Mistake Master
Hess's law
Enthalpy doesn't care how you get from reactants to products — only where you start and end. That lets you build an unknown reaction's ΔH out of known ones, as long as your steps actually add up to the target.
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Enthalpy is a state function.
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Hess's law follows from enthalpy being a state function: ΔH depends only on the initial and final states, not on the path taken between them.
So you can find an unknown reaction's ΔH by combining known reactions. Manipulate each one until the steps add up to the target: reversing a reaction flips the sign of its ΔH, and scaling it multiplies its ΔH by the same factor.
Then add the adjusted ΔH values. The one essential check: the manipulated steps must actually sum to the target reaction (intermediates canceling), or the total ΔH is meaningless.
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Assembling a target reaction.
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Manipulate, verify the sum, then add the ΔH values.
- Line up the known reactions. Identify reactions whose species overlap with the target.
- Reverse and scale as needed. Reversing flips the sign of ΔH; scaling by a factor multiplies ΔH by it.
- Check that the steps sum to the target. Add the manipulated reactions; intermediates must cancel to leave the target.
- Add the adjusted ΔH values. The sum of the adjusted ΔH values is the target's ΔH.
§3
The pieces you'll meet.
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State function, manipulate, sum.
§4
Worked example: combine two known reactions.
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Goal. Find ΔH for a target reaction using two known reactions.
Manipulate. Reverse the second known reaction (its ΔH changes sign) and double the first (its ΔH doubles), so that their sum matches the target.
Check the sum. Add the two manipulated reactions; the intermediate species cancel, leaving exactly the target reaction.
Add the ΔH values. The target's ΔH is the sum of the adjusted ΔH values. If the steps had not summed to the target, the ΔH total would be wrong — the sum check is essential.
§5
Mistakes that cost real points.
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"You can add the ΔH values without checking that the steps sum to the target."
The ΔH total is only valid if the manipulated steps actually add up to the target reaction (with intermediates canceling). Adding ΔH values from steps that do not assemble the target gives a meaningless number.
Fix. Always verify the steps sum to the target before adding their ΔH values.
"Reversing a reaction keeps its ΔH the same."
Reversing a reaction flips the sign of its ΔH. If you use a known reaction backward to build the target, you must change the sign of its ΔH, or the total will be wrong.
Fix. Flip the sign of ΔH whenever you reverse a step; keep the magnitude.
"Scaling a reaction leaves its ΔH unchanged."
Scaling a reaction by a factor multiplies its ΔH by that factor, because ΔH is extensive. Doubling a step doubles its ΔH; halving it halves the ΔH. Forgetting to scale ΔH along with the reaction is a common slip.
Fix. Multiply a step's ΔH by the same factor you scale the reaction by.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.