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2.1.5 atom economy

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Reactivity 2.1.5 - the atom economy is a measure of efficiency in green chemistry

green chemistry is the sustainable design of chemical products and chemical processes

percentage yield is an incomplete measure of efficiency of reaction, as it gives no indication of the quantity of waste produced. a synthetic route should maximise the atom economy by incorporating as many of the atoms of the reactants as possible into the desired product

also consider:

efficient processes have high atom economies, use fewer natural resources, create less waste

catalysis is also an important aspect of green chemistry, replacing stoichiometric reagents and can greatly enhance the selectivity of processes.

catalysts are effective in small quantities and can be reused, so do not contribute to chemical waste, increasing atom economy

addition polymerisation reactions have an atom economy of 100% as all reacting atoms end up in the polymer and there are no side reactions

challenge questions
  1. percentage yield and atom economy are different concepts, but both can be used to assess aspects of the overall efficiency of a chemical process. see if you can find a reaction that has a high percentage yield under certain conditions, but a low atom economy.

for 100% yield, the atom economy is

challenge questions
  1. the fertiliser triammonium phosphate is made from ‘phosphate rock’ by:
    1. reacting the phosphate rock with sulfuric acid, , to produce phosphoric acid,
    2. reacting the phosphoric acid with ammonia, , to give tri-ammonium phosphate,
      if the phosphate rock contains 90% by mass from which the overall yield of triammonium phosphate is 95%, calculate the mass of phosphate rock required to make 1000 tonnes of triammonium phosphate

1242 tonnes or 1217 tonnes

  1. sulfuric acid, , is produced from sulfur in a three-step process:


    3. assuming 100% conversion and yield for each step, what is the minimum mass of sulfur needed to produce 980 tonnes of ?

  2. the concentration of hydrogen peroxide, , in excess aqueous sulfuric acid, , can be determined by redo titration using potassium permanganate, as follows:

a $10.00\pu{ cm^{3} }$sample of $\ce{H_{2}O_{2}}$ solution requires $18.00\pu{ cm^{3}}$ of a $0.0500\pu{ mol dm^{-3}}$ solution of $\ce{KMnO_{4}}$ to reach the equivalence point in a titration. calculate the concentration of $\ce{H_{2}O_{2}}$ in the solution.

6. mixtures of sodium carbonate, , and sodium hydrogencarbonate, , in aqueous solution are determined by titration with hydrochloric acid, , in a two-step procedure.
1. titrate to the phenolphthalein end point:

2. continue titration to the methyl orange end point:

for an $\text{X}\pu{ cm^{3}}$ sample of a sodium carbonate/sodium hydrogencarbonate mixture titrated with $\text{Y}\pu{ mol dm^{-3} }\ce{HCl}$, the respective end points are Step 1 = $\text{P}\pu{ cm^{3} }\ce{HCl}$ and step 2 = $\text{Q}\pu{ cm^{-3} }\ce{HCl}$. derive relationships between $X,Y,P,Q$ to obtain the concentrations of sodium carbonate and sodium hydrogencarbonate in the original mixture

7. a sealed vessel with fixed total internal volume of contains pentane, , and oxygen only. the pentane is ignited and undergoes 100% conversion to carbon dioxide and water. subsequently the temperature and pressure in the vessel are respectively and . calculate the initial amount and mass of oxygen in the vessel

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