February 25, 2017

Homework Help: Chemistry

Posted by A Canadian on Wednesday, February 6, 2013 at 4:39pm.

This isn't exactly a homework question, I'm just trying to understand the concept we learned today...
So say we had:
4Fe + 3O2 → 2Fe2O3 + 1.65 x 10^3 kJ

Does that mean for every 4 mol of Fe, 3 mol of O2, and 2 mol of Fe2O3 we have 1.65 x 10^3 kJ?

And if I were to calculate what the enthalpy change for the formation of 23.6 g of iron(iii) oxide was,
Would it be 23.6g Fe2O3 or 23.6g of 2Fe2O3 that I would use in the equation...? When do you account for the fact that there's 2 moles of Fe2O3?

My confused attempt at solving:

Enthalpy=23.6g Fe2O3 x 1 mol Fe2O3/[(2x56) + (3x16)]g x -1.65x10^3 kJ/1 mol Fe2O3
= -243.38 kJ

Yeah, I'm pretty confused; can someone please clear things up for me??

Also, I guess it's not that important but how come when doing 1mol of Fe2O3/molar mass of Fe2O3 you can just do 1mol/160g instead of 1mol/160g/mol?? Because for molar mass we always use the unit g/mol...

Thanks so much in advance !! It's only the second lesson in grade 12 chemistry and I'm already lost! Not good.. :(

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