Responses for Ethanol Combustion and Production

Question 1: Is the fermentation of glucose to ethanol exothermic or endothermic?

Given: ΔH°rxn = -67 kJ
Required: Determine whether the reaction is exothermic or endothermic.
Analysis: The sign of the enthalpy change indicates whether heat is absorbed or released (Keifer, 2019).
Solution: The enthalpy change is negative, which means heat is released during the reaction (Keifer, 2019).
Statement: Since the enthalpy change is negative (-67 kJ), the reaction is exothermic. It means that it releases heat into the surroundings.

Question 2: Heat capacity of the calorimeter when ethanol is burned.

Given:
Mass of ethanol: 21.8 g
Temperature rise: 25.0°C to 62.3°C (ΔT = 37.3°C)
Molar mass of ethanol: 46.07 g/mol
Reaction: C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)
ΔH°rxn = -1235 kJ/mol

Required: Heat capacity of the calorimeter in kJ/°C.

Analysis:

1. Moles of ethanol: 21.8 g / 46.07 g/mol = 0.473 mol
2. Heat released: 0.473 mol × -1235 kJ/mol = -584.95 kJ
3. Heat Capacity: 584.95 kJ / 37.3°C = 15.68 kJ/°C
Solution:

The heat capacity of the calorimeter is 15.68 kJ/°C.

Statement: The heat capacity of the calorimeter is 15.68 kJ/°C. It is based on the temperature change and the energy released in time of the combustion of ethanol.

Question 3: Final temperature of the calorimeter when 12.8 g ethanol is burned.

Given:
Mass of ethanol: 12.8 g
Initial temperature: 25.0°C
Heat capacity of calorimeter: 5.65 kJ/°C
Molar mass of ethanol: 46.07 g/mol
ΔH°rxn = -1235 kJ/mol

Required: Final temperature of calorimeter.
Analysis
1. Moles of ethanol: 12.8 g / 46.07 g/mol = 0.278 mol
2. Heat released: 0.278 mol × -1235 kJ/mol = -343.43 kJ
3. Temperature change: 343.43 kJ / 5.65 kJ/°C = 60.8°C
4. Final temperature: 25.0°C + 60.8°C = 85.8°C
Solution: The final temperature of the calorimeter is 85.8°C.

Statement: The final temperature of the calorimeter after the burn of 12.8 g of ethanol is 85.8^o C.

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Question 4: Calculate ΔH°rxn for the reaction.

Given:
CH4(g) + 4 Cl2(g) → CCl4(g) + 4 HCl(g)

ΔH°f (CH4) = -75 kJ/mol
ΔH°f (CCl4) = -96 kJ/mol
ΔH°f (HCl) = -92 kJ/mol

Required: Calculate the standard enthalpy change of reaction (ΔH°rxn).

Analysis: The standard enthalpy change of reaction (ΔH°rxn) is calculated using the formula:
ΔHrxn° = ΣΔHf° (products) − ΣΔHf° (reactants)

Solution:
ΔHrxn° = [ΔHf° (CCl4) + 4 × ΔHf° (HCl)] − [ΔHf° (CH4) + 4 × ΔHf° (Cl2)]
The standard enthalpy of formation for elemental gases (like Cl2) is zero (Keifer, 2019).

ΔHrxn° = [(-96 kJ/mol) + 4 × (-92 kJ/mol)] − [(-75 kJ/mol) + 4 × (0 kJ/mol)]
ΔHrxn° = [-96 + (-368)] − (-75)
ΔHrxn° = -464 + 75 = -389 kJ/mol

Statement: The standard enthalpy change of the reaction (ΔH°rxn) is -389 kJ/mol.

Question 5a: Calculate the amount of thermal energy used to heat water.

Given:
Mass of water: 102 g
Initial temperature: 5°C
Final temperature: 67°C
Specific heat capacity of water: 4.18 J/g°C

Required: Thermal energy (q) used to heat the water.

Analysis: The amount of thermal energy is calculated using the formula:
q = m × c × ΔT (Raul et al., 2018)

where m = mass of water, c = specific heat capacity of water, and ΔT = change in temperature.

Solution:
ΔT = 67°C − 5°C = 62°C
q = 102 g × 4.18 J/g°C × 62°C = 26,431.44 J = 26.43 kJ

Statement: The amount of thermal energy used to heat 102 g of water is 26.43 kJ.

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Question 5b: Calculate the heat of combustion for the sample of biodiesel.

Given:
Thermal energy used to heat water: 26.43 kJ
Mass of biodiesel: 3.4 g

Required: Heat of combustion (in kJ/g) for the sample of biodiesel.

Analysis: The heat of combustion is calculated using the formula:
Heat of combustion = thermal energy absorbed by water / mass of biodiesel burned

Solution:
Heat of combustion = 26.43 kJ / 3.4 g = 7.77 kJ/g

Statement: The heat of combustion for the sample of biodiesel is 7.77 kJ/g.

Question 6: Which biofuel would consumers prefer to use if it had a lower thermal energy value than petroleum diesel?

