OCR B Salters - F335 Exam - 15 June 2011

Reply 100

Cyanohydrin

13

Original post by feelbetter

Does anyone have a copy of jan 11 paper please? :P

http://www.cranbrook.kent.sch.uk/site/chemistry/pastpapers/default.aspx

:biggrin:

Reply 101

Cyanohydrin

13

Do we always get the equilibrium concentrations given when calculating Kc then?

In chemical ideas all of the equilibrium concentrations have been given??

Reply 102

Rei5

could somebody plz provide me with a test answer as too why a coloured compound appears coloured

Reply 103

Cyanohydrin

13

Original post by Rei5

could somebody plz provide me with a test answer as too why a coloured compound appears coloured

you mean transition metals or the extended delocalised structure?

Reply 104

feelbetter

Original post by Cyanohydrin

http://www.cranbrook.kent.sch.uk/site/chemistry/pastpapers/default.aspx

:biggrin:

Brilliant, thanks :smile:

Reply 105

RayM

Can someone please clear up why a coloured compound appears coloured.
Is it because visible light is being absorbed and the complementary colour is reflected (what we see), or visible light is absorbed and light of the same frequency is emitted when the electrons drop back down from the excited state to ground state.

:s-smilie:

Thanks

Reply 106

ABCDemily

2

Original post by RayM

Can someone please clear up why a coloured compound appears coloured.
Is it because visible light is being absorbed and the complementary colour is reflected (what we see), or visible light is absorbed and light of the same frequency is emitted when the electrons drop back down from the excited state to ground state.

:s-smilie:

Thanks

Well, as I understand it, which isn't very well, so someone please help me out here. Obviously, the electrons are extended out over the entire molecule from the p-orbital. When these absorb energy from light (photons) they move up energy levels. When they drop back down, the wavelength that isn't absorbed is reflected. Ie, if red and yellow and green light is absorbed as light hits and moves up energy levels, as they drop back down, the blue light is reflected back out because it isn't absorbed, and therefore MUST be transmitted by the electrons.

Hopefully that helps :3

Reply 107

zahre

Original post by RayM

Can someone please clear up why a coloured compound appears coloured.
Is it because visible light is being absorbed and the complementary colour is reflected (what we see), or visible light is absorbed and light of the same frequency is emitted when the electrons drop back down from the excited state to ground state.

:s-smilie:

Thanks

Its because visible light is being absorbed and the complementary colour is reflected. Basically a compound such as dye is coloured because it absorbs light in the visible region of the spectrum. The electrons in dye molecules are part of an extended delocalised system and electrons in these systems require less energy to excite. This energy is available when the dye absorbs in the visible region. The frequency of light absorbed is related to the energy gap between the ground and excited state by E=hv. So the light that is absorbed has energy that is equal to the energy gap between the two levels. The complementary colour to that being absorbed is transmitted which is the colour we see.

(edited 12 years ago)

Reply 108

limetang

19

Original post by RayM

Can someone please clear up why a coloured compound appears coloured.
Is it because visible light is being absorbed and the complementary colour is reflected (what we see), or visible light is absorbed and light of the same frequency is emitted when the electrons drop back down from the excited state to ground state.

:s-smilie:

Thanks

When the electron is excited it absorbs light of specific frequencies (which corespond to the gap between the energy levels), basically (although I'm not quite sure how this works) light is absorbed of a certain frequency. The complementary frequency is transmitted, and that is the colour you see. (E.g. Red gets absorbed and blue gets transmitted)

Oh and as for the electrons dropping back down to ground state. They do do that, however they do not go from their excited state straight back down to their ground state. They actually go down via vibrational energy levels so no light is emmited but heat is (although you don't need to know this).

Reply 109

tkoki1993

OP

10

Original post by Cyanohydrin

Do we always get the equilibrium concentrations given when calculating Kc then?

In chemical ideas all of the equilibrium concentrations have been given??

you assume that the concentration at the start is equal to the concentration at equilibrium.

