Stankovic123's chem q's

[zvezda]

They do, it's just that since the equilibrium position is already on the right everything is happening faster, so your getting more product at a higher rate. The back reaction is also happening faster, but the overall reaction is again forward. The only thing a catalyst does is increase the rate at which the whole process happens.

But if your mixture was already in equilibrium and you added the catalyst, wouldnt there be a moment where rate of forward > rate of back?
Unless the catalyst works as one for both the forward and back, not just the forward.

[zvezda]

Ive also been going through q23 in ch 16 review of heinemann. Shouldnt the answers given for parts a and b be the other way around?
As in, answer for a should be 1.3 mol and answer for b should be 0.7mol?

[psyxwar]

But if your mixture was already in equilibrium and you added the catalyst, wouldnt there be a moment where rate of forward > rate of back?
Unless the catalyst works as one for both the forward and back, not just the forward.

Nothing should happen if it's already at equilibrium.

Actually I'm a little confused too. It doesn't seem right to have catalysts that catalyse both forward and back reactions (considering enzymes at least are highly specific catalysts and I can't imagine them being able to catalyse a back reaction as the substrate would be different). But I was told that they catalyse both back and forward reactions. Could someone provide clarification?

[scribble]

^but enzymes do catalyse the forward and back reactions.
theyre not so much specific to a particular substrate, but more to a particular reaction. so if a substrate goes in, and two products come out. those products can go back in as a substrate and come back out as the original product.

[lzxnl]

Firstly, catalysts reduce the energy of the intermediates formed by the same amount. Intermediates are a part of both the forward and backward reactions.
Secondly, rate of any reaction is proportional to . If you reduce the activation energy by the same amount, this increases each rate by the same factor due to the exponential term. Therefore both rates are still equal.

[brightsky]

yeah consider the mechanism involved. although scientists do not yet fully understand how catalysts work, they've produced a rough sketch of what they think happens. so...first you have the reactants moving around at random. chuck in a catalyst. the reactant molecules chemisorb onto the catalyst. electron rearrangement occurs. the bonds between the atoms in the reactants are weakened. therefore less energy is required to break the bonds in the reactants to form the transition state...called the activated complex, which is the state when old bonds have partially broken and new bonds have partially formed. now after the activated complex is formed, two things can happen. you either revert to the reactants, or you proceed to the products. in both cases, the molecules desorb after they are formed. so as you can see...the activated complex is the same for both the forward and the back reaction. the presence of the catalyst therefore 'lowers' the activation of both reactions (although this is technically incorrect because it doesn't actually 'lower' it...rather a different activated complex of lower enthalpy is formed in the presence of a catalyst..but i think vce accepts this way of phrasing it).

[zvezda]

Ah i see now. Thanks guys. Appreciate the help.

One thing though from unit 3, why is AAS restricted to analysis on metals only?

[teletubbies_95]

I don't know for sure but its something to do with specific atomisers in the set-up which require the analytes to be metals(ie.conductors of electricity ) to work . The atomisers excite the electrons , so the amount of specific light rays is measured accurately ,when the monochromator selects the specific wavelength of light.

Dont know for sure!!

[zvezda]

Ah ok, fair enough then!

Also, was just doing some questions from neap, and i came across this question:
"Why is water an unsuitable solvent for infrared spectroscopy?"
The answer is horrible i think. Here it is: "water itself is a strong infrared absorber"

Btw, what does the q even mean by solvent? This isnt chromatography...

[zvezda]

Why can we have multiple peaks in HNMR (eg triplet)?
I know that these are caused by neighbouring H nuclei, but if this were the case, shouldnt the number of added peaks corresponded to the number of neighbouring hydrogen environments rather than number of nuclei?
Because wouldnt it be true that these neighbouring nuclei have the same chemical shift?
Thanks

[teletubbies_95]

With regards to the first question... because a solvent is required for the sample mixture , water can't be added.
This is because water has covalent bonds , which have hydrogen bonding between them ,so they cause vibrations between bonds(O-H stretch)  when infrared light is used, thus causing interference. Solvents like hexane would be used instead.

Too tired to answer the second one ..shall do it tomoz! Hope that helped!

[lzxnl]

Ah i see now. Thanks guys. Appreciate the help.

One thing though from unit 3, why is AAS restricted to analysis on metals only?

There was a VCAA question one year that used AAS on sulfur. It's not restricted to metals.

[zvezda]

There was a VCAA question one year that used AAS on sulfur. It's not restricted to metals.

So when differentiating between UV and AAS, were just looking for which compound absorbs the energy emitted with these techniques? Because theyre basically using the same principles arent they?

[zvezda]

One question with last years VCAA unit 4 exam. With the question involving the calculation of the percentage dissociation of sorbic acid, shouldnt we be subtracting [H3O+] (when water self-ionises) from [H3O+] (calculated from the pH)?
Because we then have to calculate the concentration of the sorbate ion from this, which technically, if done so using [H3O+] (calculated from pH) would be incorrect wouldnt it?

Thanks

[lzxnl]

Do post the question next time on the internet.

I haven't seen the question, but VCAA aren't going to ask a question where the strength of the acid is comparable to that of the acid strength of water. Water's pKa is 14, which is horribly weak; you can safely ignore the dissociation of water unless the acid is weak enough, which I doubt.