Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Original post by sportycricketer

Just to confirm the colour changes for the thiosulphate, copper and iodine reaction..

Is it Brown to Colourless/Straw?

I think so yeah, just checked.

Do you have any tips? Omg im dreading this exam!!!! I dont know whether to go through theory or papers. Papers i think

Reply 1182

Original post by viksta1000

which one of the reasons why you have to use born haber to find LE...and because its impossible to measure directly

ahh ok, but sure the standard enthalpy of hydration also needs to have gaseous ions under standard conditions and this is needed to work out the lattice enthalpy?!

Reply 1183

Original post by sportycricketer

Just to confirm the colour changes for the thiosulphate, copper and iodine reaction..

Is it Brown to Colourless/Straw?

yess the iodine has a brown colour, so the CuI precipitate which is in fact white.. appears to be brown :smile:

When thiosulphate reacts with the iodine, its colour 'weakens' and when it gets to a much paler, straw coloured shade, we add starch, this turns blue/black.
When reaching the end point.. the blue/black disappears v. quickly to give a colourless solution :smile:

Reply 1184

Original post by HEC14

yess the iodine has a brown colour, so the CuI precipitate which is in fact white.. appears to be brown :smile:

When thiosulphate reacts with the iodine, its colour 'weakens' and when it gets to a much paler, straw coloured shade, we add starch, this turns blue/black.
When reaching the end point.. the blue/black disappears v. quickly to give a colourless solution :smile:

where does the white precipitate go?

Reply 1185

SmartFool

SOMEONE PLEASE GIVE ME A LIST OF THE COLOURS OF COMPOUNDS/TRANSITION METALS I NEED TO KNOW! I've tried looking through the thread to see if anyone bad posted any but can't find any!

Reply 1186

sportycricketer

Original post by HEC14

yess the iodine has a brown colour, so the CuI precipitate which is in fact white.. appears to be brown :smile:

When thiosulphate reacts with the iodine, its colour 'weakens' and when it gets to a much paler, straw coloured shade, we add starch, this turns blue/black.
When reaching the end point.. the blue/black disappears v. quickly to give a colourless solution :smile:

Original post by Rosi M

I think so yeah, just checked.

Do you have any tips? Omg im dreading this exam!!!! I dont know whether to go through theory or papers. Papers i think

Original post by ruby321

Not colourless, its pale yellow/straw I think and theres a white ppt

Thanks everyone! So basically if a question comes up about the colour change we can either say:

with starch: blue/black to colourless
without starch: brown to pale straw

Is that correct? and do we need to mention about the white ppt anywhere or is that included in the statement above?

Reply 1187

Original post by ruby321

where does the white precipitate go?

i presume it dissolves when we proceed to titrate with the thiosulphate :smile:

Reply 1188

viksta1000

15

Original post by ruby321

ahh ok, but sure the standard enthalpy of hydration also needs to have gaseous ions under standard conditions and this is needed to work out the lattice enthalpy?!

they'll tell you one of them in the question and ask you to work the other out using (clockwise arrows = anticlockwise arrow)

Reply 1189

student777

8

Original post by iSomething

Hi, im stuck on this question from the OCR practice question section about redox equations, I'm having trouble balancing it as I cant seem to arrive at the answer they have at the back of the book:

"Aqueous thiosulfate ions, S2O3^2-, are oxidised to sulfate (VI) ions, SO4^2-, by chlorine gas, which is reduced to chloride ions." Use oxidation states to construct a redox equation."

Here's my attempt but im lost as to what I should be doing to the O atoms (I could add 2H2O but as it would solve the 2O atoms and could balance with 4H+ ions on other side but the book says its 5H2O and 10H+ on other side. How is that possible unless im being really stupid.

When you do a redox equation, before you work out oxidation states you must make sure the things being redoxed are balanced. You haven't done that for Cl in the first equation. Try it again, this time balancing Cl :smile:

Reply 1190

Original post by sportycricketer

Thanks everyone! So basically if a question comes up about the colour change we can either say:

with starch: blue/black to colourless
without starch: brown to pale straw

Is that correct? and do we need to mention about the white ppt anywhere or is that included in the statement above?

I don't think that's entirley correct,
They are two different steps,
The addition of starch makes the end point more clear..
When the colour of 'straw' is still present, it means that the iodine is still present, however the point at which all the iodine has reacted will be too difficult to measure as the colour change is minuscule and hard to see, hence we add the starch to improve accuracy :smile:

Reply 1191

student777

8

Can anyone tell me why we ignore [H2O] when doing a Kstab equation?

Reply 1192

Rosi M

Original post by SmartFool

SOMEONE PLEASE GIVE ME A LIST OF THE COLOURS OF COMPOUNDS/TRANSITION METALS I NEED TO KNOW! I've tried looking through the thread to see if anyone bad posted any but can't find any!

I'll give it a go to test my revision!!! (bad motives, i know :tongue:

)

Vanadium 2+ purple 3+ green 4+ blue 5+ yellow
Chromium 3+ green +6 orange
Iron 2+ green +3 yellow
Cobalt 2+ pink
Manganese +7 purple(dark) +2 pale pink, almost colourless
Copper 2+ blue

Copper hydroxide is pale blue precipitate
Iron hydroxide (2+) is a green precipitate (rusty at surface)
Iron hydroxide (3+) is a brown precipitate
Cobalt hydroxide is a blue precipitate

Please correct me someone 'cos i dont trust myself at all. Someone add anything 'cos im sure i havent put everything down. This is all i can remember :smile:

Reply 1193

Rosi M

Original post by student777

Can anyone tell me why we ignore [H2O] when doing a Kstab equation?

Its because its dissolved in H20 and therefore the concentration of H20 is constant so we just ignore it.

Reply 1194

Original post by student777

Can anyone tell me why we ignore [H2O] when doing a Kstab equation?

because H2O is in a very high conc. i.e. it can be considered as constant in all cases so it is ignored

Reply 1195

Original post by student777

Can anyone tell me why we ignore [H2O] when doing a Kstab equation?

because almost everything is dissolved in water.. so basically that is saying everything is (aq).. Meaning water itself is large in excess.
Alsooo.. it's concentration is virtually constant..
So we don't need to bother with it :smile:

Reply 1196

Original post by Rosi M

I'll give it a go to test my revision!!! (bad motives, i know :tongue:

)

Vanadium 2+ purple 3+ green 4+ blue 5+ yellow
Chromium 3+ green +6 orange
Iron 2+ green +3 yellow
Cobalt 2+ pink
Manganese +7 purple(dark) +2 pale pink, almost colourless
Copper 2+ blue

Copper hydroxide is pale blue precipitate
Iron hydroxide (2+) is a green precipitate (rusty at surface)
Iron hydroxide (3+) is a brown precipitate
Cobalt hydroxide is a blue precipitate

Please correct me someone 'cos i dont trust myself at all. Someone add anything 'cos im sure i havent put everything down. This is all i can remember :smile:

I don't think you need to know the colour of every single ion... just the ppt. with OH- reactions and ligand substitution...

Reply 1197