Final answer:
Coal and sugar differ due to their molecular structure and bond types, with coal's complex aromatic structure contrasting with the simpler structure of sucrose. This results in different properties, including distinct enthalpies of combustion and carbon dioxide emissions during burning.
Step-by-step explanation:
Despite both coal and sugar being composed predominantly of carbon, they exhibit drastically different properties due to their distinct structures and the types of bonds present within each molecule. Coal is a complex solid material with a structure that contains a large number of oxygen atoms linking parts of the structure together in addition to carbon-carbon bonds. Its structure is similar to an aromatic hydrocarbon due to the high cellulose content from the plants it's derived from. On the other hand, sugar, such as sucrose, is a carbohydrate made up of glucose molecules joined in a specific way, resulting in a substance that is much less complex than coal but has notable biochemical uses within living organisms.
The differences in structure account for the differing enthalpies of combustion between substances like coal and sugar. Coal, having a more complex structure with oxygen atoms and carbon rings, tends to have a higher enthalpy of combustion compared to simpler organic compounds. Additionally, coal generally produces more CO2 per gram when burned than other fossil fuels which can be attributed to its higher carbon content and the presence of components that do not contribute to energy production, unlike the hydrogen content found in other fuels like natural gas.