Applications of Thermodynamics

Entropy measures the energy unavailable to do work in a system—basically the amount of disorder. According to the second law of thermodynamics, the universe naturally moves toward increased entropy over time. This explains why spontaneous processes (ones that happen naturally) always increase overall entropy.

Matter becomes more disordered as it spreads out, increasing entropy. When substances change from solid → liquid → gas, molecules spread further apart and entropy increases. Similarly, if you expand a container of gas or heat it up, entropy increases as energy and particles become more dispersed.

You can calculate the change in entropy for a reaction using this formula: $\Delta S_{system} = \sum S_{products} - \sum S_{reactants}$

Gibbs free energy (G) helps determine if a process will happen spontaneously. The equation combines changes in enthalpy (ΔH), entropy (ΔS), and temperature (T): $\Delta G = \Delta H - T\Delta S$

💡 Remember this rule: When ΔG is negative, the reaction is spontaneous (will happen on its own). When you see ΔG° on the AP exam, it refers to Gibbs free energy at standard state conditions.

Looking at the equation, you can predict if a reaction is favorable based on enthalpy, entropy, and temperature values. For instance, exothermic reactions (negative ΔH) with increasing entropy (positive ΔS) are always spontaneous at all temperatures.