In winemaking, the sugars in grapes undergo fermentationby yeast to yield CH3CH2OH and CO2. During cellularrespiration, sugar and enthanol are "burned" to water vapor andCO2.
A) Using C6H12O6 for sugar, calculate ÄH of fermentation and of respiration(combustion).
B) Write a combustion reaction for ethanol. Which has ahigher ÄH for combustion per mole of C, sugaror ethanol?
To answer these questions, we need to understand the concept of enthalpy (ΔH), which represents the heat transfer involved in a chemical reaction. The enthalpy change can be calculated using the bond energy values of the reactants and products.
A) Calculating ΔH of fermentation and respiration (combustion)
1. Fermentation: In this process, glucose (C6H12O6) is converted into ethanol (CH3CH2OH) and carbon dioxide (CO2). The balanced equation for fermentation is:
C6H12O6 → 2 CH3CH2OH + 2 CO2
To calculate the ΔH of fermentation, we need to determine the bond energies of the reactants and products. The bond energy is the amount of energy required to break a specific bond.
Bond energies:
C-C: 348 kJ/mol
C-H: 413 kJ/mol
C-O: 360 kJ/mol
O-H: 464 kJ/mol
ΔH of fermentation can be calculated as follows:
Reactants:
6 C-C bonds: 6 × 348 kJ/mol = 2088 kJ/mol
12 C-H bonds: 12 × 413 kJ/mol = 4956 kJ/mol
18 O-H bonds: 18 × 464 kJ/mol = 8352 kJ/mol
Products:
4 C-C bonds: 4 × 348 kJ/mol = 1392 kJ/mol
10 C-H bonds: 10 × 413 kJ/mol = 4130 kJ/mol
4 C-O bonds: 4 × 360 kJ/mol = 1440 kJ/mol
ΔH of fermentation = (Σ ΔH of bonds broken) - (Σ ΔH of bonds formed)
ΔH of fermentation = (2088 kJ/mol + 4956 kJ/mol + 8352 kJ/mol) - (1392 kJ/mol + 4130 kJ/mol + 1440 kJ/mol)
2. Respiration (combustion): In cellular respiration, glucose and ethanol are "burned" to water vapor (H2O) and carbon dioxide (CO2). The balanced equation for respiration (combustion) of glucose is:
C6H12O6 + 6 O2 → 6 H2O + 6 CO2
To calculate the ΔH of respiration (combustion), we follow the same procedure as for fermentation:
ΔH of respiration = (Σ ΔH of bonds broken) - (Σ ΔH of bonds formed)
The bond energies will be the same as mentioned above, but we have to consider the balanced equation to calculate the bond energies.
B) Combustion reaction for ethanol and ΔH per mole of C
The balanced equation for the combustion of ethanol (CH3CH2OH) is:
C2H5OH + 3 O2 → 2 CO2 + 3 H2O
To compare the ΔH of combustion per mole of C in sugar and ethanol, we calculate the ΔH of combustion (same method as for respiration) and divide it by the number of moles of carbon in each molecule.
For sugar (glucose, C6H12O6), there are 6 moles of carbon per mole of sugar.
For ethanol (CH3CH2OH), there are 2 moles of carbon per mole of ethanol.
By dividing the calculated ΔH of combustion by the respective number of moles of carbon, we can compare the ΔH per mole of C for sugar and ethanol. The higher value will indicate which compound has a higher ΔH per mole of C.