Entropy ( ) Entropy (S) is a measure of disorder in a system – Nature likes to create disorder (i.e., ΔS > 0) – Larger entropies mean that more energy states are available to the system (e.g., vibrational modes) Tables report standard entropies rather than entropy changes due to the third law of thermodynamics – A pure, crystalline substance at zero Kelvin has no entropy (i.e., perfect order) – Disruption of this ordering increases the entropy – For enthalpy, we can only measure changes in heat content, not absolute heat Entropy is obviously temperature dependent since increasing temp. increases molecular motion (and thus disorder) – Dependence differs from enthalpy – Entropy differences can be measured for chemical reactions (ΔS rxn )

Gibbs Free Energy and Spontaneity (6.1) Spontaneity is determined by whether a process will occur without work being done on the system – Though a process is spontaneous, that does not mean it occurs quickly (kinetics determines rate) – Spontaneous processes are processes that don’t require work (e.g., release of heat, increase in disorder) Gibbs free energy (G) determines whether a process at constant pressure and temperature is spontaneous – If the change in Gibbs free energy (ΔG) is negative, the process is spontaneous (differential form: dG < 0) – If the Gibbs free energy change is equal to zero, the process is at equilibrium (e.g., ice/water at freezing point) Gibbs free energy change is related to enthalpy and entropy – Temperature changes can cause some processes to occur spontaneously (e.g., ice melting)

Temperature and Composition Dependence of ΔG ( ) Temperature dependence of Gibbs free energy is related to enthalpy – A simple form ensues assuming the enthalpy change over the temperature range is constant For a mixture, the Gibbs energy also depends on the composition of the mixture – The dependence on the change in molar quantities of a component is referred to as the chemical potential (μ i ) Chemical potential dictates in which direction components will move in a mixture – Components will move from regions of high chemical potential to low chemical potential until the potentials are equivalent throughout the sample

Gibbs Free Energy and Equilibrium ( ) Using chemical potentials, the Gibbs free energy change in a reversible reaction can be expressed in terms of a reaction quotient (Q P ) – For gases, pressures are used instead of concentrations in Q P – ΔG 0 is related to standard chemical potentials We can get an equilibrium constant (K) from ΔG 0 since we know that at equilibrium ΔG = 0 – At equilibrium, Q P is equivalent to what we know as K – If we can get ΔG 0 from thermodynamic experiments, we can get K Since K is related to ΔG 0 we also know the temperature dependence of K