OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

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Reply 580

chemicalX

Original post by otrivine

perfect

(cu(h2o)6)2+ + 4CL- to give (cucl4)- + 6h20
pale blue solution yellow solution

lovely jubly :biggrin:

do you get why they x 1000 in the calculation, they where just being posh basically :wink:

Reply 581

Dale12

Original post by otrivine

i know how to do the calculations they are really easy but do we need to learn the table by heart?

yeah we do- but watch the video where she exaplains the table and it makes it all seem easy to just work out rather than remembering off by heart..

Reply 582

otrivine

Original post by chemicalX

lovely jubly :biggrin:

do you get why they x 1000 in the calculation, they where just being posh basically :wink:

oh right yes i think i got it i am going to do more practise on that :wink:

lool they dont need to be posh for godness sake !

ok
question
Give 3 propterties of transition metals (3)

Reply 583

The Illuminati

Original post by otrivine

oh right yes i think i got it i am going to do more practise on that :wink:

lool they dont need to be posh for godness sake !

ok
question
Give 3 propterties of transition metals (3)

variable oxidation states
form coloured compounds
act as catalysts

??

Reply 584

otrivine

Original post by Dale12

yeah we do- but watch the video where she exaplains the table and it makes it all seem easy to just work out rather than remembering off by heart..

oh right thank you :smile:

Reply 585

otrivine

Original post by The Illuminati

variable oxidation states
form coloured compounds
act as catalysts

??

correct and how are you mate :wink:

long time nice to speak to you again :smile:

can you ask me as well

Reply 586

chemicalX

Original post by otrivine

oh right yes i think i got it i am going to do more practise on that :wink:

lool they dont need to be posh for godness sake !

ok
question
Give 3 propterties of transition metals (3)

coloured compunds
lots of oxidation states
and they act as catalysts

write an equations between the reaction of fe2+ and naoh. also state the colours(2)

Reply 587

volvicstar

does anyone have good notes for the I2/S2O3 2- redox titration?

Reply 588

LifeIsGood

Original post by splendidsun

When doing Lattice enthalpy calculations, and the value for enthalpy of atomisation of Oxygen is x, would we need to double the value if its O2

No because you the atomisation is for ONE mole so won't be O2 it'll be 1/2 O2. However, say you got Na & O2 you have to have 2Na+ to react with O2- right? Then you need x2 of first ionisation energy of Na+

Reply 589

LifeIsGood

Original post by chemicalX

coloured compunds
lots of oxidation states
and they act as catalysts

write an equations between the reaction of fe2+ and naoh. also state the colours(2)

Got a question for you, Suggest why transition metals act like catalysts

Reply 590

otrivine

Original post by chemicalX

coloured compunds
lots of oxidation states
and they act as catalysts

write an equations between the reaction of fe2+ and naoh. also state the colours(2)

lovely 3/3 easy marks

Fe2+ + 0H- gives Fe(OH)2
green solution pale green PPT and then a rust brown on surface

Reply 591

volvicstar

or any good notes on p,q and r of the spec?

Reply 592

otrivine

Original post by LifeIsGood

Got a question for you, Suggest why transition metals act like catalysts

because they have variable oxidation states and adsorb onto surface materials ?

Reply 593

chemicalX

Original post by otrivine

lovely 3/3 easy marks

Fe2+ + 0H- gives Fe(OH)2
green solution pale green PPT and then a rust brown on surface

awesome! full marks :biggrin:

anyway see you in a bit im going out with family :smile:

bye

Reply 594

otrivine

Original post by chemicalX

awesome! full marks :biggrin:

anyway see you in a bit im going out with family :smile:

bye

sure at what time are u coming back :wink:

enjoy your day

Reply 595

walkbesideme

Original post by volvicstar

does anyone have good notes for the I2/S2O3 2- redox titration?

I think the things to remember with these are as follows: (the following reaction is always in the same format but the oxidising agent can change)

Cu 2+ + 4I- --> CUI + I2

The above is a fairly standard reaction and is true for several different metals (depending upon what you're testing) - you are forming the I2 for the titration with thiosulphate - the key to remember here is that you are reacting the iodine ions with an oxidising agent which is then in turn reduced hence Copper is being reduced from an oxidation state of 2+ to +

The S2O3 2- is a reducing agent therefore will reduce the I2 to I- as such (as far as i'm aware this second equation is always the same)

2 S2O3 2- + I2 --> S4O6 2- + 2 I-

Once you remember these equations it's just a matter of using your standard mole equations

Mass = mole x Mr
Moles= Conc x Volume

(sometimes along with the fact that 1mol of gas occupies 24dm^-3 at rtp) and work backwards.

You tend to be given the amount of thiosulhate which has to be added in cm^-3 and the conc it is - therefore it's fairly easy to calculate the moles added and from this it's just working backwards using the stoichiometry of the equations you're given to find what you're asked for.

It's also worth remembering that usually the titration takes a smaller sample of a greater volume solution from a conical flask - as such where you may have 0.003 mol in 25cm^-3 if this is a sample;
in 250cm^-3 you would have 0.03mol (assuming the solution is throughly mixed) but if this 250cm^-3 is a dilution of 10cm^-3 then the 10cm^-3 would still only have 0.03 mol but would have a much higher concentration.

hope this helps somewhat :smile:

(edited 11 years ago)

Reply 596

mushy1993

could someone please explain q4 e)
http://pdf.ocr.org.uk/download/pp_11_jun/ocr_66606_pp_11_jun_gce_uf325.pdf?

how do we get lactic acid?
and i got [A-]/[HA] = 0.5 , how do i comment on that result?? =/

Reply 597

--NWzD9--

Could someone help me solve this question from an OCR 2009 paper:

A chemist investigates compund E, CxHyOz, as a prospective cosmetic for skin-care

A sample of 4.362g of compound E was analysed by burning in Oxygen to form 5.119g of CO2 and 1.575g of H2O. Mass spectrometry produced a molecular ion-peak of 150

Calculate the molecular formula of Compound E. SHow working out. [5 marks]

Reply 598

walkbesideme

Original post by mushy1993

You get lactic acid from the inherant relationship between PKa and Ph - both are log relationships therefore in this situation the PKa closest to the desired Ph is selected as this will be the simplest buffer to set up at the correct and desired Ph.

The [A-]/[HA] = 0.5 means that

[A-] = 1
[HA] = 2

(as 0.5 = 1/2)
therefore you have relative concentrations of 2:1 for weak acid:conjungate base
and since the conjugate base is almost all supplied by the salt you can assume a 2:1 ratio of weak acid:salt in order to set up you buffer

Reply 599

Killjoy-

Original post by --NWzD9--

Find the moles of E reacting and the moles of combustion products.

Now write out the equation for the combustion of one mole of E (and scale everything else as well.)

Assume that the oxygen is in excess -it should be for complete combustion- and say you have n moles of it in the equation.

Now conserve mass to find n.

Once you have found n you can deduce x, y and z from the equation.

I'll post my working shortly.