IB Chemistry/Environmental Chemistry

E2 Acid Deposition

E.2.1 State what is meant by the term acid deposition and outline its origins.

Acid deposition is the process by which acidic particles, gases, and precipitation leave the atmosphere. Rain is naturally acidic with a pH of about 5.6 due to dissolved CO₂, but acid rain has a pH below 5.6 and is caused by oxides of sulfur and nitrogen. These oxides react with rain water to form acids:

CO₂ + H₂O → H₂CO₃

2NO₂ + 2H₂O → 2HNO₃ + H₂

SO₂ + H₂O → H₂SO₃

SO₃ + H₂O → H₂SO₄

E.2.2 Discuss the environmental effects of acid deposition and possible methods to counteract them.

Some effects of acid deposition include:

• Leeches important nutrients from soil such as Ca²⁺, Mg²⁺, and K⁺ which can lead to reduction in chlorophyll and therefore the ability to photosynthesize.

• Can kill aquatic life in lakes and rivers, and nitrates can lead to eutrophication.

• Erosion of stone which contains calcium carbonate (such as marble)

• Irritation of the mucus membranes increases the risk of respiratory illness such as asthma, bronchitis and emphysema

Acid deposition can be counteracted by lowering the amount of sulfur and nitrogen oxides with:

• Improved engine design

• Catalytic converters

• Removing sulfur before, during, and after use of sulfur-containing fuels

It can also include the reduction of the amount of fuel burned, alternative energy methods and the use of mass transportation. Furthermore, alkaline scrubbers, such as CaCO₃ and CaO, to remove the oxides.

Adding CaO or CaOH to lakes may also neutralize acidity, increases amount of calcium ions, and precipitate Al from the solution.

E3 Greenhouse Effect

E.3.1 Describe the greenhouse effect.

Greenhouse gases allow the passage of incoming solar short-wave radiation but absorb the longer-wavelength radiation from the earth. Some of the absorbed re-radiation is re-radiated back to earth.

E.3.2 List the main greenhouse gases and their sources, and discuss their relative effects.

Gas	Source	Heat trapping compared to CO2	Contribution to global warming
CH4	Anaerobic decay Termites Rice fields Petroleum and natural gas production	30x	18%
H2O	Evaporation Combustion of hydrocarbons	0.1x	>1%
CO2	Combustion of fossil fuels, biomass Decay of plants and animals Oxidation of soil Forest fires Internal combustion engines	1x	50%
N2O	Bacterial action Fertilizers	150x	6%
O3	Secondary pollutant smog	2000x	12%
CFCs	Refrigerants Propellants	2500-10000x	14%

E.3.3 Discuss the influence of increasing amounts of greenhouse gases on the atmosphere

Increasing greenhouse gases could increase the earth’s natural greenhouse effect and lead to global warming. With an increase of temperature, the polar ice caps could melt, resulting in the expansion of the oceans. Also, fluctuations in temperature and precipitation level may result from a possible global warming, thus leading to changes in crop production.

E4 Ozone Depletion

E.4.1 Describe the formation and depletion of ozone in the stratosphere by natural processes.

The ozone layer occurs in the stratosphere between 12km and 50 km from the surface of the Earth. Stratospheric ozone is in dynamic equilibrium with oxygen and is continually being formed and decomposed.

Formation:

O₂ + UV → 2O◦

O₂ + O◦ → O₃

Depletion:

O₃ + UV → O₂ + O◦

O₃ + O◦ → 2O₂

E.4.2 List the ozone-depleting pollutants and their sources.

Chlorofluorocarbons were previously used as refrigerants, propellants, and cleaning solvents. Unfortunately, these molecules can destroy the ozone layer.

Initiation:

CF₂Cl₂ + UV → Cl◦ + CF₂Cl◦

Propagation:

Cl◦ + O₃ → ClO◦ + O₂

ClO◦ + O◦ → O₂ + Cl◦

Termination:

ClO◦ + ClO◦ → 2Cl◦ + O₂

In this way, the CFC is acting as a catalyst—destroying the existing O₃ and preventing the formation of O₃ without being consumed. NO_x can also react catalytically with O₃.

