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Quick question on Group 7 Halogens

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    Describe two observations that you would make when concentrated sulfuric acid is added to solid sodium iodide. write an equation for a reaction that occurs in which iodid ions are oxidsed by the sulfuric acid

    Its a four mark question which i have got the first 2 marks for...the observations which are black solide iodide and bad egg smell from H2O

    but i dont understand the equation part, please help

    Thanks very much guys!
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    (Original post by HEY_101)
    bad egg smell from H2O
    The bad egg smell comes from HS, not water.

    This website should give you everything you need: http://www.chemguide.co.uk/inorganic...alideions.html
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    Oh yeh my bad typo, but the answers says this:

    2NaI + 2H2SO4 = Na2 + 2H2O + I2 + SO2

    Please explain why
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    (Original post by HEY_101)
    Describe two observations that you would make when concentrated sulfuric acid is added to solid sodium iodide. write an equation for a reaction that occurs in which iodid ions are oxidsed by the sulfuric acid

    Its a four mark question which i have got the first 2 marks for...the observations which are black solide iodide and bad egg smell from H2O

    but i dont understand the equation part, please help

    Thanks very much guys!
     2I^{-} \rightarrow I_2 + 2e^{-}
     H_2SO_4 + 2H^{+} + 2e^{-} \rightarrow SO_2 + 2H_2O (There are 3 things which sulfuric acid can make, SO2, H2S or S, just adjust the balancing accordingly)

    Put these together to get:
     2I^{-} + H_2SO_4 + 2H^{+} \rightarrow   I_2 + SO_2 + 2H_2O

    And as said above, rotton egg smell comes from H_2S, not H2O.

    If anything isnt clear let me know.
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    i thought it would be this equation:

    NaI + H2SO4 -> NaHSO4 + HI

    where did the hydrogen go from the NAso4 in the first equation i wrote?
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    (Original post by Mocking_bird)
     2I^{-} \rightarrow I_2 + 2e^{-}
     H_2SO_4 + 2H^{+} + 2e^{-} \rightarrow SO_2 + 2H_2O (There are 3 things which sulfuric acid can make, SO2, H2S or S, just adjust the balancing accordingly)

    Put these together to get:
     2I^{-} + H_2SO_4 + 2H^{+} \rightarrow   I_2 + SO_2 + 2H_2O

    And as said above, rotton egg smell comes from H_2S, not H2O.

    If anything isnt clear let me know.


    Thats so cool, how do you use the latex thing?
    and yeah my mistake i meant H2S not water, bad typo

    and why do u combine the 2 half questions here? do you not include HI?
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    (Original post by HEY_101)
    Thats so cool, how do you use the latex thing?
    and yeah my mistake i meant H2S not water, bad typo

    and why do u combine the 2 half questions here? do you not include HI?
    To get the full equation
    You can't just leave it as either or, otherwise it wouldnt be answering the question.
    & I didnt include HI because what what I wrote was the ionic equation, and also because HI is only in the acid-base reaction not the redox reaction which it was asking for.

    This is the acid base reaction:
    NI + H_2SO_4 \rightarrow NaHSO_4 + HI
    Which is what you said up there.

    Anddd... latex, either [ latex ] text here [ / latex ] or use this site if you arent familiar with how to write it fully: http://www.codecogs.com/latex/eqneditor.php

    edit: this seemed to get a little rambley from me, sorry if any confused you :laugh:
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    im sorry i still dont understand
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    (Original post by HEY_101)
    Describe two observations that you would make when concentrated sulfuric acid is added to solid sodium iodide. write an equation for a reaction that occurs in which iodid ions are oxidsed by the sulfuric acid

    Its a four mark question which i have got the first 2 marks for...the observations which are black solide iodide and bad egg smell from H2O

    but i dont understand the equation part, please help

    Thanks very much guys!
    Okay.

    Sodium iodide dissociates in solution to give sodium ions and iodide ions, NaI --> Na+ + I-.

    As you go down group (VII), the halide ions (F-, Cl-, Br-, I-) become better reducing agents.

    Therefore, iodide ions will reduce the sulfur in sulfuric acid (oxidation state of sulfur = +6) to hydrogen sulfide, H2S (oxidation state of sulfur = -2) the most easily of all the halide ions.

    Since iodide ions are the reducing agent, they must be oxidised back to iodine, I2.

    The half reactions are:

    H2SO4 + 8 H+ + 8 e- --> H2S + 4 H[sub]2[sub]O.

    2 I- --> I2 + 2 e-.

    You then balance the electrons by multiplying the second equation by 4 in order to get an equal number of electrons:

    H2SO4 + 8 H+ + 8 e- --> H2S + 4 H[sub]2[sub]O.

    8 I- --> 4 I2 + 8 e-.

    You then combine the equations to get the overall equation:

    8 I- + H2SO4 + 8 H+ --> 4 I2 + H2S + 4 H[sub]2[sub]O.
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    (Original post by HEY_101)
    im sorry i still dont understand
    Hmm What exactly is it you're not understand?
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    Ohhh now i get it thank you, and you dont have to be sorry you have been very helpful thank you,

    one more question though, this is a really bad question to ask as it would be clear that chemisrry is not my subject but why is the acid base and redox equation different and why do they ask for it seperately in different questions
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    also could you explain the formation of the equation given in the answers?

    2NaI + 2H2SO4 ->Na2 + 2H2O + I2 + SO2
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    (Original post by HEY_101)
    Ohhh now i get it thank you, and you dont have to be sorry you have been very helpful thank you,

    one more question though, this is a really bad question to ask as it would be clear that chemisrry is not my subject but why is the acid base and redox equation different and why do they ask for it seperately in different questions
    An acid base reaction is any reaction between an acid and a base, so it can occur with all of the halide ions (F-, Cl-, Br-, I-) as nothing else is stopping it from happening (unlike below).

    A redox reaction is where something is oxidised and reduced.
    This only occurs when the halide ion has a sufficient ability to act as a reducing agent (Cl- and F- cannot act as reducing agents, but Br- and I- can)

    The reason the ability to act as a reducing agent increases down the group is because:
    - Atomic radius increases
    - Outer electrons get further away from the nucleus
    - More shielding
    - So loses electrons more readily. (reducing agent loses electrons).

    So they just ask for different equations because well, they do different things to put it simply
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    (Original post by Mocking_bird)
    An acid base reaction is any reaction between an acid and a base, so it can occur with all of the halide ions (F-, Cl-, Br-, I-) as nothing else is stopping it from happening (unlike below).

    A redox reaction is where something is oxidised and reduced.
    This only occurs when the halide ion has a sufficient ability to act as a reducing agent (Cl- and F- cannot act as reducing agents, but Br- and I- can)

    The reason the ability to act as a reducing agent increases down the group is because:
    - Atomic radius increases
    - Outer electrons get further away from the nucleus
    - More shielding
    - So loses electrons more readily. (reducing agent loses electrons).

    So they just ask for different equations because well, they do different things to put it simply

    Thank you so much much, your help&advice is much apprecited really! hope there is something i could help you with in the future thanks x
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    (Original post by HEY_101)
    Thank you so much much, your help&advice is much apprecited really! hope there is something i could help you with in the future thanks x
    Glad I could help!

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Updated: April 19, 2012
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