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    (Original post by charco)
    I didn't disregard the minus...

    The enthalpy change for a combustion reaction is exothermic (ie. negative)...

    ... BUT if you perform the equal but opposite change, i.e. go from the combustion products to the compound then you are reversing the reaction, so you have to reverse the sign of the enthalpy.

    THE NEXT BIT IS VITAL TO UNDERSTANDING

    According to the law of conservation of energy the energy change going from A --> B is equal to the negative of the enthalpy change going form B --> A
    (If this were not the case we would be able to 'make' energy from nothing)

    OK got that?

    So Hess's law takes you from A to B via a third state, C

    You want to find:

    A --> B

    You know that you can go from A --> C ............. -x kJ
    and B --> C ............... -y kJ

    and you have the values for their enthalpy changes (shown as -x and -y)

    So your route is to go first from A to C. No problem, you have the value = -x kJ

    BUT now you are at C and you want to go to B.

    Well you have the energy change of B --> C

    ... soooo the value for C --> B MUST be the reverse of -y kJ, which = +y kJ

    Hence your sum becomes:

    Enthalpy (A to B) = -x + y

    The diagram I included just shows this idea on an energy level chart. You go up it's positive, you go down it's negative.
    Ok; cheers
    I get it now.
    But, another question on another question


    I'm trying to work out the enthalpy change for this reaction

    CH4 + 2H2O------------> CO2 + 4H2


    Enthalpy change of formation values

    CH4= -75
    H2O= -286
    CO2= -394




    If the definition of enthalpy change of formation is, the enthalpy change that occurs when one mole of a compound in its standard state is formed from its constituent elements, why do I have 2 moles of H20?

    Additionally, why haven't I been given a value for H2?
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    charco, for step 2 mentioned above, i dont quite get why it is from C -->B (i dont know why it is a reverse reaction)
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    (Original post by SteveCrain)
    kk; the -2219 is the bond enthalpy for combustion of propane. You got the right answer, though I still don't understand why you chuck the minus
    You need to do the reactants - the products
    (3x-394)+(4x-286) - -2219
    Which is the same as
    (3x-394)+(4x-286) - -2219
    Or
    2219 - (3x394)-(4x286)
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    (Original post by spliishie)
    You need to do the reactants - the products
    (3x-394)+(4x-286) - -2219
    Which is the same as
    (3x-394)+(4x-286) - -2219
    Or
    2219 - (3x394)-(4x286)
    have a look at the new question I posted
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    (Original post by kingsclub)
    charco, for step 2 mentioned above, i dont quite get why it is from C -->B (i dont know why it is a reverse reaction)
    The enthalpy change to get from A---B, is the same as that to get from A---C, C---B.

    A is the reactants, so to get from them to combustion products, the value is negative. To get from C---B, which is Combustion products to the products of the original equation, we are shown that the reaction is endothermic, so the value is positive. I think that's correct.
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    (Original post by SteveCrain)
    Ok; cheers
    I get it now.
    But, another question on another question

    I'm trying to work out the enthalpy change for this reaction

    CH4 + 2H2O ------> CO2 + 4H2

    Enthalpy change of formation values

    CH4= -75
    H2O= -286
    CO2= -394

    If the definition of enthalpy change of formation is, the enthalpy change that occurs when one mole of a compound in its standard state is formed from its constituent elements, why do I have 2 moles of H20?

    Additionally, why haven't I been given a value for H2?
    There is another VERY IMPORTANT concept to understrand about energy.

    Energy is an extensive property.
    This means that two moles of a substance changing has double the energy change of 1 mole.

    If you apply Hess's law to your equation: CH4 + 2H2O ------> CO2 + 4H2

    But this time let you 'intermediate' step be the elements in their standard states you get:

    CH4 + 2H2O ----> element in standard states ----> CO2 + 4H2


    Now you treat each component at a time:

    CH4 ----> elements in standard states ............ this is the REVERSE of the formation enthalpy of methane = +75 kJ

    2H2O ----> element in standard states ........... this is double the REVERSE of the enthalpy of formation of water = 2 x +286 = +572 kJ

    element in standard states ----> CO2 ........... is the enthalpy of formation of carbon dioxide = -394 kJ

    element in standard states ----> 4H2 ................. this is no energy change

    So, now you add up all of the steps:

    +75 + 572 + (-394) = +253 kJ
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    (Original post by SteveCrain)
    The enthalpy change to get from A---B, is the same as that to get from A---C, C---B.

    A is the reactants, so to get from them to combustion products, the value is negative. To get from C---B, which is Combustion products to the products of the original equation, we are shown that the reaction is endothermic, so the value is positive. I think that's correct.
    It's not even that complex.

    You don't even have to consider whether it's combustion or not:

    IF

    A --> B is exothermic and = -y kJ

    THEN

    B --> A is endothermic and = +y kJ
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    (Original post by SteveCrain)
    Ok; cheers
    I get it now.
    But, another question on another question


    I'm trying to work out the enthalpy change for this reaction

    CH4 + 2H2O------------> CO2 + 4H2


    Enthalpy change of formation values

    CH4= -75
    H2O= -286
    CO2= -394




    If the definition of enthalpy change of formation is, the enthalpy change that occurs when one mole of a compound in its standard state is formed from its constituent elements, why do I have 2 moles of H20?

    Additionally, why haven't I been given a value for H2?

    H2 is an element and not a compound (just a diatomic molecule) so it has no enthalpy change of formation as it is an element in itself
    Also, 2 moles of H2O react so you need to use 2x-286.
    The enthalpy change therefore is (formation of products) - (formation of reactants)
    (-394) - (2x-386 -75) = +453kj/mol
    Does that makes sense?
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    (Original post by spliishie)
    H2 is an element and not a compound (just a diatomic molecule) so it has no enthalpy change of formation as it is an element in itself
    Also, 2 moles of H2O react so you need to use 2x-286.
    The enthalpy change therefore is (formation of products) - (formation of reactants)
    (-394) - (2x-386 -75) = +453kj/mol
    Does that makes sense?
    Yes cheers m8; I've spent a quite a few hours on enthalpy changes this weekend. I'm feeling quite confident now.
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    (Original post by SteveCrain)
    Yes cheers m8; I've spent a quite a few hours on enthalpy changes this weekend. I'm feeling quite confident now.
    That's good :-) Just keep doing them until you understand them, they're all basically the same just with different numbers
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    (Original post by Notsocleverstudent)
    You put the bolded values as negative

    And when it comes to calculating enthalpy change, it always either Reactants - Products or Products - Reactants

    But you added the 2 values :confused:
    How do you know if its reactants - products or products - reactants??
 
 
 
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