Chemistry Questions

dkcx SG

'A' Level Qn.

'O' Level Qn (Challenging).

3 identical coins, made up of 2 metals, alloy.

1st one dissolved in concentrated nitric acid to form a blue solution.

2nd one added to excess diluted sulphuric acid, reddish brown precipitate and a colourless solution is formed.

Then it was filtered and ammonia solution added to the filtrate. White precipitate formed which dissolved in excess ammonia solution.

3rd one added to silver nitrate solution, after several hours, the solution turns pale blue and a grey solid is formed on the coin.

Identify all substances described above, including all relevant equations.

Answers :

1st experiment - copper(II) nitrate (aq) + zinc nitrate (aq)

2nd experiment - zinc sulfate (aq) + copper metal (s); zinc hydroxide (s); tetraaminezinc(II) complex ions (aq)

3rd experiment - copper(II) nitrate (aq) + zinc nitrate (aq) + silver metal (s)

Hmm, 4got about displacement of copper metal but its called ppt ah. Been years since i did such experiments to remember what it looks like since all displacement reactions done these days involves plating.

d3sT1nY SG

Sorry, I have got a question here regarding the 2nd experiment.

Why copper metal doesn't dissolve in sulphuric acid? Because it's an unreactive metal that does not react with acid?

Pardon my qualitative knowledge.

dkcx SG

Sorry, I have got a question here regarding the 2nd experiment.

Why copper metal doesn't dissolve in sulphuric acid? Because it's an unreactive metal that does not react with acid?

Pardon my qualitative knowledge.

Cu's reactivity is below that of H in the reactivity series and thus it does not react with acids.

d3sT1nY SG

Alright, then how do I explain experiment 3? And the slow reaction? Simple displacement reaction?

Wait, that doesn't explain why Cu is able to react with nitric acid. =S

Angelababy SG

Haha, my english is a bit lousy. Those kind of 'no matter how much newspaper you read also wouldn't improve' kind.

Well, not really sure about university though. But also partly because I looking for tuition to teach. Haha. I'm more well-versed in Chemistry and Biology when I was in school.

you havent seen really lousy english.

like mine!

dkcx SG

Alright, then how do I explain experiment 3? Simple displacement reaction?

Wait, that doesn't explain why Cu is able to react with nitric acid. =S

Just displacement. Cu is slightly more reactive than Ag.

Nitric acid is a very strong oxidising agent and thus will oxidise the Cu to Cu2+

d3sT1nY SG

Just displacement. Cu is slightly more reactive than Ag.

Nitric acid is a very strong oxidising agent and thus will oxidise the Cu to Cu2+

Haha. Some interests just have to be cultivated. With reference to your edited away sociology.

And thanks, I forgot that nitric acid is oxidizing. That explains the use of sulphuric acid to acidify oxidizing agents. If I didn't remember wrongly.

Erm, looking back again, I see some contradiction. I meant why Zn is able to form ZnSO4 but Cu cannot form CuSO4. If you say HNO3 is strongly oxidizing, then experiment one would fail. Am I right?

CuSO4 is blue solution if I'm not wrong. But in experiment 2, the solution is colourless due to the ZnSO4.

d3sT1nY SG

you havent seen really lousy english.

like mine!

That's kind of.. Erm.. Comforting?

dkcx SG

Haha. Some interests just have to be cultivated. With reference to your edited away sociology.

And thanks, I forgot that nitric acid is oxidizing. That explains the use of sulphuric acid to acidify oxidizing agents. If I didn't remember wrongly.

When you clear your AHSS PEs then u can try to enjoy, some courses are interesting, some are nice to know bad for exams, some are downright there to just create vacancies...

H2S04 is mostly use for acidifying things since it doesn't have the strong oxiding characteristics that HNO3 has which might affect reactions.

d3sT1nY SG

Question: What is means by hydroxy group on the phenol is strongly activating due to the delocalisation of lone pair of electrons on oxygen into the phenyl ring.

As in activate is an English term used here? Means to start something?

Question: I remember iron (III) chloride can also be used to distinguish between phenol and phenylamine. The phenol will form a violet complex with Fe3+, but what colour is phenylamine's complex.

Or I remembered wrongly?

Thanks.

HiAy3Captain SG

wow nice

d3sT1nY SG

wow nice

Wow, what's so nice. Please share?

dkcx SG

Question: What is means by hydroxy group on the phenol is strongly activating due to the delocalisation of lone pair of electrons on oxygen into the phenyl ring.

As in activate is an English term used here? Means to start something?

Don't understand your question but OH group on phenol makes it strongly activating so its easier for another functional group to attached to it since the benzene ring is now more reactive.

Chemfreak022 SG

The OH group donates electrons to the benzene ring through resonance, therefore increasing electron density of the benzene ring. Since benzene is the nucleophile in the electrophilic aromatic substitutions, increasing e density will increase the nucleophilicity of the benzene ring as compared to unsubsituted C6H6. Like what dkcx said, it makes the ring more reactive towards electrophiles and hence OH activates the ring.

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