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Help with some questions for Ethanal Chemistry Preparation

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  • 04-02-2009 3:21pm
    #1
    damienricefan
    Registered Users Posts: 165 ✭✭


    Can you help answer these? :):

    Why is cooling necessary when adding concentrated sulfuric acid to water?

    Why is heating not required once the addition of alcohol/dichromate has begun?

    Apart from a colour change, what else is observed in the flask as the ethanol/ dichromate mixture was added to hot sulfuric acid?

    What colour change is observed when ethanal reacts with acidified potassium manganate (VII) soultion? What does the colour change indicate?

    What is formed when ethanal reacts with Fehling's solution? What does this result indicate?

    In the silver mirror test, why is it essential to use a very clean test tube?

    What does the formation of silver indicate?

    : or if anyone knows a site that has answers to questions on chemistry experiments leave me a link. Understanding Chemistry by Edco has none of these answers and mock is tomorrow! :mad: ! thanks!!!


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    #2
    [Deleted User]
    Posts: 4,630 ✭✭✭ [Deleted User]


    damienricefan wrote: »
    Why is cooling necessary when adding concentrated sulfuric acid to water?

    It's an exothermic reaction. If you didn't cool it, it could react violently - spraying hot acid out of the beaker etc. etc.
    Why is heating not required once the addition of alcohol/dichromate has begun?

    Because it's an exothermic reaction. Essentially, it heats itself.
    Apart from a colour change, what else is observed in the flask as the ethanol/ dichromate mixture was added to hot sulfuric acid?

    Can't remember off the top of my head. A clear solution is collected in the conical flask, perhaps? Or which flask are you talking about?
    What colour change is observed when ethanal reacts with acidified potassium manganate (VII) soultion? What does the colour change indicate?

    The purplish colour of the KMNO4 disappears. The KMNO4 has oxidised the ethanal to ethanoic acid, so the KMNO4 itselft becomes reduced - resulting in it changing from an oxidation state of +7 (purple) to +2 (colourless).
    What is formed when ethanal reacts with Fehling's solution? What does this result indicate?

    Copper(i)oxide is formed. The result indicates that the ethanal was oxidised to ethanoic acid (as Fehlings solution is an oxidising agent).
    In the silver mirror test, why is it essential to use a very clean test tube?

    Don't actually know. Probably because any impurities would cause an inaccurate result, or something along those lines.
    What does the formation of silver indicate?

    That the ethanal has been oxidised to ethanoic acid. The Ag+ ions "react" with an e-, forming Ag (which is a precipitate).


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    #3
    Fringe
    Registered Users Posts: 1,082 ✭✭✭Fringe


    It's necessary to use a clean test tube because if you don't, the silver won't coat the sides because of whatever dirt that is already stuck there.


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