What you'll be able to do: Combine given thermochemical equations to obtain the enthalpy change of a target reaction.
Best after: Enthalpy of Reaction and Thermochemical Equations
Some enthalpies cannot be measured directly, because the reaction is too slow, too messy, or produces side products. Hess's law lets you build the answer from reactions that can be measured.
These are recommended, not required. You can start this lesson at any time.
If a reaction can be written as the sum of two or more steps, its enthalpy change is the sum of the enthalpy changes of those steps, regardless of whether the steps represent what physically happens.
ΔH(overall) = ΔH₁ + ΔH₂ + ΔH₃ + ...
Start from the target equation, not from the given ones. Take one species at a time, find the given equation containing it, and reverse or scale that equation so the species appears on the correct side with the correct coefficient. Only then add everything up.
After manipulation, any species appearing on the left of one equation and the right of another in equal amounts cancels. A leftover species that should not be in the target is a signal that a coefficient is wrong.
Enthalpies of combustion are easy to measure, so a formation reaction that cannot be run directly is often built from combustion equations. Reverse the combustion of the target compound and add the combustions of its elements.
Nothing in Hess's law claims the steps really happen. The only requirement is that the manipulated equations add to the target, because enthalpy depends on the endpoints alone.
The enthalpy change of an overall reaction is the sum of the enthalpy changes of any set of steps that sum to it.
Apply reversal and scaling to each equation and its ΔH before summing.
Species that appear on opposite sides in equal amounts drop out of the summed equation.
The summed equation must match the target exactly, coefficients and states included.
The route used is a bookkeeping device, not a proposed mechanism.
ΔH(target) = sum of manipulated step enthalpies
Find ΔH for C(s) + 1/2 O₂(g) → CO(g) given: (1) C(s) + O₂(g) → CO₂(g), ΔH = -393.5 kJ and (2) CO(g) + 1/2 O₂(g) → CO₂(g), ΔH = -283.0 kJ.
Try it first: Ask where CO has to end up in the target, then look at where it sits in the given equations.
0 of 4 steps revealed.
Find ΔH for N₂(g) + 2 O₂(g) → 2 NO₂(g) given: (1) N₂(g) + O₂(g) → 2 NO(g), ΔH = +180.5 kJ and (2) 2 NO(g) + O₂(g) → 2 NO₂(g), ΔH = -114.1 kJ.
0 of 4 steps revealed.
A student combines two equations and finds that O₂ does not cancel, leaving 1/2 O₂ on the wrong side. What went wrong and what should be checked?
0 of 3 steps revealed.
Why it's wrong: The energy flow reverses with the reaction.
Check instead: Negate ΔH the moment the arrow is flipped.
Why it's wrong: Enthalpy is an extensive quantity tied to the coefficients.
Check instead: Apply the same multiplier to both.
Why it's wrong: Hess law needs only that the equations sum correctly.
Check instead: Judge the route by the summed equation, not by plausibility.
Why it's wrong: An uncancelled species means the target was never reproduced.
Check instead: Write out the sum and compare it with the target line by line.
No practice questions are available for this topic yet. You can still practice the whole unit.
Because enthalpy is a state function, the enthalpy change of an overall reaction equals the sum of the enthalpy changes of any set of steps that add up to it. The working method is mechanical: look at the target equation, decide where each species must end up, reverse or scale each given equation to put it there, then add the manipulated ΔH values. Species that appear on both sides cancel, and if anything fails to cancel, one of the manipulations is wrong.
Chemistry 2e, OpenStax, Rice University, licensed CC BY 4.0. License