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    (Original post by A84)
    need help in gce paper unit 1 chemistry question 16 b (ii) which is Suggest how you would mix the acid and the coral to ensure that no carbn dioxide escaped from apparatus... ive found an old TSR thread with various answers.. does this look good. what is the "perfect answer"??

    From old TSR thread:

    Stick coral to a bung half fill test tube with acid put bung into test tube shake or flip the other way up so they come into contact In the sealed test tube
    That could work but the gas produced will have nowhere to go so bung could shoot out lol

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    Hi. I'm getting slightly confused over calculating the different types of enthalpy changes. I used to be OK with this, but for some reason, the way I've always used isn't getting me the right answer in this particular question.

    So, my understanding:

    ∆Hr = ∑∆Hc (reactants) - ∑∆Hc (products)


    So in a question like this:

    Given the following data:
















    Substance
    H2O(l)
    CO2(g)
    Ethane C2H6(g)
    Ethene C2H4(g)
    ∆Hf/kJmol-1
    -285.5
    -393
    -83.6
    +52.0


    Calculate the enthalpy of combustion of Ethane.

    Would you use the equation I gave above? Or would you simply use the products - reactants formula.

    Also, please can you explain why you used the particular method you did, as I believe this is where my knowledge is lacking.
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    (Original post by gabby07)
    Hi. I'm getting slightly confused over calculating the different types of enthalpy changes. I used to be OK with this, but for some reason, the way I've always used isn't getting me the right answer in this particular question.

    So, my understanding:

    ∆Hr = ∑∆Hc (reactants) - ∑∆Hc (products)


    So in a question like this:

    Given the following data:
















    Substance
    H2O(l)
    CO2(g)
    Ethane C2H6(g)
    Ethene C2H4(g)
    ∆Hf/kJmol-1
    -285.5
    -393
    -83.6
    +52.0


    Calculate the enthalpy of combustion of Ethane.

    Would you use the equation I gave above? Or would you simply use the products - reactants formula.

    Also, please can you explain why you used the particular method you did, as I believe this is where my knowledge is lacking.
    On a question like this you it's always best to draw a Hess cycle for the information you've been given and the info you want to find out

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    (Original post by samb1234)
    On a question like this you it's always best to draw a Hess cycle for the information you've been given and the info you want to find out

    Posted from TSR Mobile

    I don't think the question provides enough information to draw a Hess Cycle...

    Once I have drawn up the cycle, I find that I am missing info for Delta H combustion of Carbon :rolleyes:

    My working
    https://filetea.me/t1sTO0Q8BslQXauOLqp2w9k5A
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    (Original post by gabby07)
    I don't think the question provides enough information to draw a Hess Cycle...


    My working
    For that one you can just do product minus reactant and you get the right value. Idk why ethene data is there. It's not necessary


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    (Original post by gabby07)
    I don't think the question provides enough information to draw a Hess Cycle...


    My working
    Enthalpy change of combustion = {DeltaH products} - {DeltaH reactants}
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    Is calculating enthalpy change of combustion then different to calculating enthalpy change of reaction using combustion data? Is that where my confusion is stemming from?
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    (Original post by gabby07)
    Is calculating enthalpy change of combustion then different to calculating enthalpy change of reaction using combustion data? Is that where my confusion is stemming from?
    Yes
    There's almost no way combustion is endothermic. Your way would give endothermic however

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    (Original post by gabby07)
    I don't think the question provides enough information to draw a Hess Cycle...

    Once I have drawn up the cycle, I find that I am missing info for Delta H combustion of Carbon :rolleyes:

    My working
    https://filetea.me/t1sTO0Q8BslQXauOLqp2w9k5A
    It does. I just did it that way

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    (Original post by samb1234)
    On a question like this you it's always best to draw a Hess cycle for the information you've been given and the info you want to find out

    Posted from TSR Mobile
    I agree... i believe there is enough information to draw a thermochemical cycle. the method is sufficient as it reduces common errors.
    create a combustion equation for the given reaction and then draw the hess cycle. it should work out.

    Good Luck
    All the Best.
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    Hi,
    firstly, i think this is a great idea of a thread so thank you to the creator.
    im doing the AQA chemistry as-course.

    one of the questions wghivh appeared in the 2012 EMPA, section C, was,

    'Some alcohols can be oxidised by an acidified solution of potassium dichromate(Vl).Aldehydes can be oxidised by Tollens’ reagent or by Fehling’s solution.An unknown pure liquid A contains only a single alcohol.Outline a simple procedure to allow you to determine whether A is a primary, asecondary or a tertiary alcohol'.

    how would i approach this question. i find the mark scheme to be fairly vague and hard to follow.

    thank you,
    good luck to all
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    (Original post by samb1234)
    On a question like this you it's always best to draw a Hess cycle for the information you've been given and the info you want to find out

