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Industrial chemical reactions - HigherChoosing reaction pathways - Higher

The conditions chosen for an industrial reaction are related to producing an acceptable yield in an acceptable time. Different reaction pathways are often available to produce a given product.

Part of Chemistry (Single Science)Separate chemistry 1

Choosing reaction pathways - Higher

Usually, there is more than one way to make a particular substance. A describes the sequence of reactions needed to produce a desired . The pathway chosen for a product depends on factors such as:

  • rate of reaction
  • usefulness of

Making ethanol

The manufacture of , used as a , provides a useful example for choosing reaction pathways. Ethanol is manufactured in two ways.

  1. of plant sugars:
    • Glucose 鈫 ethanol + carbon dioxide
    • C6H12O6(aq) 鈫 2C2H5OH(aq) + 2CO2(g)
  2. of , obtained from , using steam:
    • C2H4(g) + H2O(g) 鈫 C2H5OH(l)

The table summarises some features of these two processes.

ProcessYieldAtom economyReaction rate
Fermentation of sugars15%51.1%Low
Hydration of ethene95%100%High
ProcessFermentation of sugars
Yield15%
Atom economy51.1%
Reaction rateLow
ProcessHydration of ethene
Yield95%
Atom economy100%
Reaction rateHigh

Worked example

Use information in the table above to evaluate the two processes for producing ethanol.

Fermentation has a lower percentage yield and rate of reaction than the hydration of ethene. The hydration of ethene has an atom economy of 100%, showing that all the in the form the desired product. As it also has a higher rate of reaction, the hydration of ethene appears to be the better way to make ethanol.

Equilibrium position

The hydration of ethene is a . The equilibrium position lies to the left, so only about 5% of the ethene supplied is converted to ethanol. The overall yield of 95% is achieved by recirculating unreacted ethene through the reactor.

Using by-products

Carbon dioxide is a by-product of the fermentation of plant sugars. It may be sold to fizzy drinks manufacturers to provide the bubbles in lemonade and cola. This makes it a desirable product as well, so the atom economy can be increased to 100%.

Question

Suggest how the raw materials available to manufacture ethanol affect the choice of reaction pathway.