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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: #### on March 05, 2019, 02:51:40 pm

Title: HEAT OF COMBUSTION IN DIFFERENT UNITS (???)
Post by: #### on March 05, 2019, 02:51:40 pm

Would anyone be able to explain why some fuels have a greater heat of combustion than other fuels in kJ/mol, however a lower heat of combustion than the same fuels when measured in kJ/gram???
Anyone help would be VERY much appreciate :)

Title: Re: HEAT OF COMBUSTION IN DIFFERENT UNITS (???)
Post by: Monkeymafia on March 05, 2019, 03:32:01 pm

Great question, but just the value of KJ/mol and KJ/g is the same it's just that they are in different forms.

For example, CH4 has a molar heat of combustion of 890 KJ/mol and a heat of combustion of 55.6 KJ/g.

The difference is that 890KJ is produced per mole of CH4 and 55.6KJ is produced per gram of CH4.

Hope that makes sense.

Title: Re: HEAT OF COMBUSTION IN DIFFERENT UNITS (???)
Post by: #### on March 05, 2019, 04:54:22 pm

Quote from: Monkeymafia on March 05, 2019, 03:32:01 pm

Great question, but just the value of KJ/mol and KJ/g is the same it's just that they are in different forms.

For example, CH4 has a molar heat of combustion of 890 KJ/mol and a heat of combustion of 55.6 KJ/g.

The difference is that 890KJ is produced per mole of CH4 and 55.6KJ is produced per gram of CH4.

Hope that makes sense.

Thank you! :)
Are you able to explain why for example, 1 gram of H2 releases 141 kJ of energy, which is more than 1 gram of CH4, which releases 55.6 kJ of energy. Whereas, in kJ/mol the CH4 releases more energy (890kJ) compared to H2 (which releases 282kJ)

Does that make sense?

Title: Re: HEAT OF COMBUSTION IN DIFFERENT UNITS (???)
Post by: Monkeymafia on March 05, 2019, 05:43:33 pm

Quote from: #### on March 05, 2019, 04:54:22 pm

Thank you! :)
Are you able to explain why for example, 1 gram of H2 releases 141 kJ of energy, which is more than 1 gram of CH4, which releases 55.6 kJ of energy. Whereas, in kJ/mol the CH4 releases more energy (890kJ) compared to H2 (which releases 282kJ)

Does that make sense?

Quote from: #### on March 05, 2019, 04:54:22 pm

Thank you! :)
Are you able to explain why for example, 1 gram of H2 releases 141 kJ of energy, which is more than 1 gram of CH4, which releases 55.6 kJ of energy. Whereas, in kJ/mol the CH4 releases more energy (890kJ) compared to H2 (which releases 282kJ)

Does that make sense?

Ah I see what you mean. My guess is that CH4 has a higher molar mass and so, when you convert Kj/g to Kj/mol it is a much larger number than converting kj/g to kj/mol for H2. Same thing when going backwards from kJ/mol to Kj/g, dividing a large number by a large molar mass vs dividing a large number by a small molar mass.

Think that might be it.