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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: purplecinnamon on February 08, 2016, 11:02:42 PM

Title: Help solving from change in entropy
Post by: purplecinnamon on February 08, 2016, 11:02:42 PM

So I am trying to solve a problem two different ways, and both ways get very different answers. One is right, one isn't. But I don't understand why one way works, and the other doesn't. Shouldn't they just get the same answer?

So here is the problem:

2H₂S(g) + 3 O₂ (g)  :rarrow: 2H₂O (g) +2 SO₂ (g)

Using standard thermodynamic data at 298K, calculate the entropy change for the surroundings when 1.92 moles of H₂S(g) react at standard conditions.

So the correct way to solve this is by using: ΔS=ΔH/T

And you get: -287.9 J/k

However when I use the equation: ΣΔS=ΣS(products)+ΣS(reactants),
I end up getting -152.9

So why can't we simply use a entropy values to solve for ΛS instead of ΔS=ΔH/T?

Title: Re: Help solving from change in entropy
Post by: mjc123 on February 09, 2016, 04:27:28 AM

Quote

So the correct way to solve this is by using: ΔS=ΔH/T

No it isn't. That is only the case for an equilibrium process, e.g. melting, where ΔG = ΔH - TΔS = 0;   ΔS = ΔH/T.
In other cases it's not true.

Quote

However when I use the equation: ΣΔS=ΣS(products)+ΣS(reactants)

Correctly, that's ΔS = ΣS(products) - ΣS(reactants)
Now does it work?