Green Chemistry Questions and Answers

Answer: C

Correct answer: the actual yield and the excess reagents used

Atom economy is a theoretical figure from the balanced equation, assuming stoichiometric quantities and complete conversion. Reaction mass efficiency uses what was actually charged and actually obtained, so it falls below the atom economy whenever the yield is less than complete or a reagent is used in excess.

Answer: D

In fine chemical manufacture the solvent commonly outweighs everything else charged, so reducing, recovering or replacing it moves the metrics more than any change to the reaction. Many are also volatile, flammable or toxic.

Answer: A

Correct answer: non-toxic, non-flammable and easily removed

Carbon dioxide is benign to handle, will not burn, and reverts to a gas on releasing the pressure, so the product is recovered solvent-free without distillation and the solvent is recompressed and reused. Its weakness is polarity: it dissolves non-polar material well and ionic compounds hardly at all.

Answer: B

Bulky unsymmetrical ions frustrate crystallisation, so the salt melts far below the temperature an ordinary salt would. Its negligible vapour pressure is what removes the emission problem that volatile organic solvents present.

Answer: C

Low volatility solves the emission problem but says nothing about aquatic toxicity or persistence, and some are worse on those counts than the solvents they replace. Their cost and the difficulty of purifying them are further obstacles.

Answer: D

Correct answer: removes the largest single source of waste

Solvent is usually the largest contributor to the mass of a process, often the great majority of the E-factor, so a reaction run without one removes that contribution entirely. It does not follow that the reaction will be faster or higher-yielding - mixing and heat removal are often harder without a solvent, which is why neat reactions are not universal.

Answer: A

Correct answer: cheap, non-toxic and non-flammable

Water scores on every count that makes a solvent troublesome: it costs almost nothing, poses no toxic or fire hazard, and needs no special handling or disposal. Its drawbacks are that many organic compounds will not dissolve in it and that removing it is expensive, since its latent heat of vaporisation is high.

Answer: B

Correct answer: used in small quantity and not consumed

A catalyst is regenerated in each cycle, so a small charge turns over many times and appears in neither the product nor the waste. A stoichiometric reagent is consumed in equimolar quantity and leaves as spent material, so its whole mass becomes waste. That difference in mass is why the principle is stated in terms of catalysis.

Answer: C

Chromium reagents generate a molar quantity of toxic metal residue that must be treated and disposed of, whereas air as the oxidant leaves only water. Handling oxygen safely is the engineering problem that replaces it.

Answer: D

An enzyme frequently gives one enantiomer where chemical catalysis gives a mixture requiring resolution, which halves the waste at a stroke. Its narrow tolerance of temperature, pH and solvent is the compensating limitation.