2.13f: discuss and explain, including the mechanisms for the reactions, the science community’s reasons for recommending that CFCs are no longer used due to their damaging effect on the ozone layer

The ozone (O3) layer is found in the stratosphere

• formed when oxygen (O2) molecules absorb UV light and undergo homolytic fission to creat two free radicals
• O2 --UV→ O- + O-
• a free radical then reacts with an oxygen (O2) molecule to form ozone
• O2 + O- → O3
• ozone can also absorb UV light and split back into oxygen and a free radical
• O3 → O2 + O-
• in the absence of pollutants, ozone can be formed at the same rate as it is being destroyed

Reactions with CFCs:

• CFCs are inert so make their way into the stratosphere unchanged
• UV light can break the CFCs down into other chlorine-based substances
• the conditions are just right over the Antarctic
• Cl2 --UV→ Cl- + Cl-
• the chlorine radicals destroy ozone molecules and oxygen radicals
• propagation:
• O3 + Cl- → ClO- + O2
• ClO- + O2 → Cl- + O2
• O3 + O- → 2O2

Reactions with nitrous oxides:

• O3 → O2 + O-
• O3 + NO → NO2 + O2
• NO2 + O- → NO + O2
• 2O3 → 3O2

In the stratosphere, CFCs are broke down by absorption of UV radiation to form chlorine free radicals.

The following two reactions occur:

• Cl- + O3 → ClO- + O2
• ClO- + O → Cl- + O2

Combine these two equations to give the overall equation for the reaction of ozone in the stratosphere. State the role played by the chlorine free radical in the overall reaction. Hence explain why many scientists consider the effect of CFCs on ozone to be harmful.

• overall reaction = O + O3 → 2O2
• the chlorine free radical acts as a catalyst by reacting with oxygen free radicals, preventing O3 from forming
• CFCs are harmful because they decimate the number of ozone molecules in the stratosphere, so that less of the UV radiation from the sun is absorbed, allowing more to reach the Earth’s surface, which could result in more cases of skin cancer

2.13e: apply the concept of carbon neutrality to different fuels, such as petrol, bio-ethanol and hydrogen

Biofuels are fuels that are produced from living things, making them renewable and carbon neutral

• biodiesel:
• produced from plant oils (eg. palm oil)
• the oil is hydrolysed with sodium hydroxide to break the oils down into simple fatty acids
• the fatty acids are re-esterified with methanol to produce biodiesel
• bioethanol:
• produced from plant crops such as corn and sugarcane
• the sugar in the crops can be turned into ethanol by the action of yeast in fermentation
• glucose → ethanol + carbon dioxide

Biofuels are carbon neutral because the plants used take in carbon dioxide while they are growing (for photosynthesis), then release the same volume of carbon dioxide when we are using them as fuels

Disadvantages:

• biofuels may actually not be carbon neutral in practice
• large amounts of energy are required to operate machinery, process plant material, and process biofuels
• energy is required to produce chemical fertilisers and pesticides
• destruction of rainforests to clear land for palm trees or sugar cane
• reduces the size of carbon sinks and animal habitats
• less farmland is available for food crops
• could cause a rise in food prices and malnutrition

Biodiesel from soybeans could reduce emissions by 41% compared to normal diesel as the biodiesel is very pure and does not require distillation, saving a lot of energy

• soybeans are very hardy crops, so do not require many chemical fertilisers or pesticides

Bioethanol can be produced from the action of microorganisms on cellulose, found in straw and woody plants (not part of human food supply)

Eg. Brazil manufactures bioethanol from sugarcane and mixes it with ordinary petrol, allowing cars to use it without engine modification

• the sugar cane waste is burned to produce heat and energy for the whole refinery

2.13d: demonstrate understanding of the terms ‘carbon neutrality’ and ‘carbon footprint’

Carbon footprint: the amount of greenhouse gases/carbon dioxide emitted by the activities of an individual or group during a given period

Carbon neutrality: when a fuel or activity does not contribute to nor reduce the amount of carbon in the atmosphere

2.13c: discuss the difference between anthropogenic and natural climate change over hundreds of thousands of years

Anthropogenic effects: changes brought about by human activities, such as burning fossil fuels, deforestation, and intensive agriculture

• human activity has overcome the influence of natural climate change over the last few hundred years
• the most potent greenhouse gases (CO2,NOx,CH4) are currently at their highest levels for over 650,000 years due to human activity

Natural climate change: changes brought about by orbital cycles (tilt, shape of orbit, precession) and greenhouse gases

• natural climate cycles have been occurring for millions of years

Evidence for anthropogenic climate change:

• long-term data shows that the climate has been naturally fluctuating between glacial and interglacial periods
• if climate change is natural, this pattern should continue
• short-term data shows that global temperatures have been rising since the beginning of the industrial period
• human activity is having more of an effect on climate change

