Question

synthesis of Ethanol by fermentation

(a) Calculate the atom economy and reaction efficiency of your ethanol synthesis. (b) Describe some green features of your synthesis, and any that aren't so green.

Answer 1

a. The atom economy of a reaction is a theoretical percentage measure of the amount of starting materials that ends up as the 'desired' useful reaction products. Many reactions give more than one product, and not all of them are useful, so it is useful to calculate what % of the products is theoretically useful, and we call this the atom economy of the reaction.

ATOM ECONOMY  

= 100*( TOTAL FORMULA MASSES of USEFUL PRODUCT)/( TOTAL FORMULA MASS of all REACTANTS)

Atom economy of the fermentation of sugar to make ethanol,

glucose (sugar) ---(enzyme)---> ethanol + carbon dioxide

C₆H₁₂O_6(aq) -----> 2C₂H₅OH_(aq) + 2CO_2(g)

formula mass of glucose reactant = 180

formula mass of ethanol product = 46

Total mass of desired useful product from 1 formula mass of glucose = 2 x 46 = 92

Atom economy = 100 x 92/180 = 51.1%

Reaction efficiency: The percent yield of a reaction measures the reaction's efficiency. It is the ratio between the actual yield and the theoretical yield.

Percent yield/reaction efficiency = (actual yield/theoretical yield) * 100. Percent yield measures how efficient the reaction is under certain conditions. ....[you have not given the value of theoretical yield, just put the value in the equation.]

b. 1. Sugar beet and sugar cane grow quickly, particularly in warm climates and labor may be very cheap in third world countries. So this method has a sustainable renewable resource.

2. Reaction needs normal temperature, 30-40^oC. Yeast is relatively cheap to produce, since it reproduces and grows quite naturally

3. There are no particular health and safety issues or great risks for the surrounding local communities, unlike the potential hazards of running an oil refinery.

4. The waste carbon dioxide can be safely released into the atmosphere, but it could be used in fizzy carbonated drinks or even pumped into greenhouses to increase the rate of photosynthesis.

5. It does not require any advanced technology compared to a large petrochemical complex based on crude oil.

6. It does not require the importation of expensive crude oil, a non–renewable resource and since based on an agricultural system, it ought to be sustainable with a long–term future.