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Rules of Hess's Law

Definition

The rules of Hess’s law are guidelines used to calculate the overall enthalpy change for complex reactions by breaking them down into simpler ones for which enthalpy changes are known.

Analogy

Consider these rules as instructions for building LEGO sets. If you want to build something complex (like Hogwarts Castle), it might be easier to break down the construction process into smaller parts (like building each tower separately) before putting everything together.

Related terms

Standard Enthalpy Change of Formation (∆Hf°): This refers to the energy change when one mole of compound is formed from its elements under standard conditions. It's like the energy needed to assemble a LEGO set from individual pieces.

Standard Enthalpy Change of Combustion (∆Hc°): This is the enthalpy change when one mole of substance completely burns in oxygen under standard conditions. Imagine this as the energy released when you burn all your LEGO instructions (not recommended!).

Calorimetry: This is a technique used to measure the heat absorbed or released during a chemical reaction, much like using a thermometer to measure how hot your LEGO bricks get if left in the sun.