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    heres the equation:

    N204 --> 2NO2

    im given the enthalpy of formation data:

    N204 = +9.70

    NO2 = +33.9

    the question says: using the following data calculate the enthalpy change for the forward reaction.

    HELP!?
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    Multiply NO2 enthalpy by 2. Reverse N2O4 eqn and enthalpy. And add them.
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    Okay, so you have to use a Hess' Law diagram to do this.

    Set it out with the first equation at the top, like this:



    The enthalpy change of formation is defined as "the enthalpy change when 1 mole of a substance is made from its elements in their standard states under standard conditions."

    So, as we know that enthalpy of formation means forming/making somethin from its elements, you add the elements that make N2O4 and NO2 below. These are Nitrogen and Oxygen.

    Because it says "in their standard states under standard conditions", Nitrogen and Oxygen are diatomic gases in standard conditions (ie: they go around in pairs; 2 atoms together). Their formulas are always N2 and O2, never just N and O.

    Write the elements that make up N2O4 and NO2 under the first equation, like this:




    Then you need to balance the bottom equation. Overall you have 2 Nitrogen atoms, so that is already balanced with N2 in its standard diatomic state. However you have 4 Oxygen atoms, so you need to put a "2" infront of O2 to make it balance to 4 overall, like this:



    Then, your draw arrows using the numbers you've been given.

    The enthalpy for formation of N2O4 is +9.70, which means that is take 9.70 lots of energy (lets just call it this for arguement's sake - its really kJ per mol) to form N2O4 from its elements - ie: Nitrogen (N2) and Oxygen (O2).

    So on your cycle; you draw arrows up from the elements to the N2O4, and label it 9.70 - showing it has taken 9.70 lots of energy to form it. Like this:



    Then you will need to do the same for the NO.
    To form NO from its elements (Nitrogen and Oxygen) takes 33.9 lots of energy. So you put the arrow on to the equation again, this time labelling it 33.9

    HOWEVER if you look at your top equation, you are forming 2 moles of NO, so you will need to multiply 33.9 by 2 to account for this. If you add this to your cycle it should look like this:



    Now, you are trying to get from A to B.
    A is N204 and B is N02
    The alternative route for getting from A to B is going from A to C to B on the diagram below:



    To do this you follow the arrows logically. However, your first arrow which is 9.70 is facing the wrong way so you reverse it and from Year 7 maths you'll know (I hope) that the opposite of a + is a -.
    So this become -9.70

    Your next arrow is pointing the right way to get you from C to B. You leave that as 67.8

    You get your answer by adding them together.
    So you do your -9.70 + 67.8
    = 58.1

    (I hope I've got this right!)
    • Thread Starter
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    thank you! thats helped alot
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    (Original post by Jack_wills)
    thank you! thats helped alot
    No problem! I've been revising Enthalpy today, thought I'd test myself as well as help out :thumbsup:
 
 
 
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