CBSE · Class 12 · Chemistry · Alcohols, Phenols and EthersCalculate the mass of absolute alcohol that can be prepared from 100 grams of glucose assuming 80% conversion yield, given the chemical reactions. Also, describe the industrial method of manufacturing ethanol from molasses.

Numerical Problem: Calculation of Mass of Ethanol

Chemical Equation for Fermentation of Glucose: $$\text{C}6\text{H}{12}\text{O}_6 \xrightarrow{\text{Zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$

Step 1: Determine Molecular Masses

• Molecular mass of Glucose ($\text{C}6\text{H}{12}\text{O}_6$) = $(6 \times 12) + (12 \times 1) + (6 \times 16) = 72 + 12 + 96 = 180 \text{ g/mol}$
• Molecular mass of Ethanol ($\text{C}_2\text{H}_5\text{OH}$) = $(2 \times 12) + (5 \times 1) + 16 + 1 = 24 + 5 + 16 + 1 = 46 \text{ g/mol}$

Step 2: Calculate Theoretical Yield\nFrom the balanced equation, $180 \text{ g}$ of glucose gives $2 \times 46 = 92 \text{ g}$ of ethanol. \nTherefore, $100 \text{ g}$ of glucose will theoretically give: $$\text{Theoretical yield} = \frac{92 \text{ g}}{180 \text{ g}} \times 100 \text{ g} = 51.11 \text{ g of ethanol}$$

Step 3: Calculate Actual Yield (with 80% conversion) $$\text{Actual yield} = \text{Theoretical yield} \times \frac{\text{Percentage yield}}{100}$$ $$\text{Actual yield} = 51.11 \text{ g} \times \frac{80}{100} = 40.89 \text{ g of ethanol}$$

Industrial Manufacturing of Ethanol from Molasses

\nMolasses is the mother liquor left after the crystallization of cane sugar. It is a rich source of sucrose (about 50%) and is widely used for the commercial production of ethanol through fermentation.

• Dilution of Molasses: Molasses is diluted with water until its sugar concentration falls to about 8-10% to ensure optimal microbial activity.
• Addition of Nitrogen Source: Ammonium sulfate or ammonium phosphate is added to the solution as a nutrient/food source for the yeast.
• Fermentation Process: Yeast (containing the enzyme invertase and zymase) is added to the mixture, and the temperature is maintained around $303\text{K}$ in an anaerobic environment.
• Enzymatic Reactions:
  1. Invertase converts sucrose into glucose and fructose: $$\text{C}{12}\text{H}{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{Invertase}} \text{C}6\text{H}{12}\text{O}_6 + \text{C}6\text{H}{12}\text{O}_6$$
  2. Zymase converts glucose into ethanol and carbon dioxide: $$\text{C}6\text{H}{12}\text{O}_6 \xrightarrow{\text{Zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$
• Distillation: The resulting fermented liquid (called wash) containing 10-15% ethanol is fractionally distilled to yield rectified spirit (95% ethanol by volume). Dehydration of rectified spirit over quicklime gives absolute alcohol (100% pure ethanol).