
  • Types of Chemical Bonds:
    • Ionic Bonds: Form when an atom loses or gains electrons, creating ions.
    • Covalent Bonds: Form when electrons are shared between atoms that lack an electrical charge.

Ionic Bonds

  • Formation occurs due to the transfer of electrons:
    • Cations: Positively charged ions that have lost electrons (e.g., Na+).
    • Anions: Negatively charged ions that have gained electrons (e.g., Cl-).
  • Example: Sodium Chloride (NaCl) results from the ionic bond between Na+ and Cl- due to their electrical attraction.

Covalent Bonds

  • Formed by sharing electrons, exemplified by Hydrogen gas (H2).
  • Mixtures: When atoms and molecules come together without reacting, they form mixtures, and an evenly distributed mixture is called a solution.

Types of Covalent Bonds

  • Nonpolar Covalent Bond: Electrons are shared equally.
    • Examples include hydrogen gas, oxygen gas, nitrogen gas, and methane gas.
  • Polar Covalent Bonds: Formed through the unequal sharing of electrons. Water is a prime example where oxygen is slightly electronegative, leading to its polar characteristics.

Water Molecule Structure

  • Composition: Water (H2O) consists of two hydrogen atoms and one oxygen atom.
  • Water is a polar molecule with a negative charge near oxygen and a positive charge near the hydrogen atoms.
  • Polarity allows water molecules to bond with each other and with other polar molecules and ions.

Characteristics of Polar Molecules

  • Polar molecules arise from unequal distribution of charges, resulting in a constant tug-of-war for shared electrons. Atoms with higher electronegativity draw electrons closer to their nucleus.
  • In Water:
    • Oxygen (8 protons) attracts shared electrons strongly away from Hydrogen (1 proton).
    • This results in a higher concentration of electrons near the oxygen atom, imparting a slight negative charge to oxygen and slight positive charges to hydrogen.

Properties of Water

  1. Cohesion: Due to hydrogen bonds, water adheres to itself, allowing it to transport nutrients in plants.
  2. Surface Tension: Water's cohesive properties contribute to high surface tension, which measures the difficulty of breaking the surface of a liquid.
  3. Hydrogen Bonds: Very weak compared to ionic and covalent bonds but collectively strong, enabling several properties of water.

Water as a Solvent

  • Water adheres to other polar molecules, and ionic compounds like table salt dissociate due to electrostatic attraction between water molecules and ions (Na+ and Cl-).

States of Water

  • Three States of Matter:
    1. Solid (Ice): Molecules locked in place due to stable hydrogen bonds.
    2. Liquid (Water): Molecules move freely with some hydrogen bonds present.
    3. Gas (Vapor): Rapidly moving independent molecules without hydrogen bonds.

Unique Properties of Water

  1. High Specific Heat: Requires large amounts of energy to change temperature.
  2. High Heat of Vaporization: Evaporation causes cooling.
  3. Density: Solid water is less dense than liquid water, leading to ice floating on water.
  4. Good Solvent: Can dissolve polar molecules and ions.
  5. Organizes Nonpolar Molecules: Results in hydrophilic (water-loving) and hydrophobic (water-fearing) behaviors.
  6. Ion Formation: Water can dissociate to form hydroxide (OH-) and hydrogen (H+) ions.

Implications for Marine Life

  • Water’s thermal properties prevent drastic temperature changes, allowing stable conditions for marine organisms, whereas shallow ecosystems are more vulnerable.

Hydrogen Bonding Effects

  • Water absorbs a significant amount of heat due to its hydrogen bonds, with heat increasing the movement of molecules and braking bonds, while cooling forms bonds and releases energy.

Global Thermostatic Effects

  • Water moderates Earth's surface temperature, preventing equatorial oceans from boiling and polar oceans from freezing solid, facilitating life on Earth.

Climate and Evaporation Patterns

  • The tropics are humid and receive more rainfall than evaporates; temperate regions experience less precipitation; polar regions have low evaporation relative to precipitation.

Heat Exchange Mechanisms

  • Heat removed from tropical oceans is transported towards the poles and released at higher latitudes through precipitation, impacting climate.

Composition of Seawater

  • Seawater contains sufficient salt to cover the planet with a thickness greater than 500 feet, primarily coming from geological weathering and volcanic outgassing.
  • Salinity Measurement: Measured in parts per thousand (ppt). For example, seawater typically has 35 grams of salt per 1,000 grams of water.

Seawater Salinity

  • An average salinity of 3.5% indicates 96.5% pure water and 3.5% solutes. Six ions make up 99% of seawater solids.
  • Main solutes: Sodium (Na+) and Chloride (Cl-) comprise 85% of seawater.

Sources of Seawater Salt

  • Rivers supply most of the salt; volcanic activity and weathering of sedimentary rocks contribute ions, while out-gassing from volcanic eruptions provides various gases.

Salinity Measurement Techniques

  • Conductivity of seawater indicates its salinity, which significantly influences water density.

Water Density Influences

  • Denser saltwater sinks below freshwater; cold water can trap more nutrients.

Dissolved Gases in Seawater

  • Major gases include Nitrogen (N2), Oxygen (O2), and Carbon Dioxide (CO2).

Nitrogen in Seawater

  • Comprises ~48% of dissolved gases but is mostly unusable in N2 form. Its reactive forms include nitrite (NO2), nitrate (NO3), and ammonium (NH4).

Oxygen in Seawater

  • Essential for aerobic organisms, constituting ~36% of dissolved gases, produced primarily through photosynthesis.

Carbon Dioxide in Seawater

  • Accounts for ~15% of dissolved gases and increases in concentration with water depth.

Acid-Base Balance in Seawater

  • Water separates into hydroxide (OH-) and hydrogen ions (H+); an imbalance leads to acidity or basicity as measured by pH (average seawater pH 7.8).

Carbonate Buffering System

  • CO2 dissolves in seawater forming carbonic acid, allowing the ocean to buffer changes in acidity and alkalinity, maintaining ecosystem stability.

Impact of Human Activities on Ocean Chemistry

  • Increased atmospheric CO2 leads to higher levels in seawater, promoting ocean acidification which affects calcifying organisms’ ability to form shells.

Long-term Environmental Impacts

  • Regions like New York, with significant shellfish industries, are at risk from the effects of acidification, potentially affecting economic and ecological stability.

Economic Implications for Shellfish Industry

  • New York relies heavily on shellfish, averaging $24 million annually over the past decade, highlighting the significance of aquaculture in local economies and the potential threats from ocean changes.