Le Chatelier's principle

  • #1

Homework Statement



Which process, the formation of the chloro complex, or the formation of the aquo complex, is endothermic?
Co(H2O)62+ (aq) + 4Cl- <---> CoCl4 + 6H2O (l)
Pink ----------------------------- Blue

The Attempt at a Solution



The reaction above is endothermic because it turned blue when it was heated.
But I dont know which one is chloro -complex ( CoCl4)??? and which one is aquo complex (Co(H2O)62)??
is that right? Can I state its endothermic because the color changed? or is there any other reason? o:)
 

Answers and Replies

  • #2
epenguin
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But I dont know which one is chloro -complex ( CoCl4)??? and which one is aquo complex (Co(H2O)62)??


Why yes, the words are saying so.
You ought find worrying the balance of charges between the two sides of your reaction. An examiner :devil: would.


Can I state its endothermic because the color changed? or is there any other reason? o:)
Well indirectly. It is because of what the colour change plus the information given you about the colour of the two complexes tell you how the equilibrium is shifting as temperature is increased.

As it is easy to get confused by an ambiguous-sounding phrase I always prefer to spell out explicitly 'the reaction is endothermic as it proceeds X -> Y '

Le Chatelier's principle can be understood in several ways:

1 Parrot :smile:
2 Isn't it sort of what you would expect?
3 An old schoolmaster of mine used to say there is a law of cussedness in all physics that opposes whatever you try to do. You try to heat the solution and it does a chemical reaction that absorbs heat so you heat it less than the heat you put in would if there was not that reaction. Or you try to cool it and the opposite reaction releases heat to warm it up.
4 Thermodynamic theory - there is a precise equation around heat of reaction, temperature, and equilibrium constant that tell so though you are not onto that yet I guess.
5 You can understand it at molecular level, in principle though not always in detail. The H2O molecules (are charge dipoles) point their fractionally negatively charged O atoms towards the positively charged Co2+, there is an attractive force. Why they are more attracted to it than the Cl- is is not obvious. I said you couldn't always know in detail. Perhaps someone else here can add something. But one factor is these things are always a competition. The Cl- will also be interaction with the positive ends of the H2O molecules around. Another factor is there are a lot more water molecules than Cl- for the Co2+ to interact with. The effect of these factors, these forces, predominates at low temperature. It takes energy for those waters to detach themselves from the Co2+. Now imagine you heat up so all those molecules have a lot more kinetic energy and are moving around faster. So when energy cost is no object they are freer to detach themselves. That would explain why neither complex would be preferred. Why do they get replaced and Cl- binding preferred? - (if they actually are because I don't know exactly where the equilibrium is). I'd imagine because the triangular wedge-shaped waters have to be oriented in more precise orientations round the ion to fit nicely. So they just fit into the water in ways that can be done in more ways, so more often achieved. You won't always know precisely but that is an educated guess and maybe someone here knows more.
6 And then there is an explanation which I would be shot here for saying, but it is a pity they got rid of that nice caloric theory which made some things easier. :uhh::biggrin:
 
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