Chemical Sustainability
- Created by: Emily Cartwright
- Created on: 27-05-14 19:00
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- Chemical Sustainability
- Percentage Yield
- Often in a chemical reaction, the amount of product recovered is less than might have been expected from the reactants used. This may be because;
- The reaction may not have gone to completion because it is slow and/or reversible
- There may be competing side reaction which convert a proportion of the reactants into different products
- Some product may have been lost during purification
- This loss of material may be accounted for quantitatively by calculating the percentage yield of the reaction;
- % yield = actual moles of produc/theoretical moles of product x 100
- Addition reactions have only one product, so atom economy is 100%
- Other reactions can have 100% atom economy only if all of the products are useful
- Often in a chemical reaction, the amount of product recovered is less than might have been expected from the reactants used. This may be because;
- Atom Economy
- Even if losses in a reaction are minimised so that the % yield is not a problem, the reaction may be intrinsically wasteful if there is more than one product and not all of them are useful
- atom economy = Mr of desired products/sum of Mr of all products x 100
- Atom economy measures the proportion of the mass of material used up in the reaction that could, ideally, be converted to useful product
- This takes no account of % yield, so a reaction could have a high atom economy but be of little use in practice if there is a low yield
- A reaction with a high atom economy is preferable because it can make efficient use of resources and produce a low percentage of waste products
- Even if losses in a reaction are minimised so that the % yield is not a problem, the reaction may be intrinsically wasteful if there is more than one product and not all of them are useful
- Efficiency of resource use in a chemical process
- We need to take account of both atom economy and percentage yield as well as other issues in judging whether a chemical process makes efficient use of resources
- Sustainability
- According to the UN, sustainable development is 'development that meets the needs of the present without compromising the ability of future generations to meet their own needs'
- Principles of chemical sustainability;
- Using processes that reduce the use of hazardous chemicals and which involve using fewer chemicals
- Designing processes with a high atom economy and a high % yield, in order to minimise wastage of finite raw materials and to minimise production of waste products
- Using renewable resources such as plant-based substances
- Seeking alternative, renewable energy sources, such as solar energy to reduce our rate of consumption of fossil fuels which will eventually be exhausted
- Ensuring that waste products are non-toxic and can either be recycled or biodegraded to harmless substances in the environment
- International Cooperation
- Problems of pollution and exhaustion of resources cannot be solved within one country
- Therefore it is important to establish international cooperation to promote the reduction of pollution levels
- Examples of international agreements that aim to control pollution include;
- Kyoto Protocol; Agreement to reduce CO2 emissions to try and limit climate change
- Montreal Protocol: Agreement to ban the use of chloroflurocarbons to reduce damage caused to the ozone layer
- Problems of pollution and exhaustion of resources cannot be solved within one country
- Percentage Yield
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