Green Chemistry Greenhouse effect and global warming Firstly it has to be noted that greenhouse gases aren’t all that bad for the planet. Even though greenhouse gases have a negative connotation, without these gases our planet would be covered in ice and have an average temperature of 35°C which is lower than at present. The definition of the greenhouse effect is: The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet’s surface. Greenhouse gases occur naturally in the atmosphere. Water vapour is by far the most abundant greenhouse gas, and most of it is from the evaporation of lakes and oceans. Carbon dioxide comes in as the next most abundant and this is also produced by many natural processes such as volcanic eruptions, respiration in animals and burning or decay of organic matter.
Methane is the 3rd most abundant greenhouse gas on the planet. Although it is present in smaller quantities than carbon dioxide, it makes a greater contribution to the greenhouse effect. Methane can be natural such as when it is released from certain animals as a by-product of digestion such as cows, but however most of it is made by humans such as it being emitted during the production of coal, natural gas and oil and also is a product of rotting organic waste in landfill sites. There are other greenhouse gases but these are mostly left out because their concentration in the atmosphere are much lower than those of water, carbon dioxide and methane.
Carbon dioxide is a linear molecule (O=C=O). This means the carbon dioxide molecule can absorb infrared radiation, causing the molecule to vibrate. Eventually the molecule emits some of its energy in the form of radiation and this can then be absorbed by another greenhouse gas molecule or by the earths surface. Water vapour and methane absorb energy in a similar process. In H2O the O-H bonds absorb infrared radiation and in CH4 the C-H bond absorbs infrared radiation
As it can be seen by the graph, CO2 emissions are increasing over time. This is mainly because technology is advancing and more and more countries are growing economically. Electricity is found almost everywhere in the UK and most electricity in the UK is made from fossil fuels. Also more and more people are able to afford cars and this creates more pollution. The number of companies has also increased, not only in the UK but also worldwide. This means there are more production of products which involve burning fossil fuels and also huge lorries and airplanes transporting goods around the world. The CO2 emissions produces in the 1900 looks like nothing when compared to a century later and this is because in the 1900 not everyone owned cars and only a fraction of the population had electricity. Carbon footprint and offsetting The carbon footprint of a fuel is the total mass of carbon dioxide produced by a fuel from its production to its subsequent combustion. So for a fossil fuel, the carbon footprint is the amount of carbon dioxide
produced during combustion, during the process of the extraction of the oil, coal or gas, transportation, refining and finally it’s distribution. There is no fuel which has a zero-carbon footprint and this includes hydrogen because its made from methane. The carbon footprint of an organisation or activity is the net amount of carbon dioxide emitted through the use of fossil or biofuels. For a business or factory, the carbon footprint is the net amount released as part of its daily operations. For individuals and households, it is the net amount released as part of their daily lives. For a product bought at a supermarket, the carbon footprint is the net carbon dioxide emission in all processes involved in its production and transportation to and from the market. The units of carbon footprint are tonnes of carbon dioxide per event A carbon offset is the counteracting of carbon dioxide emissions with an equivalent reduction of carbon dioxide in the atmosphere. There is some good to this such as it provides a way to remediate the effects of cases where an inevitable amount of emissions were produced, however can also be a problem. This is because people feel ok to pollute if they simply compensate by buying offsets. Some say carbon offsets are the equivalent of the medieval practice of paying for your sins.
CFC’s and the Ozone Layer Ozone, O3 is a molecule made of 3 oxygen atoms. It can either be good or bad and it depends on where it’s found. E.g Ozone near to the Earth’s surface in the troposphere is an air pollutant with harmful effects on the respiratory system of animals. However ozone in the upper atmosphere in the stratosphere protects living organisms by preventing harmful UV light reaching the earths surface. The Ozone layer is found in the resin of the atmosphere called the stratosphere, about 10-50 km above the Earth’s surface. UV radiation
from the sun has wavelengths in the range 270-400 nm. The ozone layer filters out the shorter wavelengths and these shorter wavelengths can be harmful to life. The ozone also converts this UV radiation into heat. The use of CFC’s were reduced in 1987 after major countries signed a treaty called the Montreal protocol. The graph shows the significant decrease in global CFC production. All United Nations countries, European Union members and many other countries signed the treaty. 197 countries in total. How the ozone layer is formed O2 molecules firstly absorb UV radiation with a wavelength less than 240 nm. This high energy radiation breaks the O2 into 2 atoms O2 + (Radiation < 240nm) —> 2O Then the O atoms react with other O2 molecules to form ozone molecule, O3, and this process generates heat. O2 + O —> O3 + Heat The heat produced is absorbed by air molecules in the stratosphere, raising it’s temperature. The ozone molecules formed would then absorb UV with wavelengths between 240 and 310 nm. When this happens, the O3 molecules are converted back to O2 and O atoms ( The reverse of the previous reaction) O3 + (Radiation < 310nm) —> O2 + O The O produced in this reaction immediately reacts with other O2 to make ozone again O + O2 <—> O3 + heat
and this cycle just continues. This cycle keeps the ozone layer in a stable balance and is the process which protects living organisms from the harmful effects of high-energy UV radiation. When an oxygen atom and an ozone molecule combine, they form 2 O2 molecules O3 + O —> 2O2 This obviously means that the ozone is being removed which exposes use to harmful UV radiation. Luckily for us, the removal rate is really slow due to there being a low concentration of O atoms. However, human activity can affect this balance and an example of this is CFC’s. CFC’s (Chlorofluorocarbons) were created in 1928 as a non-toxic, nonflammable refrigerant. It was also used as solvents and in air conditioners and had a low reactivity and volatility. It is now known that chlorine radicals in the stratosphere is mostly generated by human activity and the increase in these has upset the natural ozone-oxygen balance and this leads to problems in maintaining the protective ozone layer. Chlorine radicals in the stratosphere mainly come from CFC’s. The stability of CFC molecules means thatCFCs can only be broken down by extremely energetic UV radiation. When UV radiation combines with a CFC molecule in the stratosphere, a C-Cl bond breaks, producing a chlorine radical e.g:
