Ethanol
| What is Needed:
ü Ethene and steam ü 3000C ü 60-70 atmospheres ü Phosphoric acid catalyst |
All alcohols contain an –OH group attached to a carbon chain. Ethanol is C2H5OH.
Production of Ethanol
Hydration of Ethene
Ethanol can be made by reacting ethene with steam (because it contains more energy) – a process known as hydration.
CH2=CH2(g) + H2O(g) à CH3CH2OH(g)
Only a small portion of ethene reacts. The ethanol is condensed as a liquid and the unreacted ethene is recycled.
Explaining the Choice of Temperature
Reversible reactions happen in two ways – while ethene is being converted into ethanol, ethanol is also being converted back into ethene. Reversible reactions can also shift the equilibrium – or ‘alter’ the reaction. Since the reaction is exothermic – the reaction produces lots of heat. If you increase the temperature, the reaction won’t like it because it is already producing heat, therefore, it would ‘adapt’ to the conditions by making more ethene so less heat will be produced. On the contrary, if you decrease the temperature, the reaction would ‘adapt’ to this by increasing back the temperature; by producing more ethanol – in other words, push the equilibrium to the favourable/forward reaction. However, making the temperature too low would mean super slow reaction, although more ethanol would be produced. 300 degrees is therefore, a compromise temperature producing an acceptable yield of ethanol in a short time.
Explaining High Pressure
In the equation, you have two moles (one mole of ethene and one mole of water) on the left, and one mole (of ethanol) on the right. Increasing the pressure would mean the equilibrium would be shifted forwards. Why? The reaction would ‘adapt’ to the conditions by producing more ethanol because you only get one mole of ethanol – which takes less space than two moles of ethene and water.
Also, there’s the collision theory. Increasing the pressure means that there’d be less space for the atoms to move. The atoms would also move with more force. This increases the frequency of collisions.
The problem: it’s expensive and ethene might polymerise and turn into polyethene.
Fermentation
Yeast is added to a sugar or starch solution at 300C for several days in the absence of air for anaerobic respiration. Enzymes in the yeast lower the activation energy, increasing the rate of conversion of the sugar into ethanol and carbon dioxide. However, they first have to break the sugars into smaller sugars like glucose. In fact, ethanoic acid is produced and then converted into ethanol.
For example, sucrose:
C12H22O11(aq) + H20 -> C6H12O6(aq) + C6H12O6(aq) sucrose + water -> glucose + fructose
C6H12O6(aq) -> 2C2H5OH (aq) + 2CO2(g) glucose/fructose -> ethanol + carbon dioxide
The yeast then gets killed in the mixture, which means that the ethanol produced is impure. To purify it, the alcohol must undergo fractional distillation.
Comparing the two methods…
| Fermentation | Hydration | |
| Use of Resources | Uses renewable resources – sugar beet or sugar cane, corn and other starchy materials. | Uses non-renewable resources – once oil gets used up, they’re screwed. |
| Type of Process | A batch process – everything is mixed and left for several days. It is then removed and a new reaction is set up – quite inefficient. | A continuous flow process – a stream of reactants is constantly passed over the catalyst – more efficient. |
| Rate of Reaction | Slow, takes several days. | Rapid |
| Quality of Product | Produces impure ethanol that needs further processing. | Produces much purer ethanol. |
| Reaction Conditions | Uses gentle temperatures and ordinary pressure – relying on anaerobic respiration of yeast. | Uses high temperatures and pressures, needing a high input of energy – expensive. |
Common Question: Which method would poorer places like Brazil use and why? [3 marks]
Answer: Fermentation, because Brazil has the weather conditions to grow large yields of sugar cane and they don’t have access to crude oil.
Dehydration of Ethanol into Ethene
Dehydration of ethanol produces ethene and water, using hot aluminium oxide as a catalyst.
CH3CH2OH(g) -> CH2=CH2(g) + H2O(l)