Question

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. Which releases more heat from combustion per mole of C, sugar or ethanol? sugar ethanol

Answer 1

1) Combustion of Sugar Sucrose C12H22O11.

C12H22O11 + 12 O2 12 CO2 (g) + 11 H2O (g). ΔH°_rxn = ?

Using standard enthalpies of formation (ΔH°_f ) of each component & formula

ΔH°_rxn = Σ ΔH°_f (products) - Σ ΔH°_f (reactants)

ΔH°_rxn = [12 x ΔH°_f (CO2) + 11 x ΔH°_f (H2O)] - [ΔH°_f (Sucrose) + 12 ΔH°_f (O2)]

On referring literature,

ΔH°_rxn = [12 x (-393.5) + 11 x ΔH°_f (-241.8)] - [(-2226.1) + 12 x(0)]

ΔH°_rxn = -5155.7 kJ/mol.

There are 12 C in 1 sucrose mole.

ΔH°_rxn per mole of C = -5155.7 / 12 = -429.6 kJ/mol.

B) For combustion of ethanol

CH3CH2OH + 3 O2   2 CO2 (g) + 3 H2O(g). ΔH°_rxn = ?

ΔH°_rxn = [2 x ΔH°_f (CO2) + 3 x ΔH°_f (H2O)] - [ΔH°_f (Ethanol) + 3 ΔH°_f (O2)]

On referring literature,

ΔH°_rxn = [2 x (-393.5) + 3 x ΔH°_f (-241.8)] - [(-277.0) + 3 x(0)]

ΔH°_rxn = -1235 kJ/mol.

There are 2 C in ethanol molecule.

ΔH°_rxn =per mole of C = -1235 / 2 kJ/mol. = -617.5 kJ/mole.

There are enthalpies of reactions involved in combustion of Sucrose & EThanol.

It's clear by observing values that Enthalpy per mole of C of reaction is higher for combustion of Sucrose than that of for Ethanol.

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