Chemistry: Post your doubts here!

[MiniSacBall]

Color changes from --- to ---- when adding aldehyde to

1. Fehling's Solution
2. Tollens Reagent

Help me!

 

[robinhoodmustafa]

look at this

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_23.pdf

Q2 d (ii) .. why the answer is Positive not negative ??

i think they didnt ask for 'sign' usually they ask to calculate enthalpy change and give its sign along with it.

 

[♣♠ Magnanimous ♣♠]

Color changes from --- to ---- when adding aldehyde to

1. Fehling's Solution
2. Tollens Reagent

Help me!

1. blue to orange red
and 2. colorless to silver mirror type.

 

[robinhoodmustafa]

Color changes from --- to ---- when adding aldehyde to

1. Fehling's Solution
2. Tollens Reagent

Help me!

Blue to Orange

and in Tollens : Colorless to Silver Mirror

 

[♣♠ Magnanimous ♣♠]

Blue to Orange

and in Tollens : Colorless to Silver Mirror

i gave the answer .

 

[robinhoodmustafa]

1. blue to orange red
and 2. colorless to silver mirror type.

What is the color change of ketone in Iodoform test?

 

[robinhoodmustafa]

i gave the answer .

LoL. Its really feels like a race or a competition

 

[zackle09]

Oh this question has given me a lot of trouble lately.

I went with (21/1)*(1/9) = 21/9 = 7/3


My explanation was something like 21/1 = ratio of rate of tertiary to primary H. 1/9 = ratio of tertiary to primary H present. Ratio of J/K = 21/1 * 1/9 = 21/9 = 7/3

J= Tertiary Hydrogen replaced.
K= Primary Hydrogen replaced.

ohhhh! thankyouuuu so muchh!

 

[Haya Ahmed]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w13_qp_23.pdf

Q2 d (ii) .. why the answer is Positive not negative ??

robinhoodmustafa are you confident with your answer ?! >.> .. I think It is not the reason :|

 

[robinhoodmustafa]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_23.pdf

Q2 d (ii) .. why the answer is Positive not negative ??

robinhoodmustafa are you confident with your answer ?! >.> .. I think It is not the reason :|

If they meant to give a positive sign than they really show it in Ms. That is the only possible reason because we all know Combustion of Hydrocarbon is exo

 

[Haya Ahmed]

So is it wrong If we showed - sign ? robinhoodmustafa

 

[syed1995]

not directly that why i asked him if it was from past papers. You just use it to form compunds after electrophylic addition reactions :3

lol i still have no clue

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_23.pdf

Q2 d (ii) .. why the answer is Positive not negative ??

"to calculate the energy produced by the"

You just need to calculate the energy produced in Joules. And Energy itself is a scalar quantity so cannot be negative. You don't have to state the enthalpy over here. And enthalpy has the +/- sign to it to show whether the energy was produced or used up.

Since they have already told you that energy is produced and you need to calculate the amount of energy produced. The Sign doesn't matter only the magnitude does..

Even if you wrote -XX kJ I don't think you'd have been penalised for that. because sign doesn't matter in this question.

 

[syed1995]

So is it wrong If we showed - sign ? robinhoodmustafa

No. you won't be penalised for that in my opinion. If they specifically show the sign in MS or asked for Enthalpy change, then only the sign matters.. so you're good either way.

 

[MiniSacBall]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w12_qp_21.pdf
Q5 b i)

Is my answer correct especially the optical isomer one

 

[ahmed abdulla]

Cr2O7 2– + 14H+ + 6e– → 2Cr3+ + 7H2O----Eo = 1.33 V
Cl2 + 2e– → 2 Cl– ---------------------------- Eo = 1.36 V
overall ionic equation:
Cr2O7 2– + 6Cl– + 14H+ → 2Cr3+ + 3Cl2 + 7H2O

But shouldn't one with more negative E0 value be reversed?
This is from w07

 

[Haya Ahmed]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s12_qp_21.pdf


Q5 b(ii) and (iii) explain why we multiplied by 2 in (i) arent we supposed to divide by 2 ?

 

[hope4thebest]

Does CH3COOH give positive test in iodoform test? And also CH3CH2OH? there is some confusion because marking scheme gives positive with ethanol but not ethanoic acid

 

[AbbbbY]

Yes in a sense you can, however the better reason will be that C does not have empty D orbitals. just remember this one line statement it....

No! You can't!! In any sense whatsoever!
The second part. Always. Only.

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s04_qp_4.pdf Q2c(i) i get the answer but cant we state the size of carbon atom being smaller than silicon as a valid reason

You can't. Size is immaterial when Carbon has no d orbital so smaller or not, water just can't attack. It's not a matter of steric hindrance only. Thing is, you just can't. You're very right saying the carbon atom is significantly smaller, but even if it's crowded by H2O, there won't be a reaction because there are no d-orbitals present. I really hope you get what I'm trying to say.

i gave the answer .

Vaooooowwwww yaaar. Kya baat hay. Yeh lo bhai. Yeh trophy aapki hui. You're the best chemist around. Admitted. Now can we go back to 'the help thread' from the 'I know more than you' thread? Seriously guys. It's not a race here!

 

[AbbbbY]

Does CH3COOH give positive test in iodoform test? And also CH3CH2OH? there is some confusion because marking scheme gives positive with ethanol but not ethanoic acid

Ethanol yes Ethanoic Acid no.

Ethanol has an alcohol group besides a CH3- link so it'll show you a positive test with alkaline iodine.
See ethanol and think 3D. If it still doesn't make sense quote me and tell me I'll draw it out.

 

[periyasamy]

hi guys.Breaking head for this question.Just cant get it.Anyone can lend me a hand ...Thanks a lot in advance........I dont get the answer given .....
http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w12_ms_43.pdf
http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w12_qp_43.pdf
question 7 part 3.I posted the question in the picture below..Thank u ..