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AJKOER
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Aga:
I agree.
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Here is the technical definition of a catalyst (source: http://chemistry.about.com/od/chemistryglossary/a/catalystde... ):
"A catalyst is a subtance that increases the rate of a chemical reaction by reducing the activation energy, but which is left unchanged by the
reaction."
Here is a different source with discussion ( link: http://www.chemicool.com/definition/catalyst.html ), to quote:
"Definition of Catalyst
A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction; hence a catalyst can be recovered chemically
unchanged at the end of the reaction it has been used to speed up, or catalyze.
Discussion
In order for chemicals to react, the species involved in any reaction must undergo a rearrangement of chemical bonds.
The slowest step in the bond rearrangement produces what is termed a transition state - a chemical species that is neither a reactant nor a product,
but is an intermediate between the two. Energy is required to form the transition state. This energy is called the Energy of Activation, or Ea.
Reactants with energy lower than Ea cannot pass through the transition state to react and become products.
A catalyst works by providing a different route, with lower Ea, for the reaction. In any given time interval, the presence of a catalyst allows a
greater proportion of the reactant species to acquire sufficient energy to pass through the transition state and become products.
Catalysts cannot shift the position of a chemical equilibrium - the forward and backward reactions are both accelerated so that the equilibrium
constant Keq is unchanged. However, by removing products from the reaction mixture as they form, the overall rate of product formation can in practice
be increased."
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Now, here is a real world test. You have Ethyl alcohol which is not completely devoid of water. You add Aluminum, but no reaction. You add a small
amount of Iodine. The reaction starts. Assume all of the iodine can be recovered as an iodide. Is the iodine a catalyst? Is the iodide a catalyst
here? If heating the iodide liberates I2, does that constitute "recovered chemically unchanged"? How do you define here "the end of the reaction" when
there are reactions (plural)?
[Edited on 5-7-2014 by AJKOER]
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blogfast25
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Quote: Originally posted by AJKOER  |
Now, here is a real world test. You have Ethyl alcohol which is not completely devoid of water. You add Aluminum, but no reaction. You add a small
amount of Iodine. The reaction starts. Assume all of the iodine can be recovered as an iodide. Is the iodine a catalyst? Is the iodide a catalyst
here? If heating the iodide liberates I2, does that constitute "recovered chemically unchanged"? How do you define here "the end of the reaction" when
there are reactions (plural)?
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If the initial I2 is recovered as iodide then I2 is not a catalyst in the strict sense of the word.
Nor is the iodide a catalyst here: it forms by reacting I2 with the aluminium, removing its oxide layer and making it more reactive towards the
absolute alcohol. Had you added iodide instead of iodine no such effect would have been observed.
How can heating iodide yield iodine without oxidation? Only at temperatures where an iodide begins dissociating is this possible:
MI(g) + high T < === > M(g) +1/2 I2(g), shifts to the right at high T, for an exothermic iodide.
For a catalyst to qualify as a catalyst it needs to remain chemically unchanged while on the job. Iodine does change (and quite dramatically too!)
when it gets this ethanol Al lark going: it becomes iodide, permanently. Not a catalyst. Subsequent, SEPARATE, recovery of the iodide as iodine isn't
part of the (non-catalysed) reaction.
[Edited on 5-7-2014 by blogfast25]
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AJKOER
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Blogfast obviously known his stuff, but he did miss a small point. The catalyst is defined in reference to a reaction. Did anyone notice that I did
not precisely define the reaction of interest?
For example one could argue that I was looking at constructing a galvanic cell to remove the remaining water in the alcohol. Aluminum is the anode,
oxygen surface contact is the cathode and Aluminum iodide is the electrolyte! As the latter remains unchanged and promotes the reaction, it may meet
our technical definition of a catalyst for the galvanic cell, where the formation of Al(OH)3 also removes water. Now, it is most likely true that this
would be an inconsequential reaction in the overall reactions, but that not in the definition either!
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blogfast25
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| Quote: Originally posted by AJKOER | | For example one could argue that I was looking at constructing a galvanic cell to remove the remaining water in the alcohol. Aluminum is the anode,
oxygen surface contact is the cathode and Aluminum iodide is the electrolyte! As the latter remains unchanged and promotes the reaction, it may meet
our technical definition of a catalyst for the galvanic cell, where the formation of Al(OH)3 also removes water. Now, it is most likely true that this
would be an inconsequential reaction in the overall reactions, but that not in the definition either! |
You're playing with words. The electrolyte does not take part in the redox reactions of the cell, it can therefore not be considered a catalyst.
Catalysts always take part in the reaction they catalyse, no matter how subtly. The iodide ions do not, they are merely charge carriers.
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