A-level Chemistry/WJEC/Module 2/Impact

Chemists do not work in isolation; Products and ideas developed by chemists are used in the real world and can have lasting effects. "Green chemistry" is a concept that makes chemists think about the efficiency, sustainability and the potential affecting the environment.

In many areas of chemistry we need to look at how limited resources are being used. This may be fossil fuels, water or unique elements such as rare earth metals. The problem is not just that these resources will become more difficult to find and eventually run out, but also that some countries have control over access to some resources, and might want to limit how other countries can use them. In 1973, for example, a number of oil-producing countries stopped supplying oil to countries which they saw as supporting Israeli policy in the Middle East. The price of oil almost quadrupled and many countries' economies went into recession. More recently, in a different field, Chinese chemical engineers have made a concerted effort to develop alternative industrial processes which reduce reliance on catalysts and other resources which have to be imported to China.

A tragic example of the problems caused by developments in chemistry is the case of Thomas Midgley (1889-1944), who developed both tetraethyl lead and freon. Tetraethyl lead is an effective additive for petrol, helping to reduce harmful "knocking" (pre-ignition) of the fuel. Freon, and similar CFC's, are effective refrigerants, replacing toxic, flammable and explosive compounds that had been in use previously. Midgely mistakenly discounted the toxic effects of lead in the environment, and was unaware that CFC's would later be known for depleting the ozone layer. Both chemicals are now banned for any use that is not highly regulated and small-scale. Environmental historian J. R. McNeill opined that Midgley "had more adverse impact on the atmosphere than any other single organism in Earth's history".

Fritz Haber (1868-1934) has an ambiguous legacy. He developed the Haber process which has fed billions of people through the use of ammonia to make fertilisers. In the First World War he was instrumental in developing and deploying chemical weapons for Germany.

Environmental chemistry is a branch of chemistry. It is the study of chemical processes occurring in the environment which are affected by humankind’s activities.

Environment

The word environment means "surrounding". Environment means the surroundings of living beings from all sides and that affect their lives.  Environment can be defined as one's own surroundings including all of the living (biotic) and non-living (abiotic) factors that act on organism, population, or ecological community and influence its survival and development

Segments of the Environment

1. Lithosphere: the Earth's crust consisting of the soil and rocks. The soil is made up of organic, inorganic matter and water. They are responsible for the fertility of the soil and hence its productivity.
2. Hydrosphere: is the water in all states (solid, liquid and gas). This comprises of all the surface and ground water such as seas, oceans, lakes, streams, glaciers, polar ice caps, and the water locked in minerals below the Earth's crust.
3. Biosphere: parts of Earth where life exists. Life does not exist outside this zone.
4. Atmosphere: layer of gases surrounding the planet. Oxygen is used by most organisms for respiration; carbon dioxide is used by plants for photosynthesis.

Structure of the Atmosphere

The atmosphere is broadly divided into four major zones

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere

Lapse rate

• Change of temperature with height is called lapse rate.
• Decrease of temperature with height is called positive lapse rate (troposphere)
• Transition from positive to negative lapse rate at the tropopause marks what is called the temperature inversion.

Troposphere

1. It is the region nearest to the ground
2. Contains 70 % of atmospheric mass
3. Temperature decreases with height
4. 15 °C at sea level to -56.5 °C at 11,000 m
5. Contains 99% of the water vapour in the atmosphere
6. Tropopause is the layer between troposphere and stratosphere

Stratosphere

1. The region above troposphere
2. Warmer layers higher and cooler layers closer to the Earth
3. The increase in temperature is due to ozone absorbing UV rays from the sun
4. Ozone is present in the stratosphere

Mesosphere

1. Region above stratosphere
2. Temperature decreases with height
3. Menopause is the layer between Mesosphere and Thermosphere, at which is the coldest temperature (about -100 C)

Thermosphere

1. It is the region above the Mesosphere
2. Temperature increases with increasing height
3. Atmospheric gases (oxygen and nitrogen) in this region absorb solar radiation and undergo ionisation (lose electrons), forming O₂⁺, O⁺, N⁺
4. During night times, when UV radiation is not present, these species again combine with electrons to make neutral species.
5. The region from 50-100 km is called ionosphere due to presence of positive ions and electrons

Biogeochemical Cycles

Water cycle

• Evaporation: The Sun heats up the water from oceans, lakes and rivers and water changes into water vapor
• Condensation: As the water vapor rises up into the air, it starts cooling down and forms tiny water droplets. These water droplets come together to form clouds. This process is called condensation.
• Precipitation: When the clouds start getting heavy and cannot hold the water droplets anymore, they fall back to the earth in the form of rain, hail or snow. This process is called precipitation.

Some of the water that falls on the earth seeps into the ground. This water is available to us in the form of groundwater. The remaining water falls back into oceans, lakes, rivers and seas. This process is called collection. Then, the sun starts heating up this water once again. This circulation of water is called water cycle.

• Transpiration: Plants also lose water in the form of water vapor from their leaves into the air by the process of transpiration

Nitrogen cycle

The nitrogen cycle describes how nitrogen moves between plants, animals, bacteria, the atmosphere (the air), and soil in the ground. In order for nitrogen to be used by different life forms, it must change into different states. Nitrogen in the atmosphere, or air, is N₂. Other important states of nitrogen include nitrate(V) ("nitrate", NO₃^–), nitrate(III) ("nitrite", NO₂^–), and ammonium (NH₄⁺).

• Fixation: is a process by which nitrogen (N₂) in the Earth's atmosphere is converted into ammonium (NH₄⁺) by bacteria 
• Nitrification: is a process by which ammonium gets changed into nitrate(V) (NO₃^–) by bacteria. Nitrate(V) is a form of nitrogen that we can use (the plants absorb them and moves through the community by the food chain). The nitrogen is in the form of proteins, amino acids etc in the plants which will than be reassembled in other animals when the plants are eaten
• Ammonification: When a plant or animal dies, decomposers like fungi and bacteria turn the nitrogen back into ammonium so it can re-enter the nitrogen cycle.
• Denitrification: Extra nitrogen in the soil gets put back out into the air. There are specialised bacteria that perform this task as well.