CFCl3 —> Cl• + •CFCl2
Initiation
Radicals are extremely reactive and one a chlorine radical has been generated, it can react with an O3 molecule, breaking it apart and destroying the ozone. The breakdown of the ozone takes place in 2 steps Cl• + O3 —> ClO• + O2
Propagation step 1
ClO• + O —> 2O2
Propagation step 2
These 2 steps repeat in a cycle many thousands of times. A single CFC molecule can destroy over 100,000 ozone molecules. Waste Management Waste is a huge environmental problem and one solution is to use less packaging so there is less to throw away. However the other solution is to recycle as much as possible. Plastics and PVC are not biodegradable because they’re made from fossil fuels and no microorganisms exist to break them down. Plastic is insoluble and doesn’t degrade easily, they also take up a lot of space in landfill sites. they can be incinerated (burn) but this can produce harmful materials, PVC produces hydrogen chloride, so this must be done carefully. The best solution is to recycle however this is an expensive option as the plastic has to be sorted first. Another option is to make plastics that can be broken down easily in the environment. This is done by adding starch to the plastics when made. This means the plastic can be put into a landfill site and the microorganisms will be able to break down the starch, which turns the plastic into a fine powder and this also solves the problem of taking up too much space. These plastics are biodispersible and organisms cause these plastics to crumble and become dispersed in the environment. Recycling can be defined as recovering resources by collecting, separating and processing scrap materials and using them as raw materials for manufacturing new products. We need to do it because the world’s resources are running out and they’re non-renewable. Recycling reduces the expense of disposal and the expense of creating new products from raw materials. This also reduces waste in landfill sites and decreases the amount of greenhouse gases produced. Metals can be recycled easily. Iron can be separated from other rubbish by using a magnet and then can be mixed with iron ore and put in a blast furnace. Aluminium cans can be melted down and recast which saves a huge amount of energy that the process of electrolysis of alumina. Renewable Resources
Renewable resources are helpful because they can be replenished by natural processes. Their rate of replenishment is equal or greater than the rate of consumption. They don’t contribute much to global warming and is far more environmentally friendly. There are many types of renewable energy such as plant-based substances such as wood, solar energy, tidal energy, biomass, HEP and wind power. Renewable resources such as plant-based substances or solar energy must be used because the world’s finite resources such as fossil fuels will eventually run out. There are many effective ways to make what we need via renewable resources such as solar energy made with the help of solar panels. These can be used to heat water in homes however they do have some problems such as not working during night. It has been estimated that it would cost over £20000 to buy enough photovoltaic cells to provide all the energy for a single house. Wind power is much more suitable. However a problem is that it needs to be backed up by fossil fuel power stations when there isn’t enough wind and these need to be on constantly as they take too long to fire up and generate electricity. Another problem is that wind farms are normally a long way from population centres and a significant amount of energy is lost as heat during transportation. Only 1% of the world use wind power as energy and 20% of it is in Denmark The chemical company P&G announced a way to make plastic bags from renewable resources such as agricultural waste. It uses bacteria to ferment vegetable oils and sugars in the waste and converts this into plastics. Developing new catalysts A catalyst lowers the activation energy of a reaction which allows it to take place at lower temperatures. Catalysis is one of the 12 principles of green chemistry by Paul Anastas. Developing a new catalyst may take time and money however the end product produces beneficial rewards. When a catalyst is used in the production of something, the activation energy is lowered and therefore
less heat is required to make the reaction happen. This means companies can reduce the amount of water and energy required in the production of their product which means non-renewable sources of energy is conserved. Also catalysts are reusable because they aren’t used up in reactions, this means that the company will need to buy fewer chemicals and this would reduce the carbon footprint of the production of their product as there are less chemicals being transported. Catalysts are a much more efficient way for production because it uses fewer chemicals and creates less waste (therefore increasing atom economy) and mostly works under normal pressure and temperature. Greener Chemical Industry The chemical industry is now starting to understand the importance of pollution. Air and rivers can’t be polluted without acquiring a sanction and another thing helping the environment is how companies care about their public image. One way in which the effect on the environment is being reduced is by the use of less hazardous chemicals. The chemical industry produces many herbicides, insecticides etc and these are essential for agriculture however, they pose threats to the environment. DDT was the main insecticide until we realised that it had harmful effects on some higher animal species within the food chain. To solve this as a problem, new insecticides are being made and they contain natural chemicals produced by plants as a defence mechanism. Therefore these compounds do not last as long in the environment and create less of a harm. The atom economy is the percentage of the atoms in the reactants that is present in the desired product. Increasing the atom economy means that more of the desired product is made and this saves money and decreases the carbon footprint.
High temperatures are essential in almost all chemical processes and heat costs money. Therefore making efficient use of this energy saves money and decreases the amount of energy lost. One way to save heat is good thermal insulation and heat recovery.
