Help~Question (1 Viewer)

[x-ray1018]

Define the molar heat of combustion of ethanol.
thx alot~

 

[insert-username]

Hey there, welcome to BoS. I hope you find the community an enjoyable one to be a part of.

Define the molar heat of combustion of ethanol.

The molar heat of combustion of a substance is the heat liberated when one mole of that substance undergoes complete combustion with oxygen. There's more to the definition, but that's the important part. When ethanol is burned with oxygen, the heat liberated is equal to 1367kJ per mole burned.

Hope that helps,


I_F

 

[Dreamerish*~]

Oh and the formula for working out the molar heat of combustion is:

ΔH = MCΔT​

Where ΔH is the molar heat in joules, M is the mass of water used in grams, C is the constant of water - usually 4.18, and ΔT is the chance in temperature in degrees Celcius.

 

[Mountain.Dew]

Oh and the formula for working out the molar heat of combustion is:

ΔH = MCΔT​

Where ΔH is the molar heat in joules, M is the mass of water used in grams, C is the constant of water - usually 4.18, and ΔT is the chance in temperature in degrees Celcius.

uhhhhhhh that was the formula for the change in heat enthalpy, not the molar heat of combustion. however, ΔH = MCΔT is needed, with an additional equation:

molar heat of combustion = ΔH / (no. moles combusted) = MCΔT/(no. moles combusted), since molar heat of combustion is a rate, in kJ/mol.

hope you enjoy the BOS forums, x-ray1018!

 

[insert-username]

molar heat of combustion = ”H / (no. moles combusted) = MC”T/(no. moles combusted), since molar heat of combustion is a rate, in kJ/mol.

That's exactly the same formula except you divided through by the number of moles combusted. Normally for the molar heat of combustion, you work out ”H = MC”T using the change in temperature of burning one mole, anyway, so Dreamerish is right. But your formula's handy for questions looking to be tricky.

And is anyone else seeing funny? That equation looked right yesterday (i.e. it had triangles), but it's a mess today. Board upgrade problem?


I_F

 

[Mountain.Dew]

mmmmmmmm but most HSC questions wont be so easy as to give u an amount of ethanol that only 1 mol has been combusted.

molar heat of combustion = ΔH / (no. moles combusted) is a more generalised form that is applied to those tricky questions.

 

[Riviet]

And is anyone else seeing funny? That equation looked right yesterday (i.e. it had triangles), but it's a mess today. Board upgrade problem?

It was fine for a while yesterday, and the posts before are still fine, but your quote has the "?"s.

They're still fiddling around with a few things I think. Should be back to normal soon.

Also, just adding on, C is actually the heat capacity of the substance you are heating,
but we usually use C=4.18 (heat capacity of water) since we are usually heating water in these questions.