Chapter 19 Green Chemistry

What is Green Chemistry The design of chemical processes and products that reduce or eliminate the use and generation of hazardous substances in their manufacture and application Green chemistry aims to avoid problems before they happen and to consider the impacts of the manufacture use, final disposal and break-down of chemical products

12 Principles of Green Chemistry Preventing wastes It is better to prevent waste than to clean it up after it forms

12 Principles of Green Chemistry Atom economy Waste at little material as possible

12 Principles of Green Chemistry 3 Less hazardous synthesis Chemical manufacturing processes should be designed to use reactants and generate products that have little or no toxicity

12 Principles of Green Chemistry 4 Designing safer chemicals Chemical products that do their jobs well but are non-toxic will create fewer medical and environmental problems

12 Principles of Green Chemistry 5 Using safer solvents The use of additional chemicals such as solvents should be avoided, and if they are used they should be non-toxic substances

12 Principles of Green Chemistry 6 Energy efficiency Reducing energy requirements by conducting processes at room temperature and pressure reduces costs and lowers the impact on the environment.

12 Principles of Green Chemistry 7 Using renewable feedstock Starting materials for a chemical product should, where possible, be renewable resources Eg nylon from glucose instead of benzene; biodiesel from crops

12 Principles of Green Chemistry 8 Reducing use of derivatives Derivatives are chemical used for a temporary effect in the chemical process Derivatives cause a temporary change in properties and are then removed as waste

12 Principles of Green Chemistry 9 Using catalysts Catalysts speed up reactions and allow reactions to occur at a lower energy

12 Principles of Green Chemistry 10 Design for degradation The proper disposal of chemical products at the end of their use should be part of the design process. The products should be easily able to bread down into non-toxic materials

12 Principles of Green Chemistry 11 Using real-time pollution prevention This includes monitoring and controlling hazardous substances while the chemical process is occurring and monitoring them after disposal

12 Principles of Green Chemistry Preventing accidents The choice of substances and their states used in a chemical process should remove the potential for chemical accidents, including accidental releases, explosions and fires A toxic substance in solid form becomes much more dangerous if changed into its liquid or gas form

Green Chemistry Strategies Green chemistry strategies seek innovative methods to prevent the use or generation of hazardous substances initially. Risk = exposure x hazard Green chemistry involves inventing new and innovative methods of reducing chemical hazards while synthesising superior products in a more efficient and economical way.

Green Chemistry Strategies Green chemists will first analyse and assess the potential hazards associated with an existing process. They identify materials involved in every step of the chemical transformations – reactants, reagents, solvents, products and by products – as well as looking at the reaction conditions and energy input

Green Chemistry Strategies Identifying hazards and inefficiencies will lead to modification of the process The efficiency of the new process is tested to determine if it is more effective and less hazardous. This process of evaluation and testing will be repeated until a greener alternative procedure is established

Atom Economy The atom economy of a chemical reaction is a measure of the amount of starting materials that become useful products. Inefficient, wasteful processes have low atom economies. Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste. The atom economy of a reaction can be calculated:

Atom Economy Note that, because the total mass of products equals the total mass of reactants, you can put that into the bottom of the fraction in the calculation like this:

Atom Economy For example, what is the atom economy for making hydrogen by reacting coal with steam? Write the balanced equation: C(s) + 2H2O(g)    →    CO2(g) + 2H2(g) Write out the Ar and Mr values underneath: 12       2 × 18                44           2 × 2 Remember that the Ar or Mr in grams is one mole, so: total mass of products = 44 + 4 = 48g (note that this is the same as the reactants: 12 + 36 = 48g) mass of desired product (H2) = 4g % atom economy = 4⁄48 × 100 = 8.3% This process has a low atom economy and is therefore an inefficient way to make hydrogen. It also uses a non-renewable resource: coal.

Complexities and challenges for green chemistry Issues for green chemistry processes include starting materials, hazards of the process, number of steps, whether the process can be scaled up, energy use, waste produced, products produced, how the operation is to be run, and cost

Review Complete the multiple choice questions 1, 2 page 470 Complete the review questions 2, 7, page 470 Complete the worksheet – percentage yield and atom economy