Choosing reaction pathways - Higher
Usually, there is more than one way to make a particular substance. A reaction pathwayThe sequence of reactions needed to produce a desired product from a particular set of raw materials. describes the sequence of reactions needed to produce a desired productA substance formed in a chemical reaction.. The pathway chosen for a product depends on factors such as:
- percentage yieldA measure of how much product is actually made, calculated from: 100 脳 (actual yield) 梅 (theoretical yield).
- atom economyA measure of how many reactant atoms form a desired product.
- rate of reaction
- equilibrium positionA measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium.
- usefulness of by-productAn product from a side reaction that happens at the same time as the main reaction that is not needed. By-products reduce the yield of a process.
Increasingly, 'green chemistry' ideas are also included to ensure the sustainabilityThe ability to keep something going over time without harming people or the environment. of a process.
Factors which affect the sustainability of a process include:
- whether the raw materialBasic material that goods are made from such as crops, metals, wood and animal products such as wool and leather. are renewableEnergy sources that are replenished and not exhausted, eg solar power. or not
- the impact on other competing uses for the raw materials
- the type and amount of by-products
- waste
- energy inputs and outputs
Making ethanol
The manufacture of ethanolThe alcohol which is produced as a result of fermentation of sugars by yeast., used as a fuelMaterial that is used to produce heat, like coal, oil or gas., provides a useful example for choosing reaction pathways. Ethanol is manufactured in two ways.
1. fermentationType of anaerobic respiration carried out by yeast. of plant sugars:
glucose \(\rightarrow\) ethanol + carbon dioxide
C6H12O6(aq) \(\rightarrow\) 2C2H5OH(aq) + 2CO2(g)
2. hydrationChemical reaction in which water reacts with a substance. of ethene The simplest alkene, consisting of two carbon atoms joined by a double bond, surrounded by four hydrogen atoms., obtained from crude oilMixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms., using steam:
C2H4(g) + H2O(g) \(\rightarrow\) C2H5OH(l)
The table summarises some features of these two processes.
| Process | Yield (%) | Atom economy (%) | Rate of reaction |
| Fermentation of sugars | 15 | 51.1 | Low |
| Hydration of ethene | 95 | 100 | High |
| Process | Fermentation of sugars |
|---|---|
| Yield (%) | 15 |
| Atom economy (%) | 51.1 |
| Rate of reaction | Low |
| Process | Hydration of ethene |
|---|---|
| Yield (%) | 95 |
| Atom economy (%) | 100 |
| Rate of reaction | High |
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 atomThe smallest part of an element that can exist. in the reactantA substance that reacts together with another substance to form products during a chemical reaction. 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 reversible reactionA chemical reaction which can go both ways.. The equilibrium positionA measure of the relative concentrations of substances in an equilibrium, showing if there are more reactants or products at equilibrium. lies to the left, so only about 5% of the ethene supplied is converted to ethanol. The overall yield of 95% is achieved by recyclingReusing materials or reprocessing waste materials to produce new materials. 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 an explanation for why the raw materials needed may affect the reaction pathway chosen for the manufacture of ethanol.
Plant sugars are a renewableEnergy sources that are replenished and not exhausted, eg solar power. resourceA supply of something (eg money or materials) that can be used. but ethene is obtained from crude oil. Crude oil is a non-renewableA resource that cannot be replaced when it is used up, such as oil, natural gas or coal. resource that will run out if we carry on using it.