Entropy

So far in this course, we have focused on enthalpy changes to understand whether reactions release or absorb energy. But enthalpy alone does not determine whether a reaction will actually occur. Some endothermic reactions happen spontaneously, while some exothermic reactions do not. To understand why, we need the concept of entropy.

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What Is Entropy?

Entropy (symbol S) is a measure of the disorder or dispersal of energy in a system. More precisely, it quantifies the number of ways that energy can be distributed among the particles in a system.

A system with high entropy has many possible arrangements of its particles and energy — it is highly disordered. A system with low entropy has few possible arrangements — it is highly ordered.

The units of entropy are J K⁻¹ mol⁻¹ (note: joules, not kilojoules).

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Standard Molar Entropy Values

Every substance has a standard molar entropy (S°), which is the entropy of one mole of the substance under standard conditions (298 K, 100 kPa).

Some important patterns: