Amount of catalyst - affects rate of reaction ??

Does the amount/concentration of a catalyst present in a reaction affect the rate at which the reaction proceeds ?

Yes the catalyst helps lower the activation energy for the reaction to occur. And so the more you put in (i.e. higher amount/concentration) will increase the rate of reaction, till either it can't go either higher, or something else is a limiting factor. A bit vague I know, but that's all I know. Sorry if you already knew that (as it's just GCSE) :redface:

Reply 2

Ari Ben Canaan

Original post by MedicalMayhem

Are you sure about that ?

Reply 3

z0tx

Yes it does. Change in concentration affects speed of reaction. In this case, increasing the amount of catalyst increases the speed.

Reply 4

charco

Study Forum Helper

Original post by Ari Ben Canaan

Are you sure about that ?

Enzymes are biological catalysts which follow fairly simple kinetics.

E = enzyme
S = substrate

E + S <==> ES
ES ---> E + products
------------------------------ overall
S --> products

The rate of reaction is proportional to the concentration of the enzyme for low substrate concentrations.

Reply 5

MedicalMayhem

Original post by Ari Ben Canaan

Are you sure about that ?

Well if you're doing something GCSE level like me, then yes that is the explanation they'd want. But I'm sure like always when you get into A level there's a more complicated explanation.

But what I said is like the 'foundations' of what a catalyst does. But I don't know the specifics/higher leveled terminology. Sorry. :colondollar:

Reply 6

foianorsx205

The more concentration a catalyst has, the more the increase in the rate of reaction. Simples!

Reply 7

shengoc

not when your catalyst can also act as a "poison" which could speed up the rate of unwanted reaction

Reply 8

vincentiPad

the catalyst lowers the activation energy, more will speed things up, remember the temperature (if too high) can denature the peptide bonds

Reply 9

lazyswot

The more catalyst is present, the faster the rate of reaction.

Although, for the sake of completeness, that will usually only hold true up to a certain point where the rate plateaus and no additional catalyst will improve the rate any farther. And other factors may affect the reaction rate as well as this. But I don't know what level you're at in Chemistry so I don't know how much to say about it.

Reply 10

Ari Ben Canaan

Original post by lazyswot

I'm doing my A Levels actually.

Thanks for the exaplanation.

Just a quick question : only the most electropositive substances are attracted to the electrodes during electrolysis ?

Reply 11

lazyswot

Original post by Ari Ben Canaan

I'm doing my A Levels actually.

Thanks for the exaplanation.

Just a quick question : only the most electropositive substances are attracted to the electrodes during electrolysis ?

The electropositive substances (eg positive ions) are attracted to the negative electrode and the electronegative substances are attracted to the positive electrode. As far as I know the degree of electropositivity does not make a difference; any ion (or any substance that can gain or lose electrons) can react at an electrode. Which electrode it reacts at depends on whether it wants to gain or lose electrons.

Reply 12

Ari Ben Canaan

Original post by lazyswot

Supposing I am purifying a piece of Copper using the standard electrolysis method.

Whilst under electrolysis some silver is deposited in the form of 'anode sludge'.

Why dont these silver ions (let us assume they exist as silver ions in the sludge) move towards the cathode and hence be reduced ?

Reply 13

lazyswot

Original post by Ari Ben Canaan

I looked this up to make sure I wasn't explaining it wrong, so I might as well C+P what I found :smile:

"In the electrolytic refining of copper, the impure copper is made from the anode in an electrolyte bath of copper sulfate, CuSO4, and sulfuric acid H2SO4. The cathode is a sheet of very pure copper. As current is passed through the solution, positive copper ions, Cu2+, in the solution are attracted to the negative cathode, where they take on electrons and deposit themselves as neutral copper atoms, thereby building up more and more pure copper on the cathode. Meanwhile, copper atoms in the positive anode give up electrons and dissolve into the electrolyte solution as copper ions. But the impurities in the anode do not go into solution because silver, gold and platinum atoms are not as easily oxidized (converted into positive ions) as copper is. So the silver, gold and platinum simply fall from the anode to the bottom of the tank, where they can be scraped up.

Read more: Electrolysis - Refining Of Copper - Anode, Solution, Platinum, Gold, Silver, and Pure http://science.jrank.org/pages/2353/Electrolysis-Refining-copper.html#ixzz1Nl5rPNYI "

"Any metal in the impure anode which is below copper in the electrochemical series (reactivity series) doesn't go into solution as ions. It stays as a metal and falls to the bottom of the cell as an "anode sludge" together with any unreactive material left over from the ore. The anode sludge will contain valuable metals such as silver and gold.

Metals above copper in the electrochemical series (like zinc) will form ions at the anode and go into solution. However, they won't get discharged at the cathode provided their concentration doesn't get too high.

The concentration of ions like zinc will increase with time, and the concentration of the copper(II) ions in the solution will fall. For every zinc ion going into solution there will obviously be one fewer copper ion formed. (See the next note if you aren't sure about this.)

The copper(II) sulphate solution has to be continuously purified to make up for this. "

(edited 12 years ago)

Reply 14

z0tx

Original post by shengoc

not when your catalyst can also act as a "poison" which could speed up the rate of unwanted reaction

Not a very good catalyst in that case, better to simply use another one.

Reply 15

shengoc

Original post by z0tx

Not a very good catalyst in that case, better to simply use another one.

in a two step reaction, a catalyst might speed up one reaction but slows down the other. the net catalytic effect is a balance between the two. industries are constantly supporting academic research in this field.

Reply 16

Melli_

Original post by z0tx

Yes it does. Change in concentration affects speed of reaction. In this case, increasing the amount of catalyst increases the speed.

Yes, Concentration does increase the rate of reaction but that is the concentration of the reactants. The more reactants the more products.

Reply 17

Orangemonkey1

Without giving too many details, I did an experiment today. In the last part you had to decide on how you could change the experiment to show something and then do it. I got that the total volume had to be the same (not a lot of people did), however, the wording was "in the absence of ions" which I took to mean in the absence of some of the ions, not all of the ions. I didn't completely remove the thing (which I now know to be the catalyst of the reaction) but instead I halved it and it did have an affect on the rate. Do you think this will be sufficient to prove the point they wanted us to see or will I be penalised for not completely removing the catalyst?

Reply 18

charco

Study Forum Helper

Original post by Ari Ben Canaan

I'm doing my A Levels actually.

Thanks for the explanation.

Just a quick question : only the most electropositive substances are attracted to the electrodes during electrolysis ?

The terms "electropositive" and "electronegative" do not apply to electrolysis. They are used to refer to the ability to repel electrons or attract electrons, respectively, along a covalent bond.

You should be using the term electrode potential or standard reduction potential for redox processes...