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    (Original post by Georgiam247)
    Is it because oxygen is an element in its standard state therefore its standard enthalpy is 0? So it effectively doesn't count?
    The enthalpy change of combustion is defined as: the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions.
    It is expressed with the quantities:
    • energy/mole of fuel (kJ.mol) {this is primarily what we use in 2.3 }
    • energy/mass of fuel
    • energy/volume of fuel

    If you look at the example on page 193, CuSO4 (aq) is reacted with excess Magnesium. Again the figure for [delta]H is only the change for CuSO4 not Mg!

    I believe this is because the Carbon and Hydrogen, going back to your question, are the fuels that are reacted and oxygen just allows the reaction to happen. Also by definition the reacting substances must be 1 mole and in your reaction oxygen is not...

    Sorry if I complicated things even more for you, but I hope this helped
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    (Original post by Fluffay)
    The enthalpy change of combustion is defined as: the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions.
    It is expressed with the quantities:
    • energy/mole of fuel (kJ.mol) {this is primarily what we use in 2.3 }
    • energy/mass of fuel
    • energy/volume of fuel

    If you look at the example on page 193, CuSO4 (aq) is reacted with excess Magnesium. Again the figure for [delta]H is only the change for CuSO4 not Mg!

    I believe this is because the Carbon and Hydrogen, going back to your question, are the fuels that are reacted and oxygen just allows the reaction to happen. Also by definition the reacting substances must be 1 mole and in your reaction oxygen is not...

    Sorry if I complicated things even more for you, but I hope this helped

    No no that's brilliant! Thank you very much
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    Hey everyone
    What do you guys do to revise for paper 3? (Lab skills)
    Been told it's usually the easiest of the three papers, but I really suck at "suggest..." questions and all of that :C
    Are there any good sources that'll help me out?



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    (Original post by marklowell96)
    Who wants the Edexcel Chemistry Controlled Assessment?
    :lol: very funny...
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    (Original post by marklowell96)
    You think I dont have it?
    I don't know how you got it but if you did, why would you give it away on a public form, you could get in trouble MR...
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    OCR A! wow Im so late
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    Help!

    When chlorine is added to a solution containing bromide or iodide ions, a colour change occurs. What solvent would you add to the mixture to confirm the identity of the halogen produced?

    Is it not AgNO3...? and the result a cream ppt? (next question asked the result for if bromine was produced)
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    (Original post by Nautic4l)
    Help!

    When chlorine is added to a solution containing bromide or iodide ions, a colour change occurs. What solvent would you add to the mixture to confirm the identity of the halogen produced?

    Is it not AgNO3...? and the result a cream ppt? (next question asked the result for if bromine was produced)
    you add acidified silver nitrate (acidified to get rid of any unwanted ions i.e CO32-) the result would be cream ppt of silver bromide.
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    (Original post by ThatMadClown)
    you add acidified silver nitrate (acidified to get rid of any unwanted ions i.e CO32-) the result would be cream ppt of silver bromide.
    The mark scheme says 'A hydrocarbon (solvent) / volasil /named hydrocarbon solvent /tetrachloromethane' and 'Red / brown /orange / amber / yellowOr any combinationNo TE on incorrect / no reagent' for the result Q
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    (Original post by Nautic4l)
    Help!

    When chlorine is added to a solution containing bromide or iodide ions, a colour change occurs. What solvent would you add to the mixture to confirm the identity of the halogen produced?

    Is it not AgNO3...? and the result a cream ppt? (next question asked the result for if bromine was produced)
    Yeah AgNO3 is the common standard; will produce Ag+ ions in the mixture and it's the precipitate that forms with the halide ions you react it with that will give you the different colours.

    AgBr is cream coloured if I remember correctly
    AgI is yellow.
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    (Original post by James A)
    Yeah AgNO3 is the common standard; will produce Ag+ ions in the mixture and it's the precipitate that forms with the halide ions you react it with that will give you the different colours.

    AgBr is cream coloured if I remember correctly
    AgI is yellow.

    Read my other reply too please! I'm confused haha
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    (Original post by Nautic4l)
    Read my other reply too please! I'm confused haha
    You add silver nitrate as a standard method to getting a ppt

    Let's say you've got a solution of bromoethane, you add sodium hydroxide to it, these react releasing the Br- ion, I'm going to assume that you know the mechanism behind this? But now youve got bromide ions in the solution, after the addition of silver nitrate you should be getting a ppt of the silver bromide.
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    (Original post by ThatMadClown)
    You add silver nitrate as a standard method to getting a ppt

    Let's say you've got a solution of bromoethane, you add sodium hydroxide to it, these react releasing the Br- ion, I'm going to assume that you know the mechanism behind this? But now youve got bromide ions in the solution, after the addition of silver nitrate you should be getting a ppt of the silver bromide.
    That's not what it says on the mark scheme. I quoted you previously
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    (Original post by Nautic4l)
    That's not what it says on the mark scheme. I quoted you previously
    Can you post a pic of the question ?
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    (Original post by ThatMadClown)
    Can you post a pic of the question ?
    I'll link you both the paper and the mark scheme in just a second
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    (Original post by ThatMadClown)
    Can you post a pic of the question ?
    http://qualifications.pearson.com/co...e_20100121.pdf

    17ai/ii

    and mark scheme here: http://qualifications.pearson.com/co...0_US022678.pdf

    Have never come across this test before in 2 years of A-Level Chemistry.
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    (Original post by Nautic4l)
    http://qualifications.pearson.com/co...e_20100121.pdf

    17ai/ii

    and mark scheme here: http://qualifications.pearson.com/co...0_US022678.pdf

    Have never come across this test before in 2 years of A-Level Chemistry.
    the best thing to do would be writing that answer in your notes. Im on AQA and we do the silver nitrate test.
 
 
 
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