[Big-Daddy]

I recently heard that, for an upcoming thermodynamics test, I must learn to draw Hess cycles. Not merely solve Hess's Law, mind you; draw the cycles themselves, with questions saying "draw Hess cycles". Up till this point I had been perfectly happy using my own numerical method to solve the problems but now I must learn to draw the cycles themselves or lose marks.

So can anyone explain to me how to draw the cycles - once I have the final answer? After that, it should just be an exercise in drawing and labelling correctly, but I don't know how to draw the cycles.

Let's take a problem like the following, just for example's sake:

I am given the enthalpy changes of the following reactions (since they shouldn't be relevant for drawing the cycle itself, I've left them in symbolic form, e.g. ΔH1, ΔH2, etc.):

2 NaCl (s) + H₂O (l) → Na₂O (s) + 2 HCl (g) ... ΔH1
NO (g) + NO₂ (g) + Na₂O (s) → 2 NaNO₂ (s) ... ΔH2
NO (g) + NO₂ (g) → N₂O (g) + O₂ (g) ... ΔH3
2 HNO₂ (l) → N₂O (g) + O₂ (g) + H₂O (l) ... ΔH4

And I have to find the enthalpy change for the reaction:

HCl (g) + NaNO₂ (s) → HNO₂ (l) + NaCl (s) ... ΔHr

Now, using my numerical method I can easily solve the problem:

ΔHr=ΔH3-ΔH1-ΔH2-ΔH4

But now, the question is, now that I know how to transform the equations to get my final value for ΔHr, how do I draw the cycle to show this? Please bear in mind simply producing the final answer cycle will not help me much; I'd rather you explained how you arrived at it, i.e. how you decided what to write, what order, what to write over the connecting line, etc.

I have no knowledge of Hess cycles, except that the path Reactants → Products is equivalent to Reactants → Intermediates → Products. My guess is that this means the top line of the cycle is where I write HCl (g) + NaNO₂ (s) → HNO₂ (l) + NaCl (s) (my desired reaction) and below that I will transform the reactants one by one to reach the desired products, but how do I work out how to do this from the final answer (ΔHr=ΔH3-ΔH1-ΔH2-ΔH4)?

[Dan]

Start here: http://www.chemguide.co.uk/physical/energetics/sums.html and have a go.

Hint: Your cycle will have four steps.

Your answer

ΔHr=ΔH3-ΔH1-ΔH2-ΔH4

is incorrect. You are out by a factor of 2.

[Big-Daddy]

Start here: http://www.chemguide.co.uk/physical/energetics/sums.html and have a go.

Hint: Your cycle will have four steps.

Your answer

ΔHr=ΔH3-ΔH1-ΔH2-ΔH4

is incorrect. You are out by a factor of 2.

Oh sorry, yes I meant ΔHr=(ΔH3-ΔH1-ΔH2-ΔH4)/2. This was there in my original calculations, I reached 2 NaNO₂ + 2 HCl → 2 NaCl + 2 HNO₂ but forgot to divide by 2.

Like I sid, I don't need help solving the problems. I think you misunderstand my situation: what I need help with is drawing the cycles. Let's say I've got the answer (ΔHr=(ΔH3-ΔH1-ΔH2-ΔH4)/2). How do I draw the Hess cycle to represent this answer?

Four steps doesn't help much when I have no idea what they would look like. And yes, I have seen ChemGuide, but the problem there is that Mr. Clark can't offer calculations on the website itself because he wants to "protect sales of his book" (a perfectly reasonable statement). And the few cycles drawn on the website don't tell me much about how to draw them for more complicated systems like the one in my question.

If you could suggest how I should go about drawing this cycle it would be great (now that I've got my answer). I don't use cycles to find the answer, as my method is much less superfluous, but since I need to learn how to draw them I would like to know how to go from the answer to the cycle, if you know what I mean. I am not a beginner at solving these problems, but an extreme amateur at drawing the cycles for them.

[Dan]

I provided a link that briefly shows how to construct a Hess cycle. There should be more detailed examples in your textbook, and online with a bit of googleing.

I have not misunderstood your question, I am asking you to begin by attempting the question. This is a forum rule.

[Big-Daddy]

I provided a link that briefly shows how to construct a Hess cycle. There should be more detailed examples in your textbook, and online with a bit of googleing.

I have not misunderstood your question, I am asking you to begin by attempting the question. This is a forum rule.

Thank you for your help. I've got it now, though. At least I think I have.