    Posted from TSR Mobile
    Here's an easy way to understand. If it's enthalpy change of formation FPAR (products - reactants)
    If it's enthalpy change of combustion CRAP (reactants - products)
    Then after this make sure you put the correct endothermic/ ectothermic sign before the answer
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    (Original post by asifa129)
    Hi,
    firstly, i think this is a great idea of a thread so thank you to the creator.
    im doing the AQA chemistry as-course.

    one of the questions wghivh appeared in the 2012 EMPA, section C, was,

    'Some alcohols can be oxidised by an acidified solution of potassium dichromate(Vl).Aldehydes can be oxidised by Tollens’ reagent or by Fehling’s solution.An unknown pure liquid A contains only a single alcohol.Outline a simple procedure to allow you to determine whether A is a primary, asecondary or a tertiary alcohol'.

    how would i approach this question. i find the mark scheme to be fairly vague and hard to follow.

    thank you,
    good luck to all
    So there are the different tests, oxidising with potassium dichromate or tollens reagent/ feelings
    You could add potassium dichromate and if it's a:
    Primary alcohol ---> aldehyde ----> carboxylic acid
    Secondary alcohol ------> ketone
    Tertiary alcohol ----> doesn't oxidise because it has to break a C-C bond rather than a C-H
    (These are what they all oxidise too)
    The potassium dichromate in these reactions is reduced to green chromium ions

    Or you could use Tollens reagent and if a silver mirror is formed it shows an aldehyde is present
    If there's a ketone in the solution nothing happens
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    [QUOTE=gabby07;55590663]Hi. I'm getting slightly confused over calculating the different types of enthalpy changes. I used to be OK with this, but for some reason, the way I've always used isn't getting me the right answer in this particular question.

    FORGET WHAT THEY ASK YOU LOOK AT THE DATA VALUES IF Formation Values you are forming the products so Del H= Products-Reactants
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    (Original post by samwillettsxxx)
    Here's an easy way to understand. If it's enthalpy change of formation FPAR (products - reactants)
    If it's enthalpy change of combustion CRAP (reactants - products)
    Then after this make sure you put the correct endothermic/ ectothermic sign before the answer
    I don't have a problem working this kind of stuff out. Drawing a Hess cycle reduces errors as it sorts out signs for you and you don't need to remember any formulas

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    (Original post by samwillettsxxx)
    Here's an easy way to understand. If it's enthalpy change of formation FPAR (products - reactants)
    If it's enthalpy change of combustion CRAP (reactants - products)
    Then after this make sure you put the correct endothermic/ ectothermic sign before the answer
    Also, if you just learn that method you are in trouble if they ask you to do calculations with other types of enthalpy changes eg neutralisation reaction etc

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    (Original post by samwillettsxxx)
    So there are the different tests, oxidising with potassium dichromate or tollens reagent/ feelings
    You could add potassium dichromate and if it's a:
    Primary alcohol ---> aldehyde ----> carboxylic acid
    Secondary alcohol ------> ketone
    Tertiary alcohol ----> doesn't oxidise because it has to break a C-C bond rather than a C-H
    (These are what they all oxidise too)
    The potassium dichromate in these reactions is reduced to green chromium ions

    Or you could use Tollens reagent and if a silver mirror is formed it shows an aldehyde is present
    If there's a ketone in the solution nothing happens
    Thank you ever so much.
    would you have to specify 'acidified' potassium dichromate or manganate as a reagent?
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    (Original post by asifa129)
    Thank you ever so much.
    would you have to specify 'acidified' potassium dichromate or manganate as a reagent?
    Yes
    Better specify that it is sulfuric acid too

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    (Original post by C0balt)
    Yes
    Better specify that it is sulfuric acid too

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    why are tertiary alcohols more reactive than primary alcohols?
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    (Original post by gabby07)
    Hi. I'm getting slightly confused over calculating the different types of enthalpy changes. I used to be OK with this, but for some reason, the way I've always used isn't getting me the right answer in this particular question.

    So, my understanding:

    ∆Hr = ∑∆Hc (reactants) - ∑∆Hc (products)


    So in a question like this:

    Given the following data:
















    Substance
    H2O(l)
    CO2(g)
    Ethane C2H6(g)
    Ethene C2H4(g)
    ∆Hf/kJmol-1
    -285.5
    -393
    -83.6
    +52.0


    Calculate the enthalpy of combustion of Ethane.

    Would you use the equation I gave above? Or would you simply use the products - reactants formula.

    Also, please can you explain why you used the particular method you did, as I believe this is where my knowledge is lacking.
    You don't need the formation data for ethene. Learn how to manipulate Hess cycles. I find it easier and less problematic to set up a Hess cycle. You should get -1558.9 kj mol-1

    Here's my working using the longer way, my writing or explanation isn't great but that's how I did it.



    To prove it: Using the products - reactants equation. I get the same answer.

    -1642.5 -(-83.6) = 1558.9 kj mol-1

    I should mention also, the enthalpy of combustion is burning 1 mol of a substance...that's why we can't double up the equation. We are burning 1 mol of C2H6.
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