Enthalpy? (1 Viewer)

[lourai*87]

I dont know if this is HSC or not, but hopefully someone can help anyway. Its for Uni, but with any luck there might be someone... *hopes*

Ok, we are putting NH₄NO₃ with water, measuring some temperatures and then calcualting "enthalpy change for dissolution of ammonium nitrate using spec. heat capacity water and mass of solution".

Firstly, i presume that 50cm³ water does not equal 50g. Mistake number 1. So, how do i find this out?! (i have continued on though)

Then, i have my enthalpy change for the solution (1588.21 J -- wrong due to above problem), and then for each gram of ammonium nitrate (dividing this by the mass of NH₄NO₃) <--- its positive for endothermic right?

Did this twice to get the mean. All is ok-ish so far. So my enthalpy change for NH₄NO₃ is 319.87 J/g (once again, i know its wrong).

Now they want this in kJ/mol. Thats thrown me completely, I cant find conversions anywhere. i can do kJ/g but have no idea.

Damn, and i thought uni was making me feel smart. Any help would be awesome.

 

[lilchezza]

I dont know if this is HSC or not, but hopefully someone can help anyway. Its for Uni, but with any luck there might be someone... *hopes*

Ok, we are putting NH₄NO₃ with water, measuring some temperatures and then calcualting "enthalpy change for dissolution of ammonium nitrate using spec. heat capacity water and mass of solution".

Firstly, i presume that 50cm³ water does not equal 50g. Mistake number 1. So, how do i find this out?! (i have continued on though)

Then, i have my enthalpy change for the solution (1588.21 J -- wrong due to above problem), and then for each gram of ammonium nitrate (dividing this by the mass of NH₄NO₃) <--- its positive for endothermic right?

Did this twice to get the mean. All is ok-ish so far. So my enthalpy change for NH₄NO₃ is 319.87 J/g (once again, i know its wrong).

Now they want this in kJ/mol. Thats thrown me completely, I cant find conversions anywhere. i can do kJ/g but have no idea.

Damn, and i thought uni was making me feel smart. Any help would be awesome.

Okiez, kinda touched on this in prelim.

Firstly, yes endothermic reactions will be positive.

Secondly, kJ/mol issue, now we didn't do this BUT i have found an example, i don't know if it will help but hey.

eg/ 2g of a salt, of molecular weight 50, is dissolved in 100mL of water in a calorimeter. The temp drops from 17.0 C to 15. Calculate the enthalpy change in kJ, the enthalpy change per gram and the enthalpy change per mole.

rH = mCrT = 100 X 4.18 X 2 = 836J = 0.836kJ

per gram rH = 0.836kJ/2 = 0.418kJ/g (just divided it by two)

per mole rH = 0.418kJ/g = 50 X 0.418kJ/g = 20.9 kJ/mol

so no idea if that will help you, but the answer is in kJ/mol so yea

also the 50cm^3 to 50g thing, i would have presumed the same thing and been wrong also lolz. The formula we were taught to use to determine this was

grams=molarity X litres X molecular weight

 

[lourai*87]

I love you. Ok...well you know what i mean. It helps, it really really does. Thank you so much. You have no idea how stressed i am (well maybe my display of relief/affection suggests it...but nevertheless, thanks )

 

[lilchezza]

lolz.. I'm glad it helped, i nearly didn't reply coz' I thought it was wrong!