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Enthalpy Cycle

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    • Thread Starter

    I'm stuck on this question from OCR's F322 January 2012 Paper (see image below).

    Name:  Enthalpy Cycle.png
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    For A I got -168 (which was right), but then I got the other 3 wrong - I put -54 for B, -285 for C and -393 for D.

    The mark scheme says -285 (B), -393 (C) and -54 (D) and the overall ΔHf as -792.

    Could someone please explain why.


    Reaction A, is calcium and hydrochloric acid making calcium chloride, etc, which is -168

    Reaction B is the formation of water, which is -285

    Reaction C is the formation of carbon dioxide, which is -393

    Reaction D is the formation of calcium chloride from calcium carbonate, which is -54.

    Hess's law dictates that the enthalpy change of formation is the same, whether it's in 1 step or in multiple steps. If you follow the arrows around from A to B to C to D, you'll see that all the arrows orientate the same way around apart from D. When this happens, you need to reverse the sign.

    Therefore, the data we're using is
    A = -168
    B = -285
    C = -393
    D = +54

    If we let x represent the enthalpy change of formation:

    -168 - 285 - 393 + 54 = x
    x = -792

    I find that comparing what the difference is between each equation helps you work out which value represents it. For example, you picked -54 as B, but that equation requires calcium carbonate, so it can't possibly have been that. Instead, you compare each reactant and product and see what changes. The reaction forms water, so you use the value that involves the formation of water, therefore it's -285.
    • Thread Starter

    Thank you so much! That makes so much more sense - I was just sort of guessing when I did it...

    (Original post by alde123)
    Thank you so much! That makes so much more sense - I was just sort of guessing when I did it...
    You're welcome, I'm glad you do c:

    I know what you mean, but the key to doing well is understanding what's happening. When I first did Hess's law I couldn't understand a thing, but when I realised that the calculations come from the definitions, I learnt where the arrows go and thus how to work it all out. Then you follow the arrows around, switching the direction around means you switch the signs, and then you get your answer

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