Which of the following substances yields the most energy per mole when combusted under standard conditions?

• A. C2H6 (ethane)
• B. C8H18 (octane)
• C. C6H12O6 (glucose)
• D. CH4 (methane)

Answer: (B)

Verified step by step guidance

1. Identify that the problem is asking which substance releases the most energy per mole upon combustion under standard conditions. This relates to the enthalpy change of combustion (ΔH_combustion), which is the heat released when one mole of a substance burns completely in oxygen.
2. Recall that the enthalpy of combustion generally increases with the number of carbon and hydrogen atoms in the molecule because more bonds are broken and formed, releasing more energy.
3. Write the general combustion reaction for each compound, for example, for a hydrocarbon C_xH_y: \(\mathrm{C_xH_y + \left(x + \frac{y}{4}\right) O_2 \rightarrow x CO_2 + \frac{y}{2} H_2O}\).
4. Understand that larger hydrocarbons like octane (C8H18) have more C-H and C-C bonds to oxidize compared to smaller molecules like methane (CH4) or ethane (C2H6), and glucose (C6H12O6) contains oxygen already, which affects its combustion energy.
5. Conclude that by comparing the molecular size and bond content, octane (C8H18) will yield the most energy per mole upon combustion because it has the greatest number of combustible bonds among the options.