Hess's Law
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalHess's law says that because enthalpy is a state function, a reaction's ΔH depends only on the initial and final states, not the path. So you can combine known reactions — reversing one flips the sign of its ΔH, scaling one multiplies its ΔH — until they add up to the target reaction, then sum the ΔH values.
The traps assemble the steps without checking that they actually sum to the target, or mishandle the signs and factors when reversing and scaling. Manipulate each known reaction, confirm the steps add to the target, then add the adjusted ΔH values.
The work
3 ways in · any order
Lesson
Hess's Law
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Enthalpy is a state function, so ΔH sums over any path — reverse flips the sign, scaling multiplies. The lesson assembles steps to a target and sums ΔH, then closes with a ten-scenario check.
Diagnostic
10-item topic check
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Ten items spanning the Topic 6.9 misconceptions: Hess steps assembled without checking the target, ΔH treated as fixed regardless of amount, and signs mishandled when reversing or scaling equations.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.