Edexcel As Chemistry Unit 3B

Scroll to see replies

You should never, ever heat a closed system (by placing a stopper on top of a condenser as an example). Well okay, you can with specialised systems, but generally.

The increasing pressure caused by heating can lead to an explosion.

Thanks for the reply

Reply 101

The_New_Guy

Can anyone explain why I should not use Phosphorus(V)chloride to test the OH ions in Question 1.c.(iii),June 2010?

Reply 102

ro0ro0

because PCl5 is used to test for hydroxyl groups and not hydroxide ions

Reply 103

Lightuponlight

Original post by Sellord

So basically volume does not affect temperature change at all?

not a 100% sure but here how...
I looked at it through the rate of reaction terms, more concentration more collisions between particle cause increase in temperature. Increase conc increases temp!

the link between volume and temp is this :
If you increase volume does temperature increase? No although if you increase the temperature the volume increases!

i hope this is right ;P

Reply 104

Sellord

Original post by The_New_Guy

Can anyone explain why I should not use Phosphorus(V)chloride to test the OH ions in Question 1.c.(iii),June 2010?

Made same mistake today and I'm not that sure why.
Basically they are asking for a test for hydroxides, this doesnt exactly mean OH- I suppose, therefore it should be something like Universal indicator turning blue.

Reply 105

The_New_Guy

Original post by ro0ro0

because PCl5 is used to test for hydroxyl groups and not hydroxide ions

Ow..Looks like I had a big misconception.I didn't realise that OH groups form in covalent compounds and OH ions form in ionic compounds.
Thank you very much.

(edited 11 years ago)

Reply 106

The_New_Guy

Original post by Sellord

You are right.
Hope these silly mistakes don't creep in exam!

Reply 107

himi-patel

Original post by Lightuponlight

HAHAHA

i like your name "SOLITTLETIME"!!
something we all say b4 exams!!

btw...
how to calculate percentage error?
and when do u multiply by 2? :s-smilie:

percentage error is calculated by ........the uncertainity in the equipment / the reading *100.........meaning the error divide by the value multiply by 100

Reply 108

solittletime

Original post by Lightuponlight

HAHAHA

i like your name "SOLITTLETIME"!!
something we all say b4 exams!!

btw...
how to calculate percentage error?
and when do u multiply by 2? :s-smilie:

You multiply by two if you used two measurements.

For example, when finding the mass of a substance, if you subtracted the mass of container from mass of substance + container, the total uncertainty in your value for mass of substance will be 2 times the least count on the balance.

So what you are doing is compounding the uncertainties in each measurement.

Reply 109

Lightuponlight

Original post by solittletime

kk thanks (Y) :biggrin:

btw...over with the rep limit for today =P shall do laters :wink:

hows u feeling for xam, so little time?

any tips?
and watabt hess's law? is it very important?

Reply 110

solittletime

I feel like I still need to study :tongue:

Tips.. Get Anki, then use this deck I attached, and memorise everything :biggrin:

.

About Hess's law, I haven't seen it much in pastpapers, but I revised it just to be on the safe side.

Chem U3B.zip16.7KB

Reply 111

King_Arthur

Original post by Lightuponlight

actually it's like this
mass 50 cm3 solution = 50g
mass of 25cm3 solution = 25g
mol of both the solutions are the same
as the HCL is taken in excess the amount that'l react will be the same in both cases

but as the mass of the second solution is half according E=msT
the T will double

Reply 112

dunia.hatabeh

on the test, do they ever ask about rxns of alcohols such as elimination for nucleophilic substitution?

Reply 113

jerichi

Original post by dunia.hatabeh

on the test, do they ever ask about rxns of alcohols such as elimination for nucleophilic substitution?

I think they ask what type of a reaction it is sometimes so you just have to answer with it being an elimination or nucleophilic substitution. I think I came across that once or twice :smile:

Reply 114

jerichi

So...about redox reactions...how do you revise that for the exam?

Reply 115

avipsita

Does anyone now the test for an carboxylic acid ?

Reply 116

avipsita

Original post by jerichi

So...about redox reactions...how do you revise that for the exam?

I literally jst memorised them . This much better to revise them knowinf the technique .

Reply 117

King_Arthur

Original post by avipsita

Does anyone now the test for an carboxylic acid ?

add a bit of ethanol and H2SO4 and heat
sweet fruity smell

(edited 11 years ago)

Reply 118

jerichi

Original post by avipsita

I literally jst memorised them . This much better to revise them knowinf the technique .

Thank you! I'll try do that :smile:

Reply 119

Lightuponlight

Original post by jerichi

Thank you! I'll try do that :smile:

sorry bt u guys r trying to memorise which redox reactions? :s-smilie:

For the enthalpy thing i have done the questions, but havent studied the chapter or any notes for it?
what should i knoww? and how do we know if its positive or negative sign/exo or endo?
:s-smilie:

can sum1 guide be abt it?