Hess's Law

Main Content

What is Hess's Law?

Hess's Law states that:

In simpler terms: It doesn't matter whether a reaction happens in one step or in multiple steps – the total energy change will always be the same.

Think of it like climbing a mountain. Whether you take a direct steep path or a longer winding route, the height you climb from bottom to top is exactly the same. The same idea applies to chemical reactions and energy changes.

Why is Hess's Law Useful?

Some enthalpy changes are impossible or very difficult to measure directly in the lab. For example:

• Formation of propane from carbon and hydrogen:

  3C(s) + 4H₂(g) → C₃H₈(g)

  This reaction won't happen under standard laboratory conditions – carbon and hydrogen simply don't react together this way in practice.

• Bond energies – we can't break just one specific bond in a molecule and measure the energy needed without affecting other bonds.

Hess's Law lets us calculate these "impossible-to-measure" enthalpy changes by using other reactions whose enthalpy changes we can measure.

Understanding Energy Cycles

An energy cycle is a diagram that shows different routes (pathways) from reactants to products.

Here's how it works:

• Direct route = the reaction we're interested in (the one we can't measure directly)
• Indirect route = a different pathway using reactions we can measure

According to Hess's Law, the enthalpy change for the direct route equals the total enthalpy change for the indirect route.

Important principle:

• If you go forward along an arrow → use the enthalpy value as it is (same sign)
• If you go backward along an arrow ← reverse the sign of the enthalpy value