NO + O₃ → NO₂ + O₂

NO₂ + O◦ → NO + O₂

Net effect: O₃ + O◦ → 2O₂

NO₂ + UV → NO + O◦

O₃ + O◦ → 2O₂

E.4.3 Discuss the alternatives to CFCs in terms of properties.

Some options include HCFCs (Hydrochlorofluorocarbons), HFCs (Hydrofluorocarbons), and other non-chlorine containing hydrocarbons. Examples include: Chlorotrifluoromethane, 1,1,1,2-tetrafluoroethane, and 2-methylpropane. By replacing some of the chlorine atoms with fluorine, which requires more energy in breaking the bond, there is less radicalization taking place.

E5 Dissolved oxygen in water

E.5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen-demanding wastes in water.

BOD is the measure of dissolved oxygen (in parts per million) required to decompose all organic waste and ammonia in water biologically over a 5 day period at 20⁰C. The wastes demand oxygen to be decomposed.

E.5.2 Distinguish between aerobic and anaerobic decomposition of organic material in water.

If there’s sufficient oxygen present in the water, organic matter is broken down by microbes aerobically. This oxidizes the C, N, P, S, and H to produce CO₂, NO₃-, PO3−4, SO2−4, and H₂O.

If there’s an insufficient amount of oxygen present in the water, organic matter is decomposed by microbes that don’t require oxygen. They break down C, N, S, and P to form CH4, NH3, H2S, and PH3.

Element	Aerobic product	Anaerobic product
C	CO2	CH4
N	NO3-	NH3
P	PO3−4	PH3
S	SO2−4	H2S

E.5.3 Describe the process of eutrophication and its effects.

Nitrates from fertilizers and phosphates from detergents can accumulate in lakes and streams. These nutrients can increase the growth of plants and algae. This impacts the BOD because if plant growth increases too fast and the dissolved oxygen (DO) is not sufficient to decompose all organic material and waste by aerobic decomposition, anaerobic decomposition will occur. More species will die as a result of the anaerobic decay. The lake will become stagnant and devoid of life.

Eutrophication:

E.5.4 Describe the source and effects of thermal pollution in water.

If water is heated, the solubility of oxygen in the water decreases. At the same time, fish are cold-blooded, so as the temperature of the water increases, their metabolism increases. This forms a dilemma since the DO decreases as the BOD increases. This process helps to contribute to red tide.

E6 Water Treatment

E.6.1 List the primary pollutants found in waste water and identify their sources.

Waste water contains floating, suspended, and colloidal organic matter, dissolved ions with a wide range of microorganisms and bacteria as well as miscellaneous dirt, trash, grease and other chemicals.

Pesticides: DDt, herbicides, paraquat, fungicides

Dioxins: formed when organochlorine compounds are not incinerated at high enough temperatures. Very toxic and can accumulate in the liver

Polychlorobiphenyls (PCBs): used in transformers and capacitors. Persists in the environment and can accumulate in the liver, also carcinogenic

Nitrates: from fertilisers or acid rain. they are toxic at high levels, especially to babies because they have less stomach acid than adults, can cause blue baby syndrome

Heavy metals: Cadmium (Cd) (from rechargeable batteries), Mercury (Hg) (from batteries), Copper (Cu) (from household plumbing), Lead (Pb)

E.6.2 Outline primary, secondary, and tertiary stages of waste water treatment, and state the substance that is removed during each stage.

Primary Treatment: the removal of large solids

Primary treatment removes 60% of the solid material and a third of the BOD waste in the water. However, afterwards the water will still not be safe to drink.

Primary treatment involves running water through the below mechanisms in order:

1. Bar screens: these remove large objects and debris from the surface of the water and remove floating solids.

2. Settling tanks: these are used to settle out sand, dirt, and small objects from the water (as they sink to the bottom); these particles are then sent to landfills.

