ENVIRONMENTAL CHEMISTRY

CookIB Option E Chemistry

Environmental Chemistry

The effect of human activity on the environment.

How does it impact: Air Water Soil

3 Key areas of study: Atmospheric Pollutants Discarded waste in landfills Greenhouse gases

Air Pollution

Air is a mixture of the following elements and percentages:

Gas % Composition

Nitrogen (N2) 78

Oxygen (O2) 21

Argon (Ar) 1

Water Vapor 1-4

Carbon Dioxide (CO2) 0.04

Pollutants

Is a substance that has a harmful effect on the environment and is present in concentrations greater than it natural levels. Effects depend on

TOXICITY Length of time they remain in the enviroment

Air pollution is classified as Primary and Secondary Primary=emitted directly into the atmosphere Secondary=produced from primary pollutants

chemical changes.

Common Pollutants

CO2

Produced from the combustion of fossil fuels CO Smog SO2

NO VOC’s (hydrocarbons) Now its time to discuss the natural and

anthropogenic sources.

Air Pollutants

CO Toxic to humans because if affects the uptake of oxygen in blood Its absorbed by the lungs and binds to hemoglobin which prevents

oxygen from binding Hb + CO COHb Hemoglobin + Carbon monxide= Carboxyhemoglobin

This prevent oxygen from being transported This colorless and odorless compound can cause dizziness and

death at high concentrations Sources: Anthropogenic =incomplete combustion of fossil

fuels and forest fires 2C + O2 2CO CO pollution occurs in urban areas with heavy traffic CO is formed by natural sources during atomspheric oxidation of

methane gas. Methane comes from

Controlling CO

Reducing the amount of CO can be done doing the following: Lean Burn engines Catalytic converter: Thermal Exhaust reactor:

Lean Burn Engines

Follows low air/fuel ratios -(14:15 by mass) is needed for complete combustion

Max power is achieved with mixture richer in fuel, with lower air/fuel ratio around 12.5 by mass, but this makes more CO.

Engines misfire with leaner (lower ratios) Lean burn engines have ratios 18 or

higher

Catalytic Converters

Control Exhaust emissions Hot gases are mixed with air and passed

over a platinum based catalyst. Oxidation catalyst are used in lean burn

engines to convert CO to CO2. 3 way catalyst work in conventional

engines. They oxidize CO to CO2 and hydrocarbons to water and CO2

Review equations on pages 675 HL and 368 SL

Thermal Converters

Thermal Exhust Reactors Take advantage of heat of the exhaust gases

and makes CO react with more air to produce CO2

Review Figure 16.2 Know reactions for each CO method of control,

both HL and SL.

Catalytic converters

catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze.

There are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium.

Catalytic Converters

When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

2NO => N2 + O2 or 2NO2 => N2 + 2O2

Nitrogen Oxides

The known nitrogen oxides are: NO (produced from natural decomposition/lightning) NO2 (most toxic) N2O

These react with hydrocarbons to form photochemical smog, nitric acid and contribute to acid rain.

Motor vehicles are the main source (fig 16.3) Only under conditions of high temperatures

(15000C in automobile engines), nitrogen and oxygen in the air react to form NO.

NO, N2O and NO2

NO (Most pollutant) Produced naturally for decomposition of

nitrogen containing compounds Nitrogen Fixing Bacteria and Lightning Review reactions of Nitrogen and the nitrogen cycle

N2O Major source is bacteria decomposition Reacts with hydrocarbons to form

photochemical smog and also forms HNO3 making acid rain

Methods of Control of N2

Lean Burn Engines A rich mixture with a high fuel content

produces low Nox, but high CO. Lean burn engines which use air:fuel ratios : 18:1 can be used to reduce emissions of both NOx and CO.

Three way catalytic converters They oxidize CO to CO2

2CO + O2 2CO2

Sulfur Oxides

There are 2 types: SO2 and SO3

SO2 is the most dangerous, that can harm people, plants and materials (buildings) Natural sources: Volcanos and rotting veggies Secondary Pollutant Review Reaction

Anthropogenic sources: Fossil Fuels (coal)

Review all reactions

Methods of Control of SO2

Pre Combustion: The best way is to remove sulfur before or after combustion.

Sulfur can be removed by crushing the coal and washing with water. Sulfide sinks to bottom and separates from clean coal. Review Reaction

Post Combustion Alkaline scrubbing Figure 16.4

A alkaline mixture is sprayed downward unto exhaust gas. The mixture contains CaO (lime) reacts with SO2 and forms calcium sulfate which is deposited in landfill.

Fluidized Combustion Coal is mixed with powered limestone on a metal plate as air passes through

the mixture which makes particles float above a plate making a mixture behave like a fluid. The heat produced from combustion of coal causes the CaCO3 to break up into CaO and CO2 the SO2 is removed as it combust with coal.

See Reaction

Particulates

Solid particles of dust, carbon or liquid droplets of mist or fog.

Have small diameter: 0.001-10micrometers Very polar and attracted to water and form aerosols.

Gaseous suspension of very small particles of liquid Examples:

Soot (from incomplete combustion of hydrocarbons and coal burning factories)

Dust Sulfur from volcanoes Pollen, bacterial and fungal spores Arsenic from insecticides Asbestos

Material containing silicate crystals which is used to insulate building. These are released in the air when buildings are demolished.

Mercury (Fungicides, making of paper and pulp.

Methods of Control of Particulates

Gravitational Settling by rain and snow They can be prevented from entering

atmosphere byTreating industrial emissions using the following physical methods.

-Flitration-Centifugal separation-Settling tanks-Scrubbing-Electrostatic precipitation

Review these methods for quiz on weds

VOC’sVolatile Organic Compounds

These are organic air pollutants Methane Chloroethane Aromatic hydrocarbons (from incomplete combustion of coal/wood)

BenzeneVOC’s also released into air from solvents and paintsHydrocarbons from secondary pollutantsHalogen organic compounds (CFC’S)

Photochemical SmogMethane is released in large amounts by cows. It is produced by bacterial anaerobic decomposition of organic material in the water and soil. Unsaturated hydrocarbons (with double bonds), called trepenes are given out by plants.Anthropogenic sources include: Unburned Petroleum products; Gasoline emitted from car exhaust. Basicaly everything from fossil fuels

Controlling VOC’s

Using Oxidation Catalytic converters Using Thermal Exhaust Reactors

Review all reactions and methods of control

Smog

Two types of Smog Photochemical Smog:

From nitrogen oxides which oxidizes in dry sunshine Pea Soup Smog

Carbon Particulates and SO2 From reducing