Environmental Chemistry

Environmental Chemistry Week 1

  • Ideal Gas Equation: PV = nRT

    • P: Pressure

    • V: Volume

    • n: Number of moles

    • R: Gas constant

    • T: Temperature

  • Historical context:

    • Boyle’s Law: V ∝ 1/P (at constant T)

    • Charles’s Law: V ∝ T (at constant P)

    • Avogadro’s Law: V ∝ n (at constant T and P)

Gas Laws Explained

  • Boyle’s Law: Volume inversely proportional to pressure at constant temperature.

  • Charles’s Law: Volume directly proportional to absolute temperature at constant pressure.

  • Avogadro’s Law: Volume directly proportional to the number of moles at constant temperature and pressure.

  • Real gases deviate from ideal behavior at high pressures.

Pressure, Volume, and Temperature

  • Pressure: Force per unit area exerted by air.

  • Standard Pressure: Defined as 100 kPa.

  • Volume: Measured in liters (1 L = 1 dm³).

  • Moles: Concept introduced in Chemistry 1.

  • Gas Constant (R):

    • 8.314 kPa L / mol K

    • 0.08206 L·atm / mol·K

    • 8.314 J / mol·K

      • The units L atm can be expressed in terms of m3 N/m2 = N m = J

  • Temperature: Must use absolute scale (Kelvin) + 273 degrees to cel

Exercises on Ideal Gas Law

Partial Pressure and Density

  • Partial Pressure: Total pressure of a gas mixture is the sum of individual gas pressures.

  • Density: Important in meteorology; depends on temperature, pressure, and average molar mass.

  • Air Density: More dense air descends, less dense air rises

    • When examining the atmosphere we generally do not know the exact volume of air.

  • Vapour Pressure: Pressure exerted by water vapor in equilibrium with liquid water.

  • Relative Humidity: Ratio of partial pressure of water vapor to equilibrium vapor pressure.

  • Dewpoint: The temperature where relative humidity is 100%. That is, the temperature where the partial pressure of water vapour = the equilibrium vapour pressure.

Environmental Chemistry Week 2

  • Ozone Overview

    • Chemical formula: O₃ (Ozone)

    • Characteristics: Pale blue gas, melting point -192 °C

    • Resonance structures indicate stability.

  • Ozone's Dual Nature

    • "Good up high, bad nearby":

      • Stratospheric Ozone: Acts as a UV screen, protecting life on Earth.

      • Tropospheric Ozone: Toxic at concentrations above 50 ppm (30 min exposure).

  • Properties of Ozone

    • Powerful oxidizing agent, more reactive than O₂.

    • Can oxidize organic compounds and is used as a germicide in water treatment.

  • Formation of Ozone

    • Synthetic ozone is produced by passing O₂ through electrical discharge or UV light.

    • Natural formation involves photochemical reactions initiated by solar radiation.

  • Photochemical Reactions

    • Energy from light is required to break O₂ bonds, leading to ozone formation.

Calculations and Photochemical Reactions

  • Dissociation Energy of O₂

    • O₂ has a dissociation energy of 495 kJ/mol.

    • Energy required to dissociate one molecule is calculated using Avogadro's number.

Importance of Upper Atmospheric Ozone

  • Ozone Formation in the Stratosphere

    • Produced by photochemical reactions from solar radiation.

    • Cyclic process of formation and decomposition involving O₂ and O₃.

  • Significance of Upper Atmospheric Ozone

    • Essential for life; absorbs harmful UV radiation.

    • O₂ absorbs UV radiation shorter than 240 nm, while ozone absorbs between 240-290 nm.

  • Reactions Involving Ozone

    • Nitrogen oxides and chlorine atoms can catalytically destroy ozone.

    • Chlorofluorocarbons (CFCs) release chlorine in the stratosphere, contributing to ozone depletion.

Smog and Ground-Level Ozone

  • Formation of Ground-Level Ozone

    • Created through photochemical reactions between O₂ and NO₂ in the presence of sunlight.

  • Types of Smog

    • Industrial Smog: Results from combustion processes, containing SO₂ and particulates.

    • Photochemical Smog: Formed from interactions of NO and hydrocarbons in sunlight.

  • Reactions of SO₂ and NO₂

    • Contribute to acid rain and have detrimental effects on industrial areas.

Greenhouse Gases

  • Greenhouse Effect

    • Described as radiative trapping by gases, warming the Earth by about 35 degrees.

  • Albedo

    • Reflectivity of Earth's surface; affects solar energy absorption.

  • Greenhouse Gases

    • Absorb outgoing terrestrial radiation, primarily in the infrared range.

    • Gases that absorb in the wavelength range 5000-50000 nm, where most terrestrial radiation is emitted, are called greenhouse gases.

      • The absorption corresponds to vibrational and vibrational-rotational transitions.

  • Energy Absorption Mechanisms

    • Different gases absorb radiation based on their electronic, vibrational, and rotational states.

      • Electronic transitions: Generally require UV or visible radiation (< 700nm). For most atmospheric gases, there is little absorption in the range of visible radiation (400-700 nm).

      • Vibrational transitions: Require near-IR radiation (700-20000 nm), corresponding to the wavelength range of peak terrestrial radiation.

      • Rotational transitions: Require far-IR radiation (>20 μm).

Radiative Trapping by Gases

  • Role of Water Vapour

    • Constitutes a significant portion of greenhouse gases and is crucial for climate.

  • Forcing and Feedback Mechanisms

    • Forcing: Alters energy balance in the climate system.

    • Feedback: Factors that either increase or decrease the rate of a process.