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# Calculating the Entropy Change associated with the Oxidation of Magnesium watch

1. Hey all,

Somewhat stumped by what should be a very simple question.

For the reaction, Mg(s) + 2H+(aq) -> Mg2+(aq) + H2(g), I've calculated the standard entropy change of system to be -105.5 J/(mol K). Now either I've made a mistake in my math, or my understanding of entropy is lacking, since I would've predicted this change to be positive in line with the state change of solid to gas that's going on.

Could anybody shed some light on this for me?

2. You're right that should cause an increase in entropy - perhaps you got your minus sign muddled (it's easily done)?

What were you given?
3. Perhaps because there are 3 moles of reactants and only 2 of products?
4. My math is,

ΔSΘsystem = ΣSproducts – ΣSreactants
ΔSΘsystem = (–138.1 + 65.3) – (32.7 + 2(0))
ΔSΘsystem = –105.5 J mol^(-1) K^(-1)

Now as far as I'm confident in my ability to read tables, I reckon that's correct. (...I'm not overly confident!!)

LearningMath, that's something that just dawned on me too - but would it really be enough to outweight the solid to gas change?
5. Are you using the Standard molar entropy values of each species? If so, nothing can have an entropy of zero let alone negative?
6. i know that total entropy is entropy of sytem + surroundings. Your surroundings are definitely more ordered. Mg2+ is small and this contributes to high hydration enthalpy, and this would mean more ordered, so decrease in entropy.

however, the evolution of a gas is usually high enough to raise total entropy change to be greater than zero.
7. EierVonSatan, I'm just quoting values from the Nuffield Book of Data, which gives standard molar enthalpy of hydrogen ions in aqeuous solution as zero.
8. I've only just covered entropy, but i've never come across negative values... i tried to see if your standard molar entropy values are correct but i cant find all of them, maybe you'l have more luck:

http://chemtables.tk/
9. (Original post by StandardCarpet)
EierVonSatan, I'm just quoting values from the Nuffield Book of Data, which gives standard molar enthalpy of hydrogen ions in aqeuous solution as zero.
enthalpy entropy
10. (Original post by EierVonSatan)
Are you using the Standard molar entropy values of each species? If so, nothing can have an entropy of zero let alone negative?
hehe, evs, doesn't the third law of thermo state that perfect crystals have entropy of zero v(at absolute zero, of course)?
11. Nonono, I meant entropy. Sorry that was a slip! It gives standard entropy of aqueous hydrogen ions as zero!
12. Seem's you're right http://www.science.uwaterloo.ca/~cch...hermodata.html

I give up

pfft physical chemists breaking their own rules
13. Well that's bizarre.
14. (Original post by StandardCarpet)
Well that's bizarre.
found a useful site, apparently the H+ aq is taken as a reference.

check out

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