Low heat of combustion= needing more of the substance to produce energy? (1 Viewer)

[fatassmcfat]

Hey I read some site that said ethanols low heat of combustion means that you need more of it in order to go the same distance as petroleum. I thought wouldnt it be the other way around? can someone plz esplain. Thanks

 

[gwilymprice]

Heat of combustion means the amount of energy that is released when a substance is burnt. So as ethanol as a lower heat of combustion it means it will produce less energy when burnt ( combusted) by oxygen. As you're using a vehicle as an example, if you fill a 40L tank with ethanol, less energy will be created when that in completely combusted when compared to a 40L rank of petroleum or octane. Less energy = less distance travelled. Hopefully this makes sense haha

 

[anomalousdecay]

Heat of combustion means the amount of energy that is released when a substance is burnt. So as ethanol as a lower heat of combustion it means it will produce less energy when burnt ( combusted) by oxygen. As you're using a vehicle as an example, if you fill a 40L tank with ethanol, less energy will be created when that in completely combusted when compared to a 40L rank of petroleum or octane. Less energy = less distance travelled. Hopefully this makes sense haha

This.

The reason why this happens can be explained by simple Chemistry.

Unleaded Petrol's main component for releasing energy is Octane. This molecule has more of a molecular mass than ethanol. As a result, there is more potential energy in the Petrol than Ethanol. Hence, you get more energy from the higher heat of combustion of Petrol.

This is also comparable to Diesel, which is made up of Hydrocarbons of twice the molecular mass of Petrol, thus giving Diesel a higher heat of Combustion.

However this can lead to a compromise in efficiency as it is harder to completely combust fuels higher molecular mass. (This is out of syllabus scope, will post in extracurricular).

 

[gwilymprice]

Also, the reason higher mass molecules generally have higher energy potentials is because there are more bonds to be broken and bonds contain that energy. This explanation is rather simplistic but depth in this area isn't required

 

[fatassmcfat]

cool thanks So it is the heat from the combustion of ethanol which gives the engine energy, not the CO2 or H2O gas?

 

[anomalousdecay]

cool thanks So it is the heat from the combustion of ethanol which gives the engine energy, not the CO2 or H2O gas?

Yes. The reaction is explosive and increases pressure in a chamber.

 

[strawberrye]

cool thanks So it is the heat from the combustion of ethanol which gives the engine energy, not the CO2 or H2O gas?

The energy released/needed for any chemical reaction comes from the breakage and formation of bonds of the reactants and products respectively. So essentially, say for ethanol, the breakage of bonds within ethanol is an endothermic process-energy is required, however, the energy released from forming the bonds of carbon dioxide and water which is the products of the reaction is exothermic-energy is released, and because the energy from formation of bonds is greater than the energy needed to break the bonds, this is an overall exothermic reaction, resulting in a net release of energy to power the engine. But you are essentially on the right track. Check out this link for more information http://www.elmhurst.edu/~chm/vchembook/512energycombust.html. Hope this helps, if you have any more chemistry questions, you are welcome to pm me

 

[anomalousdecay]

The energy released/needed for any chemical reaction comes from the breakage and formation of bonds of the reactants and products respectively. So essentially, say for ethanol, the breakage of bonds within ethanol is an endothermic process-energy is required, however, the energy released from forming the bonds of carbon dioxide and water which is the products of the reaction is exothermic-energy is released, and because the energy from formation of bonds is greater than the energy needed to break the bonds, this is an overall exothermic reaction, resulting in a net release of energy to power the engine. But you are essentially on the right track. Check out this link for more information http://www.elmhurst.edu/~chm/vchembook/512energycombust.html. Hope this helps, if you have any more chemistry questions, you are welcome to pm me

Yeah this too.

When the water and carbon dioxide form, there is an overall increase in the energy released, in comparison to the bonds breaking in a hydrocarbon.

I'll start a new discussion in the extra curricular thread.