Ok, so apparently, after getting through Cal II, I can’t do simple algebraic calculations. I’m looking for, in the end, the resonance energy of benzene, and I have a method to get there, but I’m stuck on a certain part.
I have that ΔU of a process that I’m using to calculate this is 180.kJ/g=29214kJ/mol. (The compound has a molecular mass of 162.3, this isn’t where I’m worried about my math.)
So, I need the enthalpy now. I have:
ΔH=ΔU+Δ(PV)
ΔH=ΔU+VΔP (constant volume bomb calorimeter)
ΔH=ΔU+(VRT/V)Δn
ΔH=ΔU+RTΔn
ΔH=???+(.06214kJ/mol K)(298K)(-4.5mol)
Why question marks? How do I get from either kJ/g or kJ/mol to just plain old kJ? And the reverse is true, because I need kJ/g or kJ/mol for the next step, when I reference a table for some information needed to calculate the resonance energy.
(Homework disclaimer: This is technically homework, but it’s for a lab, and I’ve hit a wall, and done the rest of the labwork and calculations myself)
In the equation ΔH=ΔU+RTΔn, the change in internal energy (i.e. ΔU) should be in terms of joules or kJ instead of kJ/mol or kJ/g. It appears as if you already know that.
You need to provide more info.
You say that “The compound has a molecular mass of 162.3.” I assume you meant 162.3 g/mol. Correct? So, how many grams of this compound are in the system?
(180 kJ/g) * (X g) = 180X kJ = ΔU
It’s been a while since I’ve taken thermodynamics. I hope this helps.
That’s really all I do? I thought about that last night, and I didn’t see how it was OK to arbitrarily multiply by the number of grams sample just so the units canceled.
Think about this logically - any reaction that gives off, say 120 kJ/g is going to give off 120 kJ if you react 1g of the substance, 240 kJ if you react 2g, and so on.
So total energy of reaction = specific energy * mass (assuming you have the same units for mass for both specific energy and mass).
Alternatively, specific energy = total energy / mass.
Rearranging, we get: total energy = specific energy * mass.
I’m just putting this out in point form because I think it’ll jog your memory. If you’d like more detail, that can be provided.
Or in more specific terms you are mixing Intensive and Extensive properties. You are looking for H on the left (an extensive property) while using u + v(delta-P) on the right side, (intensive properties). Units rule!!! BTW how do you get greek symbols on the dope?
Point 1: After trying what you said, I still get a positive ΔH[sub]combustion[/sub], which is totally wrong for what I’m doing. This means I made a mistake a thousand steps back somewhere, or our results for the experiment put the resonance energy of benzene somewhere near 7000kJ/mol.
I’m going to go talk to the professor today. I feel like such an idiot having to get help on the calculations for a lab, though!
Point 2: Character map. The Alt code for it is supposedly in there, but I can’t get it to work in hex or in decimal.