2.13b: discuss the relative effects of different greenhouse gases as absorbers of IR and hence on global warming

The atmosphere is a layer of gases surrounding the earth

• the troposphere
• 0-15km above sea level
• densest layer, contains the most gas molecules per unit volume
• the stratosphere
• 15-50km above sea level
• enriched in ozone molecules, which absorb harmful UV radiation from the sun

Greenhouse gases:

• trap and re-radiate solar radiation to the Earth, creating the greenhouse effect, which enables life on Earth
• too many greenhouse gases in the atmosphere, in too-high abundance, causes the enhanced greenhouse effect, which is caused by human activities
• what is the greenhouse effect?
• UV and visible radiation from the Sun warms up the Earth
• the Earth loses energy as infrared radiation
• greenhouse gases absorb infrared radiation and prevent it from escaping, so less heat energy is lost to space, making the Earth steadily warmer
• what makes a greenhouse gas?
• the molecule must have a polar bond, which can absorb infrared radiation
• eg. N2 and O2 have identical electronegativities, so their bonds are not polar - they are not greenhouse gases
• eg. CH4 and CO2 do have polar bonds, so are greenhouse gases
• water is the greatest contributor to the greenhouse effect
• CFCs may only be present in small concentrations, but they are very stable and take a long time to decompose
• cattle farming increases the concentration of methane in the atmosphere
• methane absorbs more IR per molecule than CO2
• air pollutants:
• CO2 produced by the combustion of fossil fuels
• SO2 produced when sulphur (naturally present in fossil fuels) combusts
• reacts with water in the atmosphere to form acid rain
• NOx formed when nitrogen and oxygen react together in the high temperatures of car engines
• can react with water in the atmosphere to form acid rain
• produces a photochemical smog, which reduces visibility and can cause respiratory illnesses
• CFCs, used to be widely used in refrigerants and aerosols
• react with and deplete the ozone layer, allowing harmful UV light to reach the Earth’s surface

2.13a: demonstrate an understanding that the processes in the chemical industry are being reinvented to make them more sustainable (‘greener’) by: (i) changing to renewable resources (ii) finding alternatives to very hazardous chemicals (iii) discovering catalysts for reactions with higher atom economies, eg the development of methods used to produce ethanoic acid based on catalysts of cobalt, rhodium and iridium (iv) making more efficient use of energy, eg the use of microwave energy to heat reactions in the pharmaceutical industry (v) reducing waste and preventing pollution of the environment

Changing to renewable resources:

• some raw materials can be used as an alternative to crude oil
• eg. ethanol can be made from sugar cane by fermentation, and dehydration of the ethanol can make ethene, from which poly(ethene) can be polymerised
• eg. water bottles are made from poly(lactic acid) which is made in the breaking down of carbohydrates by bacteria. The lactic acid then undergoes condensation polymerisation to form PLA, which is widely recycled and is biodegradable
• renewable energy sources can be used instead of fossil fuels to generate the energy needed for chemical reactions in industry
• eg. wind power, solar power, geothermal power, etc.

Finding alternatives to very hazardous chemicals:

• using alternative reagents that are less toxic and designing synthetic routes that do not require toxic intermediates and solvents
• eg. Zoloft was initially synthesised in a four-step process using several harmful VOCs (volatile organic compounds). A new route was designed with only two steps and less toxic ethanol as a solvent
• eg. CFCs were once widely used in refrigerators and aerosols, but were found to damage the ozone layer, leading to an increase in rates of skin cancer. Now, less harmful HFCs are used, but they are still greenhouse gases

Discovering catalysts for reactions with higher atom economies:

• new catalysts may allow existing reactions to occur at lower temperatures and yields, to raise atom economies and yields
• catalysts tend to provide cleaner energy profiles and so higher yields with less waste
• catalysts are not used up, so can be recycled and used over and over again

Making more efficient use of energy:

• use reactions that occur at low temperatures and pressures
• utilise any energy released by the reaction
• catalysts are key in saving energy as they allow reactions to occur at lower temperatures
• small-scale chemical reactions can now be heated using microwave energy, giving faster reaction times and cleaner reaction profiles

Reducing waste and preventing pollution of the environment:

• reactions with high atom economies, high yields and cleaner reaction profiles
• waste products should be recycled if possible

2.12d: demonstrate an understanding that H2O, CO2, CH4 and NO molecules absorb IR radiation and are greenhouse gases, whilst O2 and N2 are not

H2O, CO2, CH4, and NO molecules are able to absorb IR as they are able to change their polarity as they vibrate. This makes them greenhouse gases, as they are able to absorb and reradiate infrared radiation from the Sun back towards the Earth. O2 and N2 are not greenhouse gases as they are not able to absorb infrared radiation.