3. Sedimentation tanks: Alum (Ca(OH)₂ and Al₂(SO₄)₃) precipitates out and carry with them solid suspended particles (this process is called flocculation)

Secondary Treatment: the removal of organic materials using microbes

• Activated sludge process:
  • Air is bubbled into sewage which has been mixed with bacteria-laden sludge.
  • Aerobic bacteria oxidize organic material in the sewage.
  • Water-containing decomposed suspended particles are passed through the sedimentation tanks where the activated sludge is collected.
  • Some of the sludge is recycled, and some is sent to landfills.
  • This removes 90% of organic oxygen-demanding waste, 50% of nitrogen, and 30% of phosphates
• Effluent is then treated with chlorine or ozone to kill pathogenic bacteria before releasing the water to lakes or rivers
• Other methods include a carbon bed to remove the remaining organics, ion exchange which removes many soluble ions, reverse osmosis and electrodialysis.

Tertiary Treatment: the removal of remaining organics, nutrients and toxic heavy metal ions

• Heavy metal ions and phosphates are removed by precipitation, for example, nickel:

Ni2+(aq) + OH_(aq)- → Ni(OH)_{2 (s)}

• Aluminum sulfate and phosphates are removed by precipitation:

Al3+(aq) + PO3−4 (aq) → AlPO_{4 (s)}

Al3+(aq) + SO2−4 (aq) → Al₂(SO₄)_{3 (s)}

• Aluminum sulfate and calcium oxide can be used to remove phosphates:

3CaO_(aq) + 2PO3−4 (aq) + 3H₂O → Ca₃(PO₄)_{2 (s)} + 6OH_(aq)-

• Heavy metals will precipitate in the presence of hydroxide:

Cr3+(aq) + 3OH_(aq)- → Cr(OH)_{3 (s)}

• Nitrates are more difficult to remove by precipitation because they’re quite soluble, however, there are some ways to remove them:
  • Anaerobic denitrifying bacteria can reduce nitrates into nitrogen

2NO2−3 (aq) → N_{2 (g)} + 3O_{2 (g)}

• Another method is to pass them into algae ponds where algae uses nitrate as a nutrient

E.6.3 Evaluate the process to obtain fresh water from sea water using multi-stage distillation and reverse osmosis.

There are also a few other treatments, such as distillation. In distillation, sea water is pumped into a reservoir, at which point it is heated. The pure water which evaporates condenses on the cool water being pumped in, leaving a salty brine, which is then pumped out.

Another method used is the reverse osmosis system. In this type of system, there is a semi-permeable membrane which the water is pumped through, thereby being the opposite of a normal osmosis system (in which water would flow from low concentration to high concentration).

E7 Soil

E.7.1 Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation.

Soil is a complex mixture of inorganic and organic materials, including living organisms. Soil degradation lowers crop production and is caused by a variety of human factors including; acidification, salinization, contamination, desertification, erosion.

We are interested in the following factors:

• Salinization: the result of continual irrigation of soil; In poorly drained soil, after the water evaporates, salt is left behind, and plants die because they are unable to take water away from the salty soil.

• Nutrient Depletion: plants remove nutrients and minerals from soil as they grow. If not properly managed by crop rotation or fertilizing the soil, nutrients will become depleted.

• Soil Depletion: caused by improper use of pesticides and over-fertilizing; chemicals can disrupt the food web, reducing soil’s biodiversity, and ultimately ruining the soil.

E.7.2 Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical and biological functions.

SOM refers to the organic constituents in the soil. This includes plant and animal tissue, partial decomposition products and soil biomass. Chemicals found in SOM from decomposition of plants are high molecular mass organics such as Polysaccharides, proteins, sugars, and amino acids. The end product of decomposition is humus. Humus is the organic decomposition layer which plants live on. It has a mixture of simple and more complex organic chemicals from plants, animals, or microbial origin.

How SOM prevents soil degradation:

• helps soil to retain moisture, and dark color helps to retain heat and warm the soil during the spring.
• contains mineral nutrients that it exchanges with plants (at the roots).
• it improves the soil structure
• it reduces soil erosion.

Biological functions of SOM:

• humus provides a source of nutrients (such as N, P, and S) to the soil. Nitrogen provides proteins, Phosphoros provides enzymes, Sulfur provides amino acids.

