Catalysts. (1 Viewer)

[mon_mon]

Catalysts are only there to speed reaction rate, but how do they do this? is it by providing spare electrons for the reaction, or is there something more sinister at work?

 

[ND]

They provide a different pathway for the same product (and the catalyst is reformed in the process). It's different for each reaction.

edit: they don't only speed it up, but lower the activation energy.

 

[mon_mon]

How so? i thought catalysts remained the same at the end of the reaction to the start of it.

 

[Constip8edSkunk]

yep, take dehydration for example

C2H5OH + H2SO4 -> C2H5HSO4 +H20
C2H5HSO4 -> C2H4 + H2SO4

the H2SO4 remains unchanged

 

[ND]

Yeh as i said, they reform.

 

[mon_mon]

So it changes, but by the end, every molecule that 'reforms' is replaced by another identical one?

 

[Frigid]

all of the above, but also sometimes provide reaction surface for the reaction to occur... but i'm not to sure about this point... something about ice crystals helping the depletion of ozone by Cl radicals during spring in antarctica...

 

[mon_mon]

Yeah, same in the Haber process. at least i assume so. Does this have ANYTHING to do with spare electrons?

 

[Frigid]

send a PM to phenol and ask (nicely of course).

 

[mon_mon]

Why Phenol? well, alright i will.

 

[spice girl]

catalysts provide alternate mechanisms for a reaction. just write that...

it lowers activation energy and increases rate of reaction (note that the two are just two sides of the same coin)

nothing to do with electron providing (cos catalysts don't give or take). i can't think of an example, but i guess it may lend or borrow electrons...

 

[inasero]

edit: they don't only speed it up, but lower the activation energy.

its basically the same thing...

you should note
1. provides an alternate pathway for the reaction to occur by lowering the AE, thus having greater propotion of reactants overcome this barrier...

2. provide a surface (if heterogenous) where molecules adsorb onto thereby weakening their intermolecular forces

 

[mon_mon]

ah, that makes alot of sense. thanks

 

[toknblackguy]

agreed with inasero
for eg, in the haber process, the magnetite (Fe3O4) is the catalyst used, and the gases adsorb onto the surface, lowering activation energy and forming products. i do'nt know how it works in reverse reaction..but i'm sure i could get away with just saying that it adsorbs reactants onto it, lowers activation energy, hence reducing the temperature required, which in turn favours forward reaction, resulting in higher yield.

but tell me, i know that catalysts do not affect the position of the equilbrium - but as i just explained..doesn't it indirectly favour yield?

 

[phenol]

spice girl is right - catalysts can "borrow" and "return" electrons

For example in the carboxylation of acetyl-CoA (a key step to lipid production in the body - this is what gets you fat)

the mechanism is complex but keep in mind the 2+ zinc ion stabilises the reaction intermediate (which is negatively charged) by temporarily forming a "bond" (i.e. borrowing an electron) and then breaking that "bond" (i.e. returning an electron)

the mechanism is beyond HSC, just keep in mind this can happen in reality

 

[mon_mon]

I think, token, that different catalysts are used for different results, and thus, yes, it will favour one side, otherwise they wouldn't use it.

 

[phenol]

nope, if it only increases the forward reaction rate and not the reverse then it will effectively change the equilibrium position

this does not happen in reality

 

[mon_mon]

Fair enough. So it just lowers reaction rate and speed up the reaction?

 

[phenol]

of both forward and reverse reaction, yup

perfect

 

[mon_mon]

Cheers. Oh. wait, this just allows it to reach the equilibrium it would normally reach, only quicker?