Ed excel Chemistry A2 12th June, Wednesday

Original post by Dolphino

Wow you've done so many exams! My first exam is on Tuesday and it's biology so yeah I'm feeling quite nervous.

In my chem mock we were given the pink booklet and it was pretty much the same as the one I use at home: http://www.edexcel.com/migrationdocuments/GCE%20New%20GCE/GCE_Chemistry_Data_booklet_Issue_2.pdf

I'm not sure of that helps you at all! But good luck once again, keep going strong!

Thanks, but this is the one I had too :frown:

The pink one I received in Jan was totally different... and one of the compounds was such a pain to find ! (literally took like over 5 minutes to figure it out, because I was trying to make do with other data & figure out the standard entropy myself :angry:

).

And thanks, best of luck to you too :smile:

I think it's about time I move onto past papers!

Original post by Dolphino

What was the pH when 24.95 cm3 NaOH(aq) had been added to 25of 1.00 mol dm-3 cm3 of 1.00 mol dm-3 HCl(aq).

Can someone work this out and tell me what they get- I get 4.3 do u guys agree?

Assuming the concentration of NaOH is also 1.00 mol dm-3 ... I got 4.3 as well :smile:

Reply 63

SKK94

1

Original post by posthumus

Assuming the concentration of NaOH is also 1.00 mol dm-3 ... I got 4.3 as well :smile:

Could you please explain how you did it? :smile:

Reply 64

Original post by Dolphino

What was the pH when 24.95 cm3 NaOH(aq) had been added to 25of 1.00 mol dm-3 cm3 of 1.00 mol dm-3 HCl(aq).

Can someone work this out and tell me what they get- I get 4.3 do u guys agree?

Original post by SKK94

Could you please explain how you did it? :smile:

Sorry I think I've done it wrong :colondollar:

Just realized I logged the moles of H+

Anyway... find moles of both OH- & H+ ... find the difference in moles... this difference is the extra H+ making the solution acidic. I got 5 x 10^-5 here. Now I negative logged this when I am supposed negative log the concentration.

The concentration = mole / volume = 5 x 10^-5 / 0.02495 + 0.025 = 1 x 10^-3 moldm^-3

pH= 3

Reply 65

Dolphino

Original post by posthumus

Sorry I think I've done it wrong :colondollar:

Just realized I logged the moles of H+

Anyway... find moles of both OH- & H+ ... find the difference in moles... this difference is the extra H+ making the solution acidic. I got 5 x 10^-5 here. Now I negative logged this when I am supposed negative log the concentration.

The concentration = mole / volume = 5 x 10^-5 / 0.02495 + 0.025 = 1 x 10^-3 moldm^-3

pH= 3

Yes that's what I realised I've done wrong too!

Reply 66

SKK94

1

Original post by posthumus

Sorry I think I've done it wrong :colondollar:

Just realized I logged the moles of H+

Anyway... find moles of both OH- & H+ ... find the difference in moles... this difference is the extra H+ making the solution acidic. I got 5 x 10^-5 here. Now I negative logged this when I am supposed negative log the concentration.

The concentration = mole / volume = 5 x 10^-5 / 0.02495 + 0.025 = 1 x 10^-3 moldm^-3

pH= 3

Thank you!
I kept using the difference in concentration of OH- and H+
xD

Reply 67

OP

does anyone wanna revise?

Reply 68

Original post by AS01

does anyone wanna revise?

What indicator would you use for weak acid & weak base titration ? :biggrin:

Reply 69

OP

Original post by posthumus

What indicator would you use for weak acid & weak base titration ? :biggrin:

yay! one is here :biggrin:

you don't

coz the vertical section of graph is rarely even there.

give an example of addition elimination reaction of acyl chloride.

Reply 70

Original post by AS01

yay! one is here :biggrin:

you don't

coz the vertical section of graph is rarely even there.

give an example of addition elimination reaction of acyl chloride.

Yup ! Now I know your well prepared, aha :tongue:

Acyl chloride + H2O ?? :smile:

Explain why LiAlH4 does not reduce the C=C bond

Reply 71

OP

Original post by posthumus

Yup ! Now I know your well prepared, aha :tongue:

Acyl chloride + H2O ?? :smile:

Explain why LiAlH4 does not reduce the C=C bond

yup!
coz its not polar.

How GC work?

Reply 72

Original post by AS01

yup!
coz its not polar.

How GC work?

Hehe, your evil... I told you that it's the one I need to go through! :tongue:

But tell me if I've missed out any points:
- if Liquid is vaporized and carried by the mobile phase (which is an inert gas I think)
- pushed through with pressure and through stationary phase
- Stationary phase consists of solvent absorbed onto solid
- The interaction between mobile & stationary phase determine how much time it takes for the mobile phase to go through, this is called the retention time.
- Retention time is unique to specific compounds, look up what type of compound it is using data resources.
- Structure can also be found using Mass or IR spectroscopy :smile:

This is also useful to purify some liquids?
I think I may have mixed HPLC with GC here :tongue:

Explain with reference to lattice & hydration enthalpies why hydroxides are more soluble as you go down group 2 & the opposite is the case for group 2.

