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# AS Hess's cycles Watch

1. So, I'm trying to answer some questions on Hess's cycles and enthalpy changes. In so far as I've been able to answer them so far, I've succeeded.

However, I now find myself unable, with the help of both the short half hour lesson we had on Hess's law, a variety of youtube videos, and the help of both the OCR book and revision guide, to even know where to start.

In a simpler problem where all equations contained only the same elements, this might not be the case, but for the first time ever I find myself completely stumped, wishing that there had been some worked examples I could find - somewhere... anywhere.

So I turn to TSR, and ask your help...

Given the enthalphy changes ΔH1 and ΔH2 below, construct a Hess's cycle that will enable you to find the enthalphy change for the reaction:

MgCO
3(s) → MgO(s) + CO[SUB]2[/SUB](g) ...ΔHR1
MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l) ...ΔH1
MgO(s) + 2HCl(aq)
→ H2O(l) + MgCl2(aq) ...ΔH2
The question is presented here exactly as it's been formatted.
2. Soy ou know the reactions of MgCO3 and MgCl2 with HCl. Just draw out the diagram with the arrows pointing to the products... if that makes sense.
3. Actually, I know the thermal decomposition of MgCO3, and the reactions of MgCO3 and MgO with HCl.

In any case, ...

That's kind of my problem. I understand the maths, and I understand what the law is basically saying. I just don't understand how to generate the cycle.

Under circumstances where I could see that one reaction from the original reactants led to intermediary products, which in turn led to the final products of the original reaction, I'd have no problem... but the presence of HCl and H2O in the supposed intermediate steps is throwing me.

Reactants go to products, or they go to an intermediate step and then products... and the sum enthalpy of that alternative pathway is the same as the enthalpy of the direct pathway ... but I'm simply not able to see ΔH1 and ΔH2 in terms of that direct pathway for which I need to find the enthalpy.

In fact, I'll go as far as to say that we didn't cover such a situation in class. Nor can I find a youtube video dealing with such a situation. It's actually driving me nuts.
4. OK... so, finally, after a whole weekend...!

1) identify which reaction you want the enthalpy change for. In this situation, it's the one given (Hr1)

2) identify which of the two remaining reactions contains reactants of products of that first reaction uniquely. If it contains unique reactants matching the equation you're looking for, leave it as is. If not, reverse the equation, thus it becomes -H1

3) Realise that because of the nature of the question, some sum of the enthalpy of the equations marked H1 and H2 must equal the enthalpy change of Hr1

4) Perform the same instruction as in step 2 upon H2 compared with Hr1, bearing in mind what's already there from the first equation.

5) Having written out the the two equations to reflect the position of products and reactants in the required equation, Hr1, it should then be possible to merge the two equations according to reactants and products such that, when you cancel out those molecules which appear on both sides, you are left with the equation for which you are trying to find the enthalpy - this proves that your answer is correct, or otherwise.

6) write your answer in terms of H1 and H2... eg, Hr1 = ... , using the the positive and negative values of H1 and H2 created by leaving an equation as is (leaving it positive), or swapping it's products and reactants around (making it negative).

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Updated: April 14, 2013
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