Chemical Sustainability

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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
    • 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
    • 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

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