IB Chemistry HL R1.3 Energy from fuels questions

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Question 1

Multiple choiceMedium1 mark

A 23.0 g sample of bioethanol, C₂H₅OH, is burned completely. Assume all its carbon was fixed from atmospheric CO₂ by recent photosynthesis, and ignore emissions from production and transport. The combustion equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Use M(C₂H₅OH) = 46.0 g mol⁻¹ and M(CO₂) = 44.0 g mol⁻¹. What are the mass of CO₂ released and the net change in atmospheric CO₂ from fixation and combustion together? Treat an increase as positive.

Show answer and explanation

Answer: C. The equation gives 2 mol CO₂ per mole of ethanol. The ethanol amount is 23.0/46.0 = 0.500 mol, so combustion releases 1.00 mol CO₂ × 44.0 g mol⁻¹ = 44.0 g. The same 1.00 mol of atmospheric CO₂ had been fixed to provide the carbon in the ethanol, so the net change is +44.0 − 44.0 = 0.0 g.

A: uses a 1:1 rather than 1:2 ethanol-to-CO₂ mole ratio, giving 22.0 g. B: calculates the released CO₂ but omits the earlier photosynthetic uptake. D: counts the uptake as a negative change but omits the CO₂ released on combustion.

Remember: for the stated carbon balance, recently fixed carbon returns to the atmosphere on combustion.

Question 2

Paper 2Medium7 marks

A pilot plant produces bioethanol from glucose in crop biomass. Figure 1 shows a simplified carbon pathway for one batch. The fermentation equation is:

\ceC6H12O6−>2C2H5OH+2CO2\ce{C6H12O6 -> 2C2H5OH + 2CO2}

Use relative atomic masses from the data booklet where needed.

Atmospheric CO2Crop biomass:glucoseFermentationBioethanolphotosynthesis;carbon fixed duringone growing season90.0 kg glucose38.6 kg ethanol collectedCO2 releasedcombustion releasesCO2
(a)

Write a balanced equation for photosynthesis in plants.

[1]
(b)

Use the fermentation equation and Figure 1 to calculate the theoretical mass of ethanol and the percentage yield for this batch. Give the mass in kg and the percentage yield to three significant figures.

[3]
(c)

Using Figure 1, explain why the ethanol can be considered a renewable fuel.

[2]
(d)

State one reason why Figure 1 alone does not establish that using this bioethanol is carbon neutral.

[1]

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