Be sure to answer all parts. In winemaking, the sugars in grapes undergo fermentation by yeast to yield CH3CH2OH (ethanol) and CO2. During cellular respiration, sugar and ethanol are "burned" to water vapor and CO2. (a) Using C6H12O6 for sugar, calculate ΔH o rxn of fermentation and of respiration (combustion). Fermentation = kJ Respiration = kJ (b) Write a combustion reaction for ethanol. Include the physical states of each reactant and product. (c) Which releases more heat from combustion per mole of C, sugar or ethanol?

a) - Fermentation = - 2 816 kJ/mol

Respiration = - 1409.2 kJ/mol

b) C₂H₅OH (l) + 3O₂ (g) → 3H₂O (g) + 2CO₂ (g)

c) combustion per mol of sugar

Explanation:

a)

The fermentation of sugar is given as follows:

C₆H₁₂O₆ (s) + 6O₂ (g) → 6O₂ (g) + 6H₂O (l)

ΔHrxn = ΔHformation (products) - ΔHformation (reactants)

= (6 mol * - 393.5kJ/mol) + (6mol * - 285.8kJ/mol) - (1 mol * - 1260kJ/mol + 6mol * 0)

= - 2 816 kJ/mol

The heat of combustion of ethanol is given as follows:

C₂H₅OH (l) + 3O₂ (g) → 3H₂O (g) + 2CO₂ (g)

Let's assume that 1 mol of ethanol is burnt. The heat of reaction, ΔHrxn is given by this equation:

ΔHrxn = ΔHformation (products) - ΔHformation (reactants)

= (2 mol * - 393.5kJ/mol) + (3mol * - 285.8kJ/mol) - [ (1mol * - 235 kJ/mol) + (3mol * 0.0000kJ) ]

= - 1644 - (-235.2)

= - 1409.2 kJ/mol

Therefore, the combustion of ethanol is exothermic. In other words, heat iis given off (this is signified by the negative sign)

b) Ethanol, like any other fuel, burns up to give water and carbon dioxide. This is based on the assumption that the combustion is complete and no side reactions take place. The combustion of ethanol is given as follows:

C₂H₅OH (l) + 3O₂ (g) → 3H₂O (g) + 2CO₂ (g)

The physical states are given in the parentheses.

c) From the comparisons of the ΔHrxn, sugar produces more energy.