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    (Original post by laurawoods)
    Please can u go through this question also with me ...I am struggling?
    25.0 cm3 of a 0.1 moldm-3 solution of KNO2 is completely oxidized by 50.0 cm3 of 0.0200 moldm-3 potassium manganate (VII) solution. To what oxidation number was the N oxidized?
    Moles of KNO2 = 0.025 x 0.1 = 0.0025 moles
    Moles of KMnO4 = 0.05 x 0.02 = 0.001 moles

    Manganate(VII) reduces:

    MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

    ∴ 0.001 moles of manganate(VII) absorbs 0.005 moles of electrons, and this is taken from 0.0025 moles of nitrite ions.

    ∴ each nitrite ion provides two electrons.

    ∴ it is oxidised from N(III) to N(V).
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    (Original post by Tullia)
    Moles of KNO2 = 0.025 x 0.1 = 0.0025 moles
    Moles of KMnO4 = 0.05 x 0.02 = 0.001 moles

    Manganate(VII) reduces:

    MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

    ∴ 0.001 moles of manganate(VII) absorbs 0.005 moles of electrons, and this is taken from 0.0025 moles of nitrite ions.

    ∴ each nitrite ion provides two electrons.

    ∴ it is oxidised from N(III) to N(V).
    is it meant to be a 5 to 2 ratio? that is the method that I was thinking of ? would that work? Please can u help me understand where this value of 0.005 moles comes from? thanks for all help offered!
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    (Original post by brittanna)
    Total=50 raw marks = 60 UMS
    ISA practical=8 raw marks
    ISA written exam=30 raw marks
    PSA=12 raw marks
    You seem to be well informed, do you know if teachers are allowed to tell us our ISA marks? It's just that my teacher won't but I don't know if he just won't or if he's not allowed. Thanks
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    (Original post by laurawoods)
    is it meant to be a 5 to 2 ratio? that is the method that I was thinking of ? would that work? Please can u help me understand where this value of 0.005 moles comes from? thanks for all help offered!
    Half-equation:

    MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

    Half-equation:

    NO2- + H2O --> NO3- + 2H+ + 2e-

    Gives:

    2MnO4- + 16H+ + 10e- --> 2Mn2+ + 8H2O

    5NO2- + 5H2O --> 5NO3- + 10H+ + 10e-

    Which gives:

    2MnO4- + 5NO2- + 6H+ --> 2Mn2+ + 5NO3- + 3H2O
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    (Original post by Tullia)
    Half-equation:

    MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

    Half-equation:

    NO2- + H2O --> NO3- + 2H+ + 2e-

    Gives:

    2MnO4- + 16H+ + 10e- --> 2Mn2+ + 8H2O

    5NO2- + 5H2O --> 5NO3- + 10H+ + 10e-

    Which gives:

    2MnO4- + 5NO2- + 6H+ --> 2Mn2+ + 5NO3- + 3H2O

    Hello on the last line that should be 16 protons. But what i don't still understand is how did u know it was going to be NO3- ions???? it is not given in the question, right?
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    (Original post by neha6)
    anyone done the empa yet? i mean task3, im so scared for it, any hints/tips would be great!
    me too, I've got mine on tuesday!!
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    (Original post by laurawoods)
    Hello on the last line that should be 16 protons. But what i don't still understand is how did u know it was going to be NO3- ions???? it is not given in the question, right?
    The electrons, protons and waters cancel; this does leave you with six protons.

    Why do you think it goes to NO3-? I'm not being awkward, but want you to think about it rather than me just feeding you answers.
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    (Original post by Tullia)
    The electrons, protons and waters cancel; this does leave you with six protons.

    Why do you think it goes to NO3-? I'm not being awkward, but want you to think about it rather than me just feeding you answers.
    Hmm... all i know is that the nitrogen must be being oxidised. But i don't really know why it is NO3- ? Sorry! I am not too clever!
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    What are the observations when Silicon reacts with Oxygen? In the text book it doesnt talk about the observation for this reaction.
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    (Original post by Hamzi)
    What are observations when Silicon reacts with Oxygen? In the text book it doesnt talk about the observation for this reaction.
    Hello, according to my teacher we don't need to know any of the actual observations!
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    (Original post by Mariah246)
    You seem to be well informed, do you know if teachers are allowed to tell us our ISA marks? It's just that my teacher won't but I don't know if he just won't or if he's not allowed. Thanks
    My teacher told me what our class got in ours (what raw marks we got). However, they don't know what grade it will be or how many UMS it will be as the grade boundaries haven't been produced yet. But I don't see why they can't tell you what raw mark you got. I'm guessing that he won't tell you because he doesn't know which grade it is or because he hasn't finished marking all of them yet.
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    (Original post by laurawoods)
    Hello, according to my teacher we don't need to know any of the actual observations!
    But in one of the past papers they did ask questions to do with observations for these reactions so I am guessing you have to know them but its just that for Silicon the AQA text book just mentions a vigorous reaction and nothing else for Silicon.
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    (Original post by Hamzi)
    But in one of the past papers they did ask questions to do with observations for these reactions so I am guessing you have to know them but its just that for Silicon the AQA text book just mentions a vigorous reaction and nothing else for Silicon.
    Was it a legacy paper question ? Or was it from one of the new papers?
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    (Original post by laurawoods)
    Was it a legacy paper question ? Or was it from one of the new papers?
    It was the new ones.
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    (Original post by Hamzi)
    It was the new ones.
    Hmmm...so our teacher was misleading us! :mad:
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    (Original post by laurawoods)
    Hmmm...so our teacher was misleading us! :mad:
    Its on q4 on the June 2011 paper

    Here is the link to the paper:
    http://filestore.aqa.org.uk/subjects...W-QP-JUN11.PDF
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    So can anyone tell me the observations for the reaction of Silicon with Oxygen??
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    (Original post by Hamzi)
    So can anyone tell me the observations for the reaction of Silicon with Oxygen??
    You don't need to know this, it's not even mentioned in the nelson thorne textbook

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    (Original post by popnit)
    You don't need to know this, it's not even mentioned in the nelson thorne textbook

    Posted from TSR Mobile
    I guess you do because if they give you all the observations for the other period 3 elements with oxygen in the text book then why did they just skip observations for silicon.

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    (Original post by Hamzi)
    I guess you do because if they give you all the observations for the other period 3 elements with oxygen in the text book then why did they just skip observations for silicon.

    Posted from TSR Mobile
    Basically both the reactions of Al and Si with oxygen is very slow, so slow in the case of the AQA course anyway, that no flame can even be observed. So really the only observations you need to know in relation to the course are Na, Mg, P, and S
 
 
 
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