why is KJ/g a better measure of 'heat efficiency' of fuels than... (1 Viewer)

[RG11]

molar heats of combustion?

thanks in advance guys

 

[kev-kun]

I believe you mean compare to kJ/mol right? and heat of combustion?
Well kJ/mol is measured by one mole of a substance. For example 1 mol of methanol = about 32 g, ethanol is about 46 g etc. However this isn't a fair comparison in the sense that one mole of a substance is dependent on its MM.

kJ/g is the better measure since you have 1 g of each substance. Comparing one gram of ethanol to one gram of 1-propanol gives a fair test between the two alkanols.

To put it into perspective, say if you have 2 trees, each with varying height. You take one mole of each tree and burn it, you would get different amounts of heat of combustion since one mole of the largest tree is much greater than the little tree. However, if you take one gram of each tree, then you have one gram of tree to test as a comparison. Therefore its a fair test.

Hope that explains a bit?

 

[HSC2014]

To reiterate kev-kun to an extent:

The energy content of fuels is dependent on the stored energy within the intramolecular bonds. Now, fuels vary greatly in molecular mass (think hydrocarbon chains) and hence comparison by molar heat of combustion (kJ/mol) is MISLEADING - not wrong. It's misleading in the sense that the higher molecular mass fuels are more "efficient" since the numbers are bigger but this is not true - they are only bigger as the fuels have larger molecules and hence more stored energy per mol.

Therefore, we use KJ/g to standardise the "heat efficiency" of fuels i.e. the amount of energy released per unit of mass by combustion.

 

[RG11]

ohh, so just to make sure im understanding it properly... a mole of a substance can vary and because of this the heat released is not accurate and there fore me use KJ/g because one gram is a common unit

 

[HSC2014]

Sought of... I wouldn't use the word "accurate" but rather less insightful.

 

[anomalousdecay]

We had in our HSC this year a four marker on finding the specific heat capacity of a substance. It is worth finding out how to do these calculations.
Also make sure you use the form given in the standard integrals table:

J per K per kg. (Or for this case J/g).

Its just a technicality really, and the Board of Studies prefers you to use the right units or else you can lose marks.

 

[RG11]

thanks guys, also one more question does the fact that in the heat of combustion of alkanols experiment the glass conical flask used is an isulator relate to the validity of the experiment or the accuracy?

 

[QZP]

Validity. If the container released a lot of heat energy, the experiment would not be "valid" since it DID NOT achieve the aim of the experiment which was to determine the heat of combustion of various alkanols. It would just give some random value

 

[anomalousdecay]

Validity. If the container released a lot of heat energy, the experiment would not be "valid" since it DID NOT achieve the aim of the experiment which was to determine the heat of combustion of various alkanols. It would just give some random value

This is true. Validity refers to keeping everything of the method the same to accepted standards.

Accuracy is just a measure of how close the experimental value obtained is relevant to the recognised standard values by established science organisations (IUPAC, ANSTO, etc.)

Reliability ---> Repetition

 

[RG11]

thanks everyone, i had my prac exam and i did very well.