9.3Enthalpy
Section · module m68727Read §9.3 at OpenStax ↗
Study this section
Written from the section, each card quoting the span it came from. The ones you miss come back sooner.
What this needs
Learn these first — this section will not make sense without them.
Hess's law is linear combination of equations: scale a reaction (and its ΔH) by a constant, reverse another (negating its ΔH), then add so intermediates cancel — exactly the multiply-then-add elimination method for two-variable systems, just eliminating a chemical species instead of x.
What this touches
Algebra that turns up here. Not a blocker, but this is where you will see it used.
Reversing a reaction negates its ΔH, and multiplying a reaction by a fraction multiplies ΔH by that same fraction — signed-number bookkeeping that, if fumbled on one step of a multi-step Hess's law sum, flips the final answer's sign.