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8.45- why do you multiply?

Posted: Mon Jan 22, 2018 12:44 pm
by sofiakavanaugh
The question is
4C(s) + S8(s) ---> 4CS2 (l) deltaH=358.8 KJ, and its asking how much heat is absorbed at constant P with 1.25 mol S8

My understanding of this is that the 358.8 KJ implies that it is for 1 mol of rxn even though it doesn't say KJ/mol, and so you have to use the 1.25 mol of S8 to find the heat. In the answers it has you multiply the deltaH by 1.25 mol, but I would think you would divide it:
(358.8 KJ/1 mol rxn)(1 mol rxn/1.25 mol S8), so that the mols of rxn cancel and you are left with the KJ per mol of S8. Can someone please explain to me why this logic is incorrect please, and how we would know when to multiply and when to divide in problems like these?

Re: 8.45- why do you multiply?

Posted: Mon Jan 22, 2018 12:51 pm
by Clara Rehmann 1K
In the original equation, we have 1 mol of S8. We want to calculate the enthalpy when there's 1.25 mol of S8. To get 1.25 mol S8, you multiply 1 mol of S8 by 1.25. Multiply the reaction's enthalpy by the same amount to get the enthalpy for 1.25 moles.

Re: 8.45- why do you multiply?

Posted: Mon Jan 22, 2018 2:01 pm
by Chem_Mod
Enthalpy is an extensive property, meaning that its value is different (dependent) on how much you have. There fore if there's a certain amount of enthalpy released or absorbed for every mole of a reaction or substance we should expect more enthalpy for more than one mole.

Re: 8.45- why do you multiply?

Posted: Mon Jan 22, 2018 8:58 pm
by Nicole Anisgard Parra 2H
The delta H value tells you the kj of heat per 1 mol of S8/4 mol of C/4 mol of CS2. If you were to have the same reaction occur with 1.25 moles of S8, you know the kj required for 1 mol (it was given to you). Thus, you would multiply the delta H value by 1.25 to get the kj for 1.25 moles of S8.

Re: 8.45- why do you multiply?

Posted: Mon Jan 22, 2018 11:29 pm
by Veronica Rasmusen 2B
The delta H of 358.8 tells us the heat absorbed per mole of every substance in the reaction, this would mean there is 358.8 Kj/4 mol CS2, 358.8 KJ/4 mol C, and 358.8 KJ/ 1 mol S8. Therefore to find the heat absorbed by 1.25 mol of S8 we would multiply (358.8 KJ/ 1 mol S8) times (1.25 mol S8)

Re: 8.45- why do you multiply?

Posted: Tue Jan 23, 2018 2:07 pm
by 404995677
so for part B the solutions manual divides the enthalpy of reaction by 4 mol C (358.8kJ/4 mol C) to find the enthalpy of one mole of C?