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    If you have the Oxford Chemistry book for the new syllabus, have you noticed any wrong answers??

    I am more than 100% sure that the answer to question 5 in the end of topic questions for chapter 5 is WRONG.

    Please be a hero and help me figure this out! I'd really appreciate anyone's take on this.
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    Alternatively, if you copy the Q and A... I (or someone else) will tell you.
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    (Original post by Pigster)
    Alternatively, if you copy the Q and A... I (or someone else) will tell you.
    Sure!

    Question:

    Use data from section 12 of the Data Booklet to calculate the enthalpy change of reaction for each of these chemical reactions:

    (data booklet can be found here: http://www.ibchem.com/root_pdf/data_booklet_2016.pdf)

    a) C2H2 (g) + H2 (g) --> C2H6 (g)

    b) CH4 (g) + H2O (g) --> CO(g) + 3H2 (g)

    Answer:

    a) 2706 - 2830 = -124 kJ

    b) 2582 - 2380 = 202 kJ (NB C---C =1072 kJ mol-1)

    In addition to the fact that I don't know if I'm applying the rules correctly, I don't understand where these values come from since they are nowhere to be found in section 12 of the data booklet, leading me to think that they might be using other information?
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    You need section 11 of the Data booklet.

    They are bond enthalpy calculations.
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    (Original post by Pigster)
    You need section 11 of the Data booklet.

    They are bond enthalpy calculations.
    Thanks, but am I not supposed to use the following equation since the question asks for the enthalpy change of reaction?

    ΔHo = Σ(νΔHo f) (products) − Σ(νΔHo f) (reactants)

    aka:

    Would I not use the data from seaction 12 to complete this formula like this:

    (-84) - ((52)+9) = +136 kJ

    ((-110.5)+(0)) - ((-74)+(-285)) = 249 kJ
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    Why are there no column headers in section 12? The first column is DfH, IDK what the second value is, the third is entropy.

    I guess you meant to put +0 rather than +9 on the first.

    Since the Q said to use section 12 data, then you have done it right (my layout:
    -(+52) -(0) + (-84) = -136
    -(-74) -(-285.8) +(-110.5) + 3(0) = 249.3

    The answers assumed section 11 data.

    Just to check your understanding, I'll ask: why are the two values (-124 and -136) different?
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    (Original post by Pigster)
    Why are there no column headers in section 12? The first column is DfH, IDK what the second value is, the third is entropy.

    I guess you meant to put +0 rather than +9 on the first.

    Since the Q said to use section 12 data, then you have done it right (my layout:
    -(+52) -(0) + (-84) = -136
    -(-74) -(-285.8) +(-110.5) + 3(0) = 249.3

    The answers assumed section 11 data.

    Just to check your understanding, I'll ask: why are the two values (-124 and -136) different?
    Absolutely!

    Well, I can't claim that I have reached a perfect understanding, but I believe that the difference between bond enthalpy and the enthalpy change of reaction is that bond enthalpy deals with the energy required to break a bond/energy released when a bond is formed, while the enthalpy change of reaction deals with the energy that is either released from a system/consumed by a system in a chemical reaction.

    Is this correct?

    Also, thank you very much!
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    Nope: bond enthalpy values are averages* (except for things like H-H bonds), so calculations based on bond enthalpies are only estimates.

    *The C-H bond occurs in lots of different chemicals and can have a different bond strength in each molecule, so we use an average derived from all the different values.
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    (Original post by Pigster)
    Nope: bond enthalpy values are averages* (except for things like H-H bonds), so calculations based on bond enthalpies are only estimates.

    *The C-H bond occurs in lots of different chemicals and can have a different bond strength in each molecule, so we use an average derived from all the different values.
    Aaah that makes sense! So, can we say that enthalpy change of a reaction is more "correct" or closer to the real value?
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    (Original post by IDKY)
    Aaah that makes sense! So, can we say that enthalpy change of a reaction is more "correct" or closer to the real value?
    Yep.
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    (Original post by Pigster)
    Why are there no column headers in section 12? The first column is DfH, IDK what the second value is, the third is entropy.

    I guess you meant to put +0 rather than +9 on the first.

    Since the Q said to use section 12 data, then you have done it right (my layout:
    -(+52) -(0) + (-84) = -136
    -(-74) -(-285.8) +(-110.5) + 3(0) = 249.3

    The answers assumed section 11 data.

    Just to check your understanding, I'll ask: why are the two values (-124 and -136) different?
    There are no headings as the shown databooklet was the first edition. This was subsequently corrected in the second edition.

    The first data column is standard enthalpy change of formation.
    The second data column is standard Gibbs free enthalpy change of formation
    The third data column is standard entropy.
 
 
 
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