/sci/ - Science & Math

[Anonymous No.2582012]

Call me stupid, but I'm having trouble understanding what I view as an otherwise piss-easy concept.

Yeah, I'm having trouble using Hess' Law to calculate the standard enthalpy of a reaction. Could one of you good gentlemen kindly explain to a plebeian such as myself how to work through a problem, step-by-step?

I would greatly appreciate your help, and fully understand that I must be missing something here. Google isn't doing me much help, at any rate.

[Anonymous No.2582029]

>implying we're gentlemen

[Anonymous No.2582042]

>>2582029
Well shit, that's just an assumption I had operated on. Fuck.

[Anonymous No.2582058]

>>2582012

Take your reaction. Break it up into the simplest steps as possible. Minimize the coefficients if need be to keep the enthalpy change of the reaction in the standard form given in your tables. Add the simple reactions to each other and cross out the terms common to both sides. This should leave you with the reaction formula. Add the enthalpy change of each reaction and you should have your answer.

[Anonymous No.2582139]

>>2582058
So, would the following problem pan out this way:
2Al(s) + Fe2O3(s) --> 2Fe(s) + Al203(s)

a) 2Al(s) + 3/2O2(g) --> Al2O3(s) [-1669.8kJ]
b) 2Fe(s) + 3/2O2(g) --> Fe2O3(s) [-824.2kJ]

So then I'd just make "a" into "2a" and "b" into "2-b." Then do the same to the delta H's, respectively, and add the delta H's together?

[Anonymous No.2582144]

>>2582139
That would mean the outcome is -1691.2kJ, right? Or am I mistaken?

[Anonymous No.2582222]

Someone please give me some confirmation :D

In return, cool pictures.

[Anonymous No.2582233]

[Anonymous No.2582270]

>>2582144
Bamping for confirmation.

[Anonymous No.2582283]

>>2582144
I got 865.6 but I haven't done one of these in forever. Just substituted it like I was a goon doing a math equation, but idk. What was your process?

[Anonymous No.2582322]

>>2582283
Since I doubled "a" I also doubled its heat energy, making it 2 * -1669.8kJ.

For "b" I doubled it and inverted its value, meaning I did the same to its heat energy, making it -(-824.2kJ) * 2

Then I added those values together.