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Topic: how to determine if a chemical reaction is spontaneous  (Read 6160 times)

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Offline Isomer

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how to determine if a chemical reaction is spontaneous
« on: July 16, 2006, 09:02:09 PM »
How do I figure out which is the strongest oxidizing agent to the weakest when I have a question that consists of only variables? I know how to use the table...but when my teacher gives me a question with only variables ....I'm stumped!


« Last Edit: July 17, 2006, 09:56:02 PM by geodome »

Offline sdekivit

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Re: SPONTANEITY RULE..
« Reply #1 on: July 17, 2006, 01:10:10 AM »
give an example and show where you're stumped ;) then there are people who can help you

Offline Isomer

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Re: SPONTANEITY RULE..
« Reply #2 on: July 17, 2006, 07:39:34 PM »
I'll try. It's hard because I can't use a subscrpt. Here we go..

REACTION....

A2 (aq) + 2B- ---> B2 (aq) + 2A- (aq)

C2(aq) + 2B- (aq) ---> no reaction

D2 (aq) + 2A- (aq) ---> 2D- (aq) + A2 (aq)


Anything after a letter is an small number, and the negatives you see behind some of the terms are ion charges.

The question is when these elements are arranged in order from strongest oxidizing agent to the weakest oxidizing agent, the order is...

Using these...A2, B2, C2, D2

The answer is D2, A2, B2, C2.

I understand why C2 is last, because there is no reaction, but I don't know how to figure out the other three!

Offline Donaldson Tan

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Re: how to determine if a chemical reaction is spontaneous
« Reply #3 on: July 17, 2006, 09:59:01 PM »
In general, if dG for a chemical reaction is less than zero, then the reaction is said to be spontaneous.

For aqueous redox system, Ecell = -dG/nF (where n is the number of moles of electron transferred for 1 mole of reaction, and F is faraday's constant).

This means Ecell is positive for a spontaneous redox system

A2 (aq) + 2B- ---> B2 (aq) + 2A- (aq)

C2(aq) + 2B- (aq) ---> no reaction

D2 (aq) + 2A- (aq) ---> 2D- (aq) + A2 (aq)

With reference to your reaction system, can you see any similarity to the displacement reactions of halogens, ie.
1. F2 + 2Cl- => Cl2 + 2F-
2. Cl2 + 2Br- => Br2 + 2Cl-
3. Br2 + 2I- => I2 + 2Br-

In terms of oxidising power, F > Cl > Br > I
« Last Edit: July 17, 2006, 10:04:25 PM by geodome »
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Offline Isomer

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Re: how to determine if a chemical reaction is spontaneous
« Reply #4 on: July 18, 2006, 12:10:08 AM »
Thanks for the responses! I figured it out finally!!!! ;)

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