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

Answer:

a. ΔH rxn (fermentation) = -68.9 kJ

   ΔH rxn (respiration) = -2538.5 kJ

b. CH₃CH₂OH(l) + 3O₂(g)  →  2CO₂(g) + 3H₂O(l)

Explanation:

The enthalpy of reaction ($\Delta H_{rxn}$ can be calculated using the standard enthalpy of formation (ΔHf) of products and reactants as follows:

$\Delta H_{rxn} = \Sigma n_{p} \Delta H_{p} - \Sigma n_{r} \Delta H_{r}$                            (1)

where ΔHp and ΔHr are the enthalpy of formation of the products and reactants, respectively, and np and nr are the stoichiometric coefficient of the product and reactants, respectively, from the balanced reaction.

a. The ΔH rxn of fermentation of sugar is:

C₆H₁₂O₆  →  2CO₂ + 2CH₃CH₂OH            

ΔHf (C₆H₁₂O₆) = -1273.3 kJ/mole

ΔHf (CO₂) = -393.5 kJ/mole

ΔHf (CH₃CH₂OH) = -277.6 kJ/mole  

Using equation (1):

$\Delta H_{rxn} = 2 \Delta H_{f}(CO_{2}) + 2 \Delta H_{f}(CH_{3}CH_{2}OH) - \Delta H_{f}(C_{6}H_{12}O_{6})$

$\Delta H_{rxn} = 2(-393.5) + 2(-277.6) - (-1273.3)$

$\Delta H_{rxn} = -68.9 kJ$

The ΔH rxn of respiration (combustion) of sugar is:

C₆H₁₂O₆ + 6O₂  →  6CO₂ + 6H₂O    

ΔHf (H₂O) = -241.8 kJ/mole

$\Delta H_{rxn} = 6 \Delta H_{f}(CO_{2}) + 6 \Delta H_{f}(H_{2}O) - \Delta H_{f}(C_{6}H_{12}O_{6})$  

$\Delta H_{rxn} = 6 (-393.5) + 6 (-241.8) - (-1273.3)$  

$\Delta H_{rxn} = -2538.5 kJ$  

b. The ΔH rxn of combustion of ethanol is:

CH₃CH₂OH(l) + 3O₂(g)  →  2CO₂(g) + 3H₂O(l)    

$\Delta H_{rxn} = 2 \Delta H_{f}(CO_{2}) + 3 \Delta H_{f}(H_{2}O) - \Delta H_{f}(CH_{3}CH_{2}OH)$  

$\Delta H_{rxn} = 2 (-393.5) + 3 (-241.8) - (-277.6)$  

$\Delta H_{rxn} = -1234.8 kJ$    

We can see that the combustion of sugar releases more heat than the combustion of ethanol.    

I hope it helps you!