Physical functions of SOM:

• SOM can retain several times its mass of water (like a sponge). Therefore more SOM means more water, making the soil more stable.

Chemically, SOM acts like clay with cation exchange capacity (CEC): it contains active sites that enable it to bind to nutrient cations. Humus also has the ability to maintain a constant pH by acting as a buffer.

E.7.3 List common organic soil pollutants and their sources

Here is a list of common soil pollutants and their major sources:

• Agrichemicals: from pesticides, herbicides and fungicides.
• Polyaromatic hydrocarbons: from incomplete combustion of coal, oil, gas, wood and garbage.
• Polychlorinated biphenyls (PCBs): from transformers and generators (they are used as a coolant).
• Organotin compounds: from bactericides and fungicides (used in paper, wood, textile and anti-fouling paint).
• Hydrocarbons and other VOCS: from transport, solvents and industrial processes.

E8 Waste

E.8.1 Outline and compare various methods for waste disposal.

Method of disposal | Advantages (+) | Disadvantages (-)

Landfill | (+)Cheap, leaves large amount of land reused after fill | (-)Leaches into soil and ground water; needs time to settle, maintenance for methane

Open Dumping | (+)Extremely cheap, convenient | (-)Unsightly; causes disease, odor, ground water pollution

Ocean Dumping | (+)Cheap, convenient | (-)Toxic in oceans, dangerous to fish, pollutes the sea

Incineration | (+)Provides source of energy, takes up little space, has stable residue | (-)Causes air pollution

Recycling | (+)Produces new raw materials, creates a sustainable environment | (-)Expensive, still causes some air pollution

E.8.2 Describe the recycling of metal, glass, plastic and paper products, and outline its benefits.

There are 3 main benefits to recycling that apply to metal, glass, plastic and paper. These are:

• Saving raw materials
• Saving energy (as energy is required to produce new materials)
• Saving space (in landfills)

In addition, glass and metals can be constantly recycled (over and over) without much degradation in the material.

The processes of recycling for each of the materials are as follows:

• Metals: sorted (by magnets or flotation) --> melted --> re-moulded --> re-used.
• Glass: sorted (colour) --> washed --> crushed --> re-moulded --> re-used.
• Plastics: sorted --> degraded to monomers (through pyrolysis, hydrogenation, gasification and thermal cracking) --> repolymerised --> re-used.
• Paper: mixed into water and chemicals (to form pulp) --> pulp is spun (removes staples/paper clips) --> washed to remove ink --> dried and bleached white --> re-used.

E.8.3 Describe the characteristics and sources of different types of radioactive waste.

Low-level waste includes any gloves, paper towels or protective clothing that has been used in areas where radioactive materials have been handled. The level of activity is low and the half lives are short. This waste generally comes from hospitals due to cancer treatment, and includes any items that have come in contact with the radioactive material.

High-level waste is generated by nuclear power plants and the military. It demonstrates a high level of activity and generally isotopes have long half-lives. High-level waste also comes from fuel rods or the reprocessing of spent fuel (power companies, military)

E.8.4 Compare the storage and disposal methods for different types of radioactive waste.

The nuclear decay process produces heat and energy. Low-level waste is stored in cooling ponds until the activity has fallen to safe levels (generally a few years). The water is then passed through ion exchange resins which remove isotopes responsible for activity. The water is then diluted and released into the sea.

High-level waste takes thousands of years to lose activity. Much of spent radioactive fuel is recovered for reuse. If not, the waste, generally a liquid mixture of radioactive waste, is converted into a solid glass component through a vitrification process: The waste is dried in a furnace and fed into a melting pot together with glass-making material (sand). The molten material is then poured into a stainless steel container where it cools and solidifies. These containers will remain radioactive for thousands of years. The containers are currently stored in concrete vaults, but it is hoped that they will later be transferred to salt chambers one day to be stored for thousands of years until the activity falls to safe levels.

E9 Ozone Depletion (HL)

E.9.1 Explain the dependence of O2 and O3 dissociation on the wavelength of light.

