Hess's Law

Why can’t all reactions be tested with a calorimeter

Because some reactions produce multiple products at once or are too small/fast to measure accurately.

What assumption do we make about enthalpy change in theory

That the enthalpy change depends only on initial and final conditions — not on the pathway taken.

What does it mean that enthalpy is path independent

No matter how many intermediate steps a reaction takes, the total ΔH is always the same for the same reactants → products.

Who proposed Hess’ Law and when

G.H. Hess, in 1840.

State Hess’ Law in words.

“The enthalpy change of an overall chemical reaction is equal to the sum of the enthalpy changes of the individual steps.”

Why is Hess’ Law useful

It allows us to calculate enthalpy changes without direct calorimetry, by combining known equations and their ΔH values.

What happens to ΔH if you reverse a chemical equation

The sign of ΔH flips.

What happens to ΔH if you multiply or divide a chemical equation by a factor

ΔH is multiplied or divided by the same factor.

What’s the logic behind changing ΔH when you change coefficients

Enthalpy is extensive, meaning it depends on the amount of substance reacting.

Step 1 of solving a Hess’ Law problem

Identify the target equation (the one you need to find ΔH for).

Step 2 of solving a Hess’ Law problem

List the given equations and their ΔH values.

Step 3 of solving a Hess’ Law problem

Manipulate the given equations (reverse, multiply/divide) so that when added, they yield the target equation.

Step 4 of solving a Hess’ Law problem

Add the manipulated equations together, canceling anything that appears on both sides.

Step 5 of solving a Hess’ Law problem

Add all the ΔH values for the manipulated equations to find the net ΔH.

What’s key to mastering Hess’ Law problems

Being systematic and patient — use trial and error to make equations fit the target.

What’s the rule about combining enthalpies in this example

Use given enthalpy data (from calorimetry), adjust the equations, cancel terms, and sum all ΔH values.

What’s the core concept behind Hess’ Law

Energy conservation — total enthalpy change depends only on start and end states, not the route.

What makes Hess’ Law powerful in thermochemistry

It allows calculation of unknown enthalpies using known reactions instead of lab measurements.

In simple words, what does Hess’ Law say

“If you can’t measure it, build it.”