Molar heat of combustion (1 Viewer)

[tennille]

I'm having a bit of trouble with the molar heat of combustion. Say the HSC exam supplies you with data on molar heat of combustions of 3 fuels (methanol, ethanol and propanol). If they asked you what would be the most suitable fuel out of those three, would it be the one with a lower molar heat of combustion, or a higher molar heat of combustion?

 

[Xayma]

Most suitable fuel would be the one with the highest heat of combustion per gram.

It is the one that releases the most energy per gram of fuel burnt.

Molar heat of combustion isn't as good an indicator as the molar mass changes significantly.

 

[Tommy_Lamp]

it makes sense, you would want the fuel that gives you the most energy per gram, that way its more efficient.

 

[nit]

Technically it's the one with lower heat of comb, since heat of combustions have a negative value

 

[xeriphic]

I'm confused now

 

[wind]

Technically it's the one with lower heat of comb, since heat of combustions have a negative value

lol

It's the fuel that gives the most negative number. There we go!

 

[sHin]

My understanding is that molar heat of combustion of is a positive value.

Enthalpy change (delta H) is a negative value, as it directly relates to the energy of the products, so molar heat of combustion is = - (enthalphy). Can someone confirm this?

 

[helper]

Enthalpy change can be positive (endothermic reaction) or negative (exothermic).
Combustion is an exothermic process so the enthalpy change will be negative.

However, by convention molar heats of combustion have been traditionally quoted as positives even though they are a negative enthalpy change.

 

[Shards]

just use the absolute value

so, whichever one gives you the biggest number without looking at the sign (provided that the signs are the same, actually)

 

[Gesus]

well, you cant exactly have a combustion that sucks up energy

 

[d_elmo]

well considering the molar heat of combustion is defined as the energy required to completely combust one mole of a substance, id say it would be the lowest one.

 

[smallcattle]

huh?? someone say the highest while others say the lowest... which one should it be??

 

[wind]

The substance that gives the greatest discrepancy from zero.

 

[~*HSC 4 life*~]

largest in magnitude...absolute valu whatever you like to call it. the negative sign is almost like "the units" indicating it is exothermic, that is all.

and as we know, combustion is an extothermic process .: we take the fuel which has the (highest magnitude) KJ/mol

 

[angelicdevil]

is it just me? or did all that just majorly confuzzle other ppl too?

ive done a practice question like the orginal question posted on this thread...
the question was...
a student used each of the follwing spirit burners (being methanol, ethanol and propanol) to determine the hear of combustion of each fuel. they did this by heating 200ml of water in a beaker over each burner for 2 minutes...
the results are:
fuel methanol ethanol propanol
mass of burner before burning (g) 212.11 213.27 219.85
mass of burner after burning (g) 211.47 212.27 219.25
mass of fuel used (g) 0.64 0.69 0.60
mass of water heated (g) 200.00 200.00 200.00
temperature of water before heating (°C) 18.00 18.00 19.00
temperature of water after heating (°C) 34.00 40.00 43.00

to find out which was the most suitable fuel out of those three using calcs..

eg..for methanol...
C2H5OH (l) + 3O2 -----> 2CO2 (g) + 3H2O (L)
h=mc T
= 200 x 4.2 x 22
= 18.48 kJ
H= h / m/M
= 18.48 / (0.69/46)
= 1246 kJ mol -¹ released...
so would i have to do this for each spirit and then which eva has the least amount of heat released would be the most effiecient?

 

[Paroissien]

You would, and it is the most amount of energy released that is the most effecient.
Answer would be propanol

 

[tina_goes_doo]

My understanding is that molar heat of combustion of is a positive value.

Enthalpy change (delta H) is a negative value, as it directly relates to the energy of the products, so molar heat of combustion is = - (enthalphy). Can someone confirm this?

The way you describe it is from the maquarie study guide. Just had a look at that today

But they got a negative answer because they subtracted the final temperature from the initial to get a positive change in temp.

It really depends on the method you use - initial minus final or the other way around. If you get a negative molar heat of combustion then it is exothermic with the first one.

 

[Wilko2564]

Im pretty sure its always final minus intial.