Chemistry Paper 51

[csn_aras_97]

Hey guys, so how'd it go?

 

[mohammad hurani]

it was easy

 

[randomcod]

I thought it was one of the easiest papers

 

[Abdel Moniem]

my answers for the paper
HCO2-->CO2+H+ +2e-
MnO4+5e-+8H+-->4H2O+Mn2+
I think concentration was 1. something where i used Mr*Concentration(mol/dm^3)=Concentration(g/dm^3) and for the methanoate ions it was 2*1. something i don't quite remember the calculation part.
to prepare a concentration which is 50 times dilute, concentration/50 and a volume of 250 is required i used dilution's law. fill the burette with the saturated solution, run 5.00 cm^3 into a beaker, add to graduated flask through the funnel, add deionisaed water till we reach the 250 cm^3 mark of the graduated flask. i said that sulphuric acid was used before titration to increase the solubility of solution. concentration was 0.051 and in the 250cm^3 solution it was 0.0051 i used C1*V1=C2*V2 where V1=5cm^3 V2=250 and C2=0.0051. i said that delta H is -ve(exothermic) because solubility increases with increase in temperature. a white ppt of barium sulphate is formed.
Kc=[HI]^2/[H2][I2]
H2+I2-->2HI
a a --
a/2-y a/2-y 2y
I drew a line of best fit joined from the second point on to the last part gradient was 0.750 then set my gradient to be y/a in to calculate the kc i think it was 36.
using a flask of 500cm^3 insted of a dm^3 will increase the rate of reaction ONLY! because the reactant molecules will collide more frequently and break to form HI molecule at a faster rate. Kc is not affected. I said using a temperature of 1000k will decrease the yield of HI because equilbrium position shifts to the left to decrease the increase in temperature and i drew from the same point as 760K but less Y was formed

 

[Alex10]

If solubility increases with temperature doesn't it mean the forward reaction is endothermic? So delta H would be positive?

 

[Abdel Moniem]

When the Delta H of solution is positive the solubility is low, but it clearly stated in the question that solubility increases with temperature so it must be exothermic. I am not sure too.

 

[randomcod]

When the Delta H of solution is positive the solubility is low, but it clearly stated in the question that solubility increases with temperature so it must be exothermic. I am not sure too.

If solubility increases with temp, it's endo.

I skimmed over your answer before. I'm afraid you need 245 cm^3 of water to dilute 50 times what you were given!
There's a couple other things but I noticed that.

 

[Abdel Moniem]

If solubility increases with temp, it's endo.

I skimmed over your answer before. I'm afraid you need 245 cm^3 of water to dilute 50 times what you were given!
There's a couple other things but I noticed that.

yes you are right about the endothermic thing just thought about it now i am sorry guys. why is it 245cm^3. You got me worried is there many incorrect answers?. if there is an incorrect answer i am sure its the 2nd part of q2).

 

[Senorita.K]

If solubility increases with temperature doesn't it mean the forward reaction is endothermic? So delta H would be positive?

yes exactly
i wrote the same

 

[Senorita.K]

for diluting part i made reference to c1v1=c2v2 formula so i got the answer as 15 cm3 ( of water needed to dilute it 50 times )