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Aqa chem 4/ chem 5 june 2016 thread

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Reply 2060

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN13.PDF
For 2cii) why do you times 0.0607 by 17.2?

I used q=mcΔT and ΔH=q/no. mol. q=(4.18/1000)(20)ΔT no. mol is n=m/mr which is n=5/74.6 then sub into second equation 17.2=(20)(4.18/1000)ΔT/0.0607 rearrange and then get an answer of 13.8 and as ΔH is positive its a decrease in temp so do 298-13.8 to get an answer. c is 4.18/1000 to get it into J not KJ hope this helped.

Reply 2061

jammypancake

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN13.PDF
3bi anyone?

Reply 2062

SubwayLover1

Original post by SirRaza97

Sure no problem. The Cl atom is smaller compared to bromine and has less electron sheilding due to less shells around the atom. So the attraction between the nuclues of the chlorine atom and the bond pair of electrons is much stronger because the distance between them is smaller. (Think of it like two magnets attracting. The closer they are the stronger they attract and so the harder it is you pull them away from each other).

Thus the energy required to overcome this force is much greater. So Cl-Cl bond has requires more energy to break than Br-Br bond.

I thought Br2 had a higher boiling point??

Reply 2063

jammypancake

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN12.PDF
2b? how to calculate the gradient?

Reply 2064

SirRaza97

Original post by SubwayLover1

I thought Br2 had a higher boiling point??

I'm talking about breaking bonds not intermolecular forces.

Reply 2065

johnson_01

Original post by jammypancake

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN13.PDF
3bi anyone?

Just think of it in terms of ΔG=ΔH-TΔS where where ΔG is y T is the value which varies so is x the ΔS is connected to T so is your gradient m and ΔH is you y intercept c. To get the marks all you do is quote the equation and state what each term corresponds to in y=mx+c. Dont know whether this helped or not.

Reply 2066

badaman

Original post by Bloom77

And how would you do that
I normally combine them to find the ratio like you know for those maths chem questions at the end

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1466534952945.jpg35.6KB

Reply 2067

Seclusion

Original post by jammypancake

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN13.PDF
3bi anyone?

Gibbs free energy matches to it. G is y, -DeltaS is gradient, T is x and DeltaH is the y intercept,

Reply 2068

thomaarrss

is a reaction spontaneous if delta G is below or equal to zero or just below 0???

Reply 2069

zombaldia

Original post by GO97

hm im pretty sure bonds forming release energy and u have put in energy to break bonds so bonds breaking would take up energy

I meant it uses up energy. It's still endothermic

Reply 2070

Seclusion

Original post by thomaarrss

is a reaction spontaneous if delta G is below or equal to zero or just below 0???

Equal and below.

Reply 2071

zombaldia

Original post by thomaarrss

is a reaction spontaneous if delta G is below or equal to zero or just below 0???

Think below but you set it =0 as -0.00000000001 and 0 makes no difference. In practice it's the same

Reply 2072

johnson_01

Original post by jammypancake

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN12.PDF
2b? how to calculate the gradient?

Take the point (460,0) and the point (700,23.5) then do change in y over change in x to get 23.5/240 to get an answer of 0.0979 which rounds to 0.098. Its hard to see but it is at 23.5 just a bad question for accuracy

Reply 2073

GO97

Original post by zombaldia

I meant it uses up energy. It's still endothermic

ah ok sorry if i misread

Reply 2074

YoloSwagginz

Original post by GO97

hey sorry your quiz is right, its the cr2o7^2-/cro4^2- ions that are orange/yellow. the hexa-aqua ions are violet, this could have cost me big time in tommorows exam haha thanks for that

http://m.imgur.com/ulgdobg

It's yellow and the exams consider them yellow check 2013 or the 2014 pap

Reply 2075

shamk123

Original post by thomaarrss

is a reaction spontaneous if delta G is below or equal to zero or just below 0???

i think it is just below 0 but for calculation purposes its classed as 0, i could be wrong though correct if i am please

Reply 2076

zombaldia

Original post by Chembio123

Only starting to revise tonight, am I doomed?

No I think chem 5 easy. Just read over chemrevise

Reply 2077

YoloSwagginz

Original post by shamk123

i think it is just below 0 but for calculation purposes its classed as 0, i could be wrong though correct if i am please

It has to be below 0 and in calculations when you round it make sure you choose the rounding that gives make delta g below 0 if you round the wrong way it will be positive

Reply 2078

Jmedi

Original post by Bloom77

And how would you do that
I normally combine them to find the ratio like you know for those maths chem questions at the end

Posted from TSR Mobile

1MnO4- for 5 Fe 2+
2MnO4- for 5C2O4 2-
3:5

Reply 2079

shamk123

Original post by Chembio123

Only starting to revise tonight, am I doomed?

Im the same as you i started about an hour ago after 7-8 weeks of neglecting chem5 its safe to say best option is to pull an all nighter, so i took some modafinil
to keep me awake for the whole night, im just gonna get some sleep after the exam