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Topic: Two practice problems - Heat, entrophy and spontaneity  (Read 3020 times)

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Offline kensher

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Two practice problems - Heat, entrophy and spontaneity
« on: May 14, 2016, 05:48:32 PM »
There is two practice problems which I don't really get. I will just jump right to it.

Practice problem number 1

Which of these spontaneous procecces below are:

2) Magnesium bonds burns

a) exothermic and leads to an increase in entropy in the system
b) exothermic and leads to a decrease in entropy in the system
c) endothermic and leads to an increase in entropy in the system
d) Neither exothermic nor endothermic, but leads to an increase in entropy in the system

I figured that the answer had to be b) based on elimination; I also checked it with the answer from the student book. However, I don't understand how you can see that the reaction is «exothermic and leads to a decrease in entropy in the system» based on the text. Can anybody please clarify this for me?


Practice problem number 2

Show with calculations if this reaction goes spontaneously at room temperature (25oC)

2Al(s) + 3Cl(g)  :rarrow: 2AlCl3(s)

ΔHf(AlCl3(s)) = -706 kj/mol
S(Al(s)) = 28,0 j/(mol * K)
S(cl2(g)) = 223 j/(mol * K)
S(AlCl3(s)) = 109 j/(mol * K)

After some calculations, I got:

ΔG = -1412 kj -(298K * (-0.507 kj/K)) = -1260,1 kj

The process goes spontaneously at room temperature.

However the answer in my student book was: - 1560 kj, and I was wondering if the book itself had made a mistake?

Offline mjc123

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #1 on: May 16, 2016, 04:26:01 AM »
1. How did you "figure that it had to be b) based on elimination" if you don't understand the problem? What exactly don't you understand?

2. It looks like the book has made a mistake; they appear to have got the sign of the entropy change wrong.

Offline Enthalpy

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #2 on: May 16, 2016, 05:38:33 AM »
I don't like the wording "goes spontaneously". Whatever the entropy, Gibbs energy and any thermo quantity, if you put a chunk of aluminium in chlorine at room temperature, nothing spectacular will happen.

You could make the same computation with aluminium in air and deduce that an aluminium chunk ignites spontaneously - but obviously aluminium doesn't understand the computation.

Perhaps "thermo quantities would permit the reaction to proceed under some very special conditions". It may require to alloy the aluminium with gallium to destroy the oxide layer for good, grind the aluminium to a fine powder, and similar measures.

Offline kensher

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #3 on: May 16, 2016, 07:43:18 AM »
1. How did you "figure that it had to be b) based on elimination" if you don't understand the problem? What exactly don't you understand?

2. It looks like the book has made a mistake; they appear to have got the sign of the entropy change wrong.

1)
There were also three other reactions, and I figured out the answer to these first. Therefore, the only answer left was «b) exothermic and leads to a decrease in entropy in the system». All the reactions were:
1) exhaust gas from a motor engine combines with air
2) Magnesium bonds burns
3) Water evaporates from a pothole
4) A candle burns

I don't understand why the reaction «2) Magnesium bonds burns» is «b) exothermic and leads to a decrease in entropy in the system».
It seems as the book expects you to figure that out just based on the text alone (without being presented with a balanced equation).

2)
Glad to know that the fault was assigned to the book, and not me :)
« Last Edit: May 16, 2016, 08:08:49 AM by kensher »

Offline mjc123

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #4 on: May 16, 2016, 10:23:47 AM »
First, "magnesium bonds burns" is eaningless. Bonds don't burn. Magnesium burns. Burning is an exothermic reaction - if something is endothermic, it doesn't burn. Why does it lead to a decrease in entropy? Can you see what is different from a candle burning?

Offline kensher

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #5 on: May 16, 2016, 10:39:34 AM »
No, I can't see why the entropy is decreasing. That's basically what I don't get.

Offline mjc123

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #6 on: May 16, 2016, 12:51:37 PM »
What is the reaction for the burning of magnesium?

Offline kensher

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #7 on: May 16, 2016, 01:20:42 PM »
I would guess:

2Mg (s) + 02 (g) :rarrow: 2MgO

Edit:

And I go from 3 moles to 2 mole and thereby reducing the entropy. Got it now :)
Thanks for asking the leading questions!
« Last Edit: May 16, 2016, 04:35:05 PM by kensher »

Offline mjc123

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #8 on: May 17, 2016, 04:30:20 AM »
More particularly, you go from 1 mole of gas to 0 mole. Gases have much higher entropy than liquids or solids, so if you change the number of moles of gas, that usually determines the sign of the entropy change.

Offline Enthalpy

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Re: Two practice problems - Heat, entrophy and spontaneity
« Reply #9 on: May 17, 2016, 11:40:58 AM »
I don't see why magnesium is always cited as a burning metal. Except maybe as ultrafine turnings, it's badly difficult to light.

I tried once with a small ingot: I had to use an acetylene burner, heat the metal to orange, then I could stop the acetylene and the magnesium did burn in pure oxygen blast, with a pale green flame far less spectacular than steel cutting. As soon as I turned the oxygen off, the fired choked under its ash and stopped.

Neither had I ever an accidental magnesium fire when milling nor turning magnesium, despite high cutting speed and low feed. Under such conditions, steel chips catch fire, magnesium doesn't. And if one must fear burning chips when machining, he should be worried of titanium.

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