EDIT: opposite is the case for sulfates, sorry :smile:

(edited 10 years ago)

Reply 73

OP

Original post by posthumus

Hehe, your evil... I told you that it's the one I need to go through! :tongue:

But tell me if I've missed out any points:
- if Liquid is vaporized and carried by the mobile phase (which is an inert gas I think)
- pushed through with pressure and through stationary phase
- Stationary phase consists of solvent absorbed onto solid
- The interaction between mobile & stationary phase determine how much time it takes for the mobile phase to go through, this is called the retention time.
- Retention time is unique to specific compounds, look up what type of compound it is using data resources.
- Structure can also be found using Mass or IR spectroscopy :smile:

This is also useful to purify some liquids?
I think I may have mixed HPLC with GC here :tongue:

Explain with reference to lattice & hydration enthalpies why hydroxides are more soluble as you go down group 2 & the opposite is the case for group 2.

I thought whats the point of revision form the stuff u know :tongue:

GC- uses gas as mobile phase and liquid adsorbed on the surface of solid as stationary phase. gas passed under pressure and the rest is alright :smile:

Its used for volatile compounds :smile:

Increase in ionic radii so less attarction to OH. Lattice enthalpy decreases as the ionic radii goes on increasing hence less negative. Hydration enthalpy decreases as well as ionic radii increases down the group so again this is less negative.
Since Hsolution = -Hlatt+ Hhydration H solution becomes lmore exo down the group so Stotal becomes more positive hence solubility increases.

Reply 74

OP

Original post by posthumus

Hehe, your evil... I told you that it's the one I need to go through! :tongue:

But tell me if I've missed out any points:
- if Liquid is vaporized and carried by the mobile phase (which is an inert gas I think)
- pushed through with pressure and through stationary phase
- Stationary phase consists of solvent absorbed onto solid
- The interaction between mobile & stationary phase determine how much time it takes for the mobile phase to go through, this is called the retention time.
- Retention time is unique to specific compounds, look up what type of compound it is using data resources.
- Structure can also be found using Mass or IR spectroscopy :smile:

This is also useful to purify some liquids?
I think I may have mixed HPLC with GC here :tongue:

Explain with reference to lattice & hydration enthalpies why hydroxides are more soluble as you go down group 2 & the opposite is the case for group 2.

EDIT: opposite is the case for sulfates, sorry :smile:

why is aldehyde more reactive than ketone?

Reply 75

Original post by AS01

I thought whats the point of revision form the stuff u know :tongue:

GC- uses gas as mobile phase and liquid adsorbed on the surface of solid as stationary phase. gas passed under pressure and the rest is alright :smile:

Its used for volatile compounds :smile:

Increase in ionic radii so less attarction to OH. Lattice enthalpy decreases as the ionic radii goes on increasing hence less negative. Hydration enthalpy decreases as well as ionic radii increases down the group so again this is less negative.
Since Hsolution = -Hlatt+ Hhydration H solution becomes lmore exo down the group so Stotal becomes more positive hence solubility increases.

Aah good point ! I need lessons on how to revise :redface:

Erm lattice enthalpy depends on sum of ionic radiis.... {r(+) + r(-)}
The OH group has a very small radii therefore lattice enthalpy increases considerably as you go down :smile:

since its negative lattice enthalpy... H solution becomes more exothermic favoring solubility as you go down

However sulfate ions are much large {r(+) + r(-)} as cation radii increases as you go down the group the lattice enthalpy does not increase by a lot. This does not outweigh the hydration enthalpy so it's more endothermic as you go down. Hence solubility is less favored down group 2 sulfates.

Is the C=O less polar in ketones due to alkyl groups pushing electrons towards the carbon and making it less delta positive. Therefore less likely to be attacked by nucelophile? Whereas aldehyde has a polar C=O bond.
That's about all I have to say :colondollar:

I don't know why aldehyde C=O is more polar though

Reply 76

OP

Original post by posthumus

Aah good point ! I need lessons on how to revise :redface:

Erm lattice enthalpy depends on sum of ionic radiis.... {r(+) + r(-)}
The OH group has a very small radii therefore lattice enthalpy increases considerably as you go down :smile:

since its negative lattice enthalpy... H solution becomes more exothermic favoring solubility as you go down

However sulfate ions are much large {r(+) + r(-)} as cation radii increases as you go down the group the lattice enthalpy does not increase by a lot. This does not outweigh the hydration enthalpy so it's more endothermic as you go down. Hence solubility is less favored down group 2 sulfates.

Is the C=O less polar in ketones due to alkyl groups pushing electrons towards the carbon and making it less delta positive. Therefore less likely to be attacked by nucelophile? Whereas aldehyde has a polar C=O bond.
That's about all I have to say :colondollar:

I don't know why aldehyde C=O is more polar though

um hydration enthalpy is always exo though
yup! thats right and also aldehydes are less sterically hindered :smile:

It is polar coz of difference in electronegativity :smile:

Reply 77

Original post by AS01

um hydration enthalpy is always exo though
yup! thats right and also aldehydes are less sterically hindered :smile:

It is polar coz of difference in electronegativity :smile:

Yh but hydration enthalpy decreases as you go down :smile:

Think of it as the lattice enthalpy can't outweigh it, or keep up with the change (to outweight it) because it has a big anion {r(+) + r(-)}

Okay now I see the flaw, it said this in george facer book.... is lattice enthalpy endothermic or exothermic ? (disregarding the negative sign) :confused:

Reply 78

OP

Original post by posthumus

Yh but hydration enthalpy decreases as you go down :smile:

Think of it as the lattice enthalpy can't outweigh it, or keep up with the change (to outweight it) because it has a big anion {r(+) + r(-)}

Okay now I see the flaw, it said this in george facer book.... is lattice enthalpy endothermic or exothermic ? (disregarding the negative sign) :confused:

lol yeah! it becomes more positive for lattice enthalpy as it is -Hlatt so prolly the hydration enthalpy is more negative than lattice enthalpy

Reply 79