june 01 paper 3 chemistry help!!

[amaghraby]

guys i really need help in june 01 paper 3 question 5c
if u know a website with the mark scheme please supply!!
thanks in advanced

 

[HyDraZin3]

Give me the link of the paper, i will try to help

 

[amaghraby]

srry man there us no link on xtrmepapers...ill try to scan the question and upload it

 

[amaghraby]

here is the question

 

[HyDraZin3]

I dont know if my answer is right, but i think that starch gets used up as it reacts with the water outside it and forms glucose and so the colour goes lighter as the concentration of starch decreases but as it is of 4 marks i dont know whether its right to write that HCl acts as a catalyst in this reaction

 

[amaghraby]

ok but why does it turn colourless?....iodine is brown :S and tehre is a similar question in june 10 32 but there, there was potassuim iodide which is colourless

 

[HyDraZin3]

Here they have written that starch reacts with iodine to form deep blue colour, so as the concentration of starch decreases the color will ofcourse go paler right?

 

[amaghraby]

ya but this doesn't explain why it goes COLOURLESS!!!!it should stay brown is iodine is brown

 

[Violettamee]

guys ok i know the answer to that question (nd i'll also show the mark scheme - i have it)

anyway ...
in the question, it says AQUEOUS starch solution (with HCl) .. this shows that some amount of water IS present in the solution .. so starch is HYDROLYSED to glucose .. u should know the reaction :-

glucose - H2O --> starch
and
starch + H2O --> glucose.

so, at 0min, not much starch was hydrolysed and when it was tested with iodine, the solution turned deep blue.. coz iodine reacts with the starch and turns the solution deep blue while the HCl acts as a catalyst (just like in the stomach!!! the stomach gas HCl as a catalyst !!!! biology LOL)

then at 5min, most of the starch got hydrolysed to glucose, so wen it was tested with iodine, the solution only turned pale blue as there was not MUCH starch present ....

then finally, at 20 mins, ALL the starch got hydrolysed, and wen it was tested with iodine, the solution was colorless as there was NO starch present nd only glucose ..... remember in the whole process HCl acts as a catalyst !!!!!!!!

so yeah there we go...
here's wt the mark scheme says :-

 

[Violettamee]

Here they have written that starch reacts with iodine to form deep blue colour, so as the concentration of starch decreases the color will ofcourse go paler right?

yup thats true...