In the question it is already given that petroleum diesel produces 43 Kj/g of thermal energy. As suggested by Steg et al., (2018), consumers usually prefer fuels with higher energy output. This is as they provide more energy per gram. Therefore, fuels that provide higher energy output have improved efficiency. However, if a biofuel has a lower energy value, however, is more environmentally friendly, some customers might prefer it for ethical reasons if the alternative biofuel reduces their carbon footprint. In terms of rationale, it can be stated that consumers might prefer to utilize biofuels even with a lower thermal energy value than petroleum diesel when it is environmentally friendly.

Question 7: Explain why biodiesel is labeled as “carbon-neutral.”

Biodiesel is considered carbon neutral as the carbon dioxide or CO2 it releases in time of combustion is roughly equal to the amount of CO2 absorbed by the plants which are used to produce it in time of their growth (Alami et al., 2021). This creates a closed carbon cycle, and it indicates that the net increase of CO2 in the atmosphere is minimal. Differently, it can also be stated that biodiesel is labeled as carbon neutral as the CO2 emitted in time of the combustion is often balanced by the absorption of CO2 in time of the growth of the biomass from where it is produced. Thus, it leads to no net increase in the atmospheric level of carbon.

Question 8: What type of respiration does yeast undergo in the fermentation process to produce ethanol?

Yeast undergoes anaerobic respiration or fermentation process in time of the production of ethanol. The said process often takes place in the absence of oxygen (Tse et al., 2021). This process leads to the conversion of glucose into ethanol and carbon dioxide. As a whole, it can be stated that Yeast undergoes anaerobic respiration in time of the fermentation process to produce ethanol.

Question 9: Calculate the standard enthalpy change of combustion of ethanol.

Given:
ΔHf (CO2) = -393.5 kJ/mol
ΔHf (H2O) = -285.8 kJ/mol
ΔHf (C2H5OH) = -277.1 kJ/mol

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

Required: Standard enthalpy change of combustion of ethanol.

Analysis: Using Hess’s Law, the standard enthalpy change of combustion (ΔH°combustion) is calculated as:
ΔHcombustion° = ΣΔHf° (products) − ΣΔHf° (reactants) (Feng et al., 2018)

Solution:
ΔHcombustion° = [2 × (-393.5 kJ/mol) + 3 × (-285.8 kJ/mol)] − (-277.1 kJ/mol)
ΔHcombustion° = [-787 kJ/mol + (-857.4 kJ/mol)] − (-277.1 kJ/mol)
ΔHcombustion° = -1644.4 kJ/mol + 277.1 kJ/mol = -1367.3 kJ/mol

Statement: The standard enthalpy change of combustion of ethanol is -1367.3 kJ/mol.

Question 10: Two societal challenges that may occur if food crops were diverted from a raw material to produce ethanol.

Diversion of food crops, for example, sugarcane and corn to produce ethanol could result into a significant reduction in the overall availability of food for consumption (Mohanty & Swain, 2019). This could further lead to higher prices of food. The said event can also increase food insecurity, and this is more specific in regions that are already struggling with the supply of food. The transition from the use of crops for production of food to production of fuel could also disrupt the agricultural market. Farmers might prioritise growing crops for biofuel because of higher profitability. It might lead to a significant reduction in the production of crops. The negative impact on the production of crops might also affect industries that heavily rely on crop-based sectors.

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Reference

Keifer, D. (2019). Enthalpy and the second law of Thermodynamics. Journal of Chemical Education, 96(7), 1407-1411. https://pubs.acs.org/doi/abs/10.1021/acs.jchemed.9b00326

Raul, A., Jain, M., Gaikwad, S., & Saha, S. K. (2018). Modelling and experimental study of latent heat thermal energy storage with encapsulated PCMs for solar thermal applications. Applied Thermal Engineering, 143, 415-428. https://doi.org/10.1016/j.applthermaleng.2018.07.123

Steg, L., Shwom, R., & Dietz, T. (2018). What drives energy consumers?: Engaging people in a sustainable energy transition. IEEE Power and Energy Magazine, 16(1), 20-28. DOI: 10.1109/MPE.2017.2762379

Alami, A. H., Alasad, S., Ali, M., & Alshamsi, M. (2021). Investigating algae for CO2 capture and accumulation and simultaneous production of biomass for biodiesel production. Science of the total environment, 759, 143529. https://doi.org/10.1016/j.scitotenv.2020.143529

Tse, T. J., Wiens, D. J., & Reaney, M. J. (2021). Production of bioethanol—A review of factors affecting ethanol yield. Fermentation, 7(4), 268. https://doi.org/10.3390/fermentation7040268

Feng, J., Shang, Y., & Zhang, Y. (2018). Research on synthesis and thermodynamic properties of 2-methoxycyclohexanol: Synthesis and determination of the specific heat capacity, standard enthalpy change of combustion and standard enthalpy change of formation. Journal of Thermal Analysis and Calorimetry, 131, 2197-2203. https://doi.org/10.1007/s10973-017-6784-4

Mohanty, S. K., & Swain, M. R. (2019). Bioethanol production from corn and wheat: food, fuel, and future. In Bioethanol production from food crops (pp. 45-59). Academic Press. https://doi.org/10.1016/B978-0-12-813766-6.00003-5