O₂ has a bond order of 2 (a double bond), therefore it is more difficult to break. This means that it will require more energy to do so, so a shorter wavelength (242 nm).

O₂ + UV (242 nm) → 2O◦

O₃, due to its resonant structure, has a bond order of 1.5, meaning it is less difficult to break than the double bond in O₂. This means that it will require less energy to do so, meaning a longer wavelength (330 nm).

O₃ + UV (330 nm) → O₂ + O◦

E.9.2 Describe the mechanism in the catalysis of O3 depletions by CFCs and NOx

Some of the pollutants in the atmosphere such as nitrogen oxide and chlorofluorocarbons (CFCs) function as catalysts of depletion of the ozone.

For example, there are chlorofluorocarbons (CFCs), which are used for refrigeration and propellants, in the stratosphere. UV light is capable of breaking weaker C-Cl bonds by homolysis to produce chlorine free radicals. CFCs usually remain in the troposphere but the CFCs molecules eventually diffuse into the upper atmosphere, stratosphere where they gain higher energy from UV light. When the CFCs is exposed to high energy UV radiation, photochemical decomposition occurs producing reactive chlorine gas atoms. Instead of C-F bond, which is more electronegative, thus stronger than C-Cl, the weaker C-Cl bond is broken down first.

The chlorine free radical produced from the photochemical decomposition of the CFCs functions as a catalyst in the decomposition of ozone.

Then, the newly formed ClO molecule enters the termination step, by reacting with oxygen free radical to form oxygen molecule and chlorine free radical.

Since the chlorine free radical is regenerated, one chlorine free radical can destroy numerous ozone molecules following the same step as shown above. The nitrogen oxides, which are used for supersonic aircraft engine, react similar to how CFCs react with the ozone.

From the initial step, the nitrogen monoxide reacts with ozone to produce nitrogen dioxide and an oxygen molecule. Then the nitrogen dioxide reacts with an oxygen free radical to regenerate nitrogen monoxide and an oxygen molecule.

As a result overall, an ozone molecule reacts with an oxygen free radical to form 2 oxygen molecules.

E.9.3 Outline the reasons for greater ozone depletion in the polar regions.

A hole in the ozone layer is found above Antarctica. Depletion is seasonal with the largest holes occurring during the early spring (October/November). This decrease is due to chemicals produced by man. In the winter (Jun-Sept), NO2 and CH4 are trapped with ClO and Cl2 on the surface of ice. A catalytic reaction on the surface of the ice converts the ClO into Cl2 and HClO forming a “chlorine reservoir.” This temporarily lessens the amount of chlorine released into the atmosphere. When the ice melts in the spring, the chlorine contained in the ice crystals surges into the atmosphere, thus causing the hole in the ozone layer to temporarily expand.

E10 Smog (HL)

E.10.1 State the source of primary pollutants and the conditions necessary for the formation of photochemical smog.

Smog is a poisonous combination of smoke, fog, air and other chemicals. Photochemical smog occurs in cities where exhaust from internal combustion engines concentrates in the atmosphere. Oxides of nitrogen and hydrocarbon give the air a characteristic yellow/brown color. In the sunlight, these chemicals are converted into secondary pollutants. Smog tends to form in large cities and is favored by a lack of wind. It also occurs more often in bowl-shaped cities because the higher ground surrounding these cities prevents the movement of air. Smogs typically occur where there is a temperature inversion. Normally, the temperature decreases with altitude. Warm air typically rises, takes the pollutants with it, and is then replaced by cleaner cooler air. However, typically in areas which are notorious for smog, the atmospheric conditions cause a layer of still warm air to layer a blanket of cooler air. The trapped pollutants cannot rise, and if the condition persists, the amount of pollutants in the warm air near the ground can rise to dangerous levels.

E.10.2 Outline the formation of secondary pollutants in photochemical smog.

Photochemical smog is a chemical soup containing hundreds of different chemicals formed in the atmosphere as a result of free radical reactions caused by UV light.

In the early morning hours, a buildup of hydrocarbons (VOCs) and NOx from car exhaust occurs. As the sun comes out, the NO2 absorbs sunlight and forms free radicals.

