molar heat of combustion (1 Viewer)

[lilchezza]

Okiez, i know this has come up in many other threads, but i am studying and really busy, i couldn't find what i needed so here goes
I know it is
rH = -mcrt
And what they stand for, what i wanted to know is when your workin it out, what steps do you take... do you just
do
rH = -mcrt
then find molecular weight
no of moles
and then divide the answer of rH = -mcrt by the no of moles?

 

[Mountain.Dew]

Okiez, i know this has come up in many other threads, but i am studying and really busy, i couldn't find what i needed so here goes
I know it is
rH = -mcrt
And what they stand for, what i wanted to know is when your workin it out, what steps do you take... do you just
do
rH = -mcrt
then find molecular weight
no of moles
and then divide the answer of rH = -mcrt by the no of moles?

m = mass of substance heated or cooled
rH = change in enthalpy
C = heat capacity, usually of water
rt = change in temperature (final T- inital T)

when u said "then find molecular weight
no of moles
and then divide the answer of rH = -mcrt by the no of moles?"

this involves the molar heat of combustion.

that is, molar heat of combustion = rH / (no. moles of substance combusted)

molar heat of combustion is in kj/mol

 

[Riviet]

Remember that the molar heat of combustion is the heat releasd by the complete reaction of 1 mole of a fuel with oxygen. Recall that the units for molar heat of combustion are KJ/mol.
When using the formula for heat of combustion, the answer you get will be always be in joules per amount of the fuel in grams/kilograms, depending on what the units of m were when you substituted it into the equation. For example, if m=1.45g, then your answer after substituting into the equation would be in J/1.45g of the fuel.
We want to how many joules per gram, so we divide by the mass (1.45g in this example) and now your answer will be in J/gram.
We want 1 mole of the fuel so we use the molecular mass of the fuel, say ethanol for example, which is 46g per 1 mole of it. Therefore, we multiply this by your last answer in J/gram to get J/mol. But the units of molar heat of combustion are in KJ/mol so we divide your latest answer by 1000 to convert J/mol to KJ/mol.
I hope that helps.

 

[lilchezza]

lolz riviet you seem to be answering alot of my questions, with the exact info i need, you rock
thankyou both for replying, helped alot.
:wave:

 

[funking_you]

I've created a thread which you can access at:

http://community.boredofstudies.org...-questions-answered-assistance-resources.html

If you look under the Module One section, you will find a fully worked example oof calculating the molar heat of combustion, which included an explanation of all the formulas and what they mean.


Cheers,
George

 

[lilchezza]

DUDE, i had like nearly exactly the same question in my 1/2 yrly today, and am glad to say i done it right . Even with the test out of the way, this info is really helpful, as i'm really bad at molar heat of combustion, so thankyou very muchly!

 

[sando]

Already finished Production of Metals, and for that assesment i came 1st. lol. wat a fluke
But seriously i will hav no idea when it comes to the equations in the half hearly.

Do any of u guys hav techniques u use that make it so easy to apply to any questions u get????

My teacher just said start u should start every question with workin out the number of mols. But then i get stuck lol.

 

[Riviet]

It's The Identification and Production of Materials.

Do any of u guys hav techniques u use that make it so easy to apply to any questions u get????

If it's a question about things reacting, I always write an equation. Write down what I know for the products/reactants and work out the required unknown.

 

[YBK]

you shouldn't think of it as a specif method.

It's more of a logical type q..

 

[lilchezza]

logical doesn't work for me