In equilibrium calculations, when they ask you to state the assumption you made, thats the answer OR [H+] = [A-]

Reply 110

bajito

I am worried about conjugate base reactions and working out ka. In the specimen paper there was a question that asked you to work out the value of Ka telling you a 0.1 moldm-3 solution of ghb has a ph of 2.9. In the mark scheme they seem to get the number 1.26 x 10-3 moldm-3 out of nowhere for the conc of the H+ ions (and therefore the A- ions). Can someone tell me where it comes from?
Thanks

Reply 111

RayM

Original post by bajito

I am worried about conjugate base reactions and working out ka. In the specimen paper there was a question that asked you to work out the value of Ka telling you a 0.1 moldm-3 solution of ghb has a ph of 2.9. In the mark scheme they seem to get the number 1.26 x 10-3 moldm-3 out of nowhere for the conc of the H+ ions (and therefore the A- ions). Can someone tell me where it comes from?
Thanks

PH 2.9

[H+] = 10^-2.9 = 1.26x10^-3

:smile:

Reply 112

bajito

Original post by RayM

PH 2.9

[H+] = 10^-2.9 = 1.26x10^-3

:smile:

It's all so obvious now! Thanks i was getting worried... :colondollar:

Reply 113

RayM

Original post by bajito

It's all so obvious now! Thanks i was getting worried... :colondollar:

I had the same problem!
Couldnt work it out for days!
:redface:

Reply 114

Jamesrb

So who has memorised all of the organic toolkit? Also, how did you do it?!

Reply 115

DCRichards

Original post by Jamesrb

So who has memorised all of the organic toolkit? Also, how did you do it?!

Nooooppee, I'm not sure it's worth trying to cram it and learn it at this stage. Friedel-Craft, benzene reactions, alcohols to aldehydes/ketones, hydrogenation (from fats and oils) and esterification are the most likely to come up I'd say, so it's worth trying to remember those if you can (and knowing what's ob your data sheet). You could be asked any of them, and OCR love to mess with you and ask very difficult and awkward questions from every corner of the spec, but I think it's unproductive to learn them all and time would be better spent covering topics and ideas.

Reply 116

Jamesrb

Original post by DCRichards

Nooooppee, I'm not sure it's worth trying to cram it and learn it at this stage. Friedel-Craft, benzene reactions, alcohols to aldehydes/ketones, hydrogenation (from fats and oils) and esterification are the most likely to come up I'd say, so it's worth trying to remember those if you can (and knowing what's ob your data sheet). You could be asked any of them, and OCR love to mess with you and ask very difficult and awkward questions from every corner of the spec, but I think it's unproductive to learn them all and time would be better spent covering topics and ideas.

I think that's some solid advice, thanks! :smile:

Reply 117

DCRichards

Original post by Cyanohydrin

http://www.cranbrook.kent.sch.uk/site/chemistry/pastpapers/default.aspx

:biggrin:

Nice one, thanks! Love the name by the way, couldn't really be any more fitting for this thread to be honest.

Reply 118

DCRichards

Original post by Jamesrb

I think that's some solid advice, thanks! :smile:

No worries, don't hold me to it though! If you are gonna try and memorise some of them I'd say try finding something that will remind you. For example, for the Friedel-Craft ones, they are Alkylation and Acylation so you have AlCl3 as a catalyst. You could also think of the Acidified Dichromate(IV) for alcohol reactions as something like those weird coloured alcopop shots you get, as dichromate is orange, and turns green when it oxidises the alcohol. Revision cards are also a good way as long as you look at them all the time :smile:

Reply 119

Jamesrb

Original post by DCRichards

Nooooppee, I'm not sure it's worth trying to cram it and learn it at this stage. Friedel-Craft, benzene reactions, alcohols to aldehydes/ketones, hydrogenation (from fats and oils) and esterification are the most likely to come up I'd say, so it's worth trying to remember those if you can (and knowing what's ob your data sheet). You could be asked any of them, and OCR love to mess with you and ask very difficult and awkward questions from every corner of the spec, but I think it's unproductive to learn them all and time would be better spent covering topics and ideas.

When you say benzene reactions and Friedel-Craft, do you mean that we should know the following off by heart:

Nitration, Sulfonation, Chlorination, Bromination, Alkylation and Acylation including reagents and conditions?

OCR B Salters - F335 Exam - 15 June 2011

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