N₂ + O₂ → NO₂ (Primary Pollutant)

NO₂ + UV → NO + O◦

These radicals react with O2 to form ozone and water or to form hydroxyl radicals.

O◦ + O₂ → O₃ (Secondary Pollutant)

O◦ + H₂O → 2OH◦

Secondary photochemical oxidants react with a variety of molecules and hydrocarbons to form peroxides, aldehydes and ketones.

OH◦ + NO₂ → HNO₃

OH◦ + RH → R◦ + H₂O

R◦ + O₂ → ROO◦ (Peroxide Radical)

Chain termination can occur when peroxide radicals react with NO2 to form Peroxyacylnitrates (PANs) which are irritants to the eyes and skin.

ROO◦ + NO₂ → ROONO₂ (PAN)

E11 Acid Deposition (HL)

E.11.1 Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur.

NOx and SOx are converted into acid via free radical reactions:

H₂O + O₃ → 2HO◦ + O₂

The hydroxyl radicals then react directly with SO_x and NO_x in the presence of water to form acids.

HO◦ + NO₂ → HNO₃

HO◦ + NO → HNO₂

or,

HO◦ + SO₂ → HOSO₂◦

HOSO₂◦ + O₂ → HO₂◦ + SO₃

SO₃ + H₂O → H₂SO₄

E.11.2 Explain the role of ammonia in acid deposition.

Ammonia gas can to a small extent neutralize the effect of acid rain in the atmosphere by the formation of (NH₄)₂SO₄ and NH₄NO₃:

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

NH₃ + HNO₃ → NH₄NO₃

NH+4 is a strong conjugate acid, so when ammonium salts sink into the ground, the NH+4 enters the soil where acidification and nitration can occur.

E12 Water and Soil (HL)

E.12.1 Solve problems relating to the removal of heavy-metal ions, the phosphates and nitrates from water by chemical precipitation.

Some of the salt in the soil dissolves, however most remains in solid form, thus creating equilibrium. Remember these 5 steps:

1. Write a balanced equation
2. Find the equilibrium equation (K_sp = [ion conc.][ion conc.])
3. Identify the direction of the change in equilibrium
4. ICE it! Initial, Change, Equilibrium
5. Plug the E line into the equilibrium equation and solve for x, then follow through to find what the question is asking for (i.e. plug back into 2x if concentration, or find Ksp if appropriate.)

E.12.2 State what is meant by the term cation-exchange capacity (CEC) and outline its importance.

Both SOM and clay have negatively charged particles which will bond to cations such as Ca+2, Mg+2, Na+, K+, Al+3. The amount of positively charged cations that soil can hold is called the cation-exchange capacity(CEC). A larger CEC indicates a larger capacity to hold cations. These cations are exchanged with cations such as hydrogen on the root hairs of a plant to provide it with nutrients.

E.12.3 Discuss the effects of soil pH on cation-exchange capacity and availability of nutrients.

If soil is more acidic, there is a higher percentage of acidic cations found in the soil. Soil pH is important because acid cations such as aluminum ions are harmful to the plants. Although soil has some buffering capacity, it is sometimes necessary to add lime to the soil to raise the pH and increase the concentration of basic cations held by the clay and the SOM.

When soil is analyzed, the total concentration of basic cations is compared to the total concentration of acidic cations. Cations such as Al+3 are harmful to plants. Soil pH is important because above pH=5, Al+3 will precipitate out of solution. If there is acid in the rain which lowers the pH of the soil, the Al+3 will no longer precipitate out of the solution. This cation is toxic to plants, so in essence, acid rain would be killing the plants.

E.12.4 Describe the chemical functions of soil organic matter (SOM).

In addition to the nutrient cations required by the plants and organic matter, SOM can also bind to organic and inorganic compounds in the soil which helps to reduce the negative environmental effects of contaminants such as pesticides, heavy metal ions and other pollutants. SOM contributes to CEC, enhances the ability of soil to buffer changes in pH, and forms stable complexes with cations. SOM also reduces the effect of pesticide, heavy metals, and other pollutants.