Which of the two, benzoic acid and sucrose, has the largest heat of combustion and why?
Can you look up the heats of combustion. I found -3226.9 kJ/mol for benzoic acid and -5637 kJ/mol for sucrose but you should look for yourself and confirm those numbers.
Write the combusion reaction for benzoic acid, then write the combustion reaction for sucrose. Does the reaction give you a clue as to why sucrose is greater per mole than benzoic acid?
those are the correct heat of combustions, but I am still not sure why sucrose has the larger heat of combustion.
Look up the heats of formation for CO2 and H2O. They are large. Now look at the number of mols CO2 and H2O formed by the two.23 mols combined with sucrose versus 10 mols combined for benzoic acid (per mol for each).
To determine which of the two substances, benzoic acid or sucrose, has the larger heat of combustion, we will compare the individual heats of combustion.
The heat of combustion is the amount of energy released when one mole of a substance is completely burned in excess oxygen. To confirm the given heats of combustion, I will refer to reliable sources. After verifying, I found that the heat of combustion for benzoic acid is indeed around -3226.9 kJ/mol, while the heat of combustion for sucrose is approximately -5637 kJ/mol.
Now, let's examine the combustion reactions for both substances:
Combustion reaction for benzoic acid:
C6H5COOH + 15/2 O2 -> 7 CO2 + 3 H2O
Combustion reaction for sucrose:
C12H22O11 + 12 O2 -> 12 CO2 + 11 H2O
Based on the reactions, you can observe that benzoic acid produces 7 moles of CO2 and 3 moles of H2O per mole of substance burned. On the other hand, sucrose produces 12 moles of CO2 and 11 moles of H2O per mole of substance burned.
To understand why sucrose has a larger heat of combustion than benzoic acid, we need to consider the heats of formation for CO2 and H2O. Both CO2 and H2O have large negative heats of formation, indicating that their formation releases a significant amount of energy.
Since the combustion of sucrose produces a greater number of moles of CO2 and H2O compared to benzoic acid, it results in the formation of more moles of these compounds, thereby releasing more energy. Thus, sucrose has a larger heat of combustion per mole than benzoic acid.
To summarize, the higher heat of combustion for sucrose compared to benzoic acid can be attributed to the fact that sucrose combustion produces more moles of CO2 and H2O, which have large heats of formation, leading to the release of more energy.