10-8 Entropy, Free Energy, and Spontaneity (Section 10.10) And you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Entropy is a measure of disorder. The greater the disorder or randomness, the greater the entropy and vice versa. The symbol for entropy is S and the change in entropy is ∆S.

Examples of +∆S (increasing entropy): ice melting, perfume filling a room, your room becoming quite messy.

There are two trends or “driving forces” in nature; that is, reactions will move toward these two states or conditions: 1) Lower enthalpy (H) ~ most reactions are exothermic with products at lower H. 2) Higher entropy (S) ~ a trend toward greater disorder. These two forces combine to determine the Gibbs Free Energy (G) of a system and can best be explained by the important equation: ∆G = ∆H – T∆S

When the sign of ∆G is negative a reaction will occur spontaneously, which means it will occur without continual assistance. Spontaneous does NOT mean a reaction will just occur on its own; it takes energy to break chemical bonds and get a reaction started (activation energy). Enthalpy and entropy changes allow chemists to predict whether a process will be spontaneous: (Note: temperatures for T must be in Kelvin, that is, no negative values)

∆G = ∆H – T∆S Supplemental work on board here......

The last two situations illustrate the importance of the temperature. ∆G will be negative and the reaction spontaneous if: Case 3: the temperature is low, making the ∆H term more dominant Case 4: the temperature is high, making the T∆S term more dominant

Example of Case 4: H 2 O(g) + C(s) → CO 2 (g) + H 2 (g)  H = +131 kJ/mol g + s = g + g  S = kJ/mol K At 25ºC = 298 K  G = +131 –[298(0.133)] = +91 will not happen At 900ºC = 1173 K  G = +131 –[1173(0.133)] = -25 will happen

Summary Higher temperature makes entropy more important. Lower temperature makes enthalpy more important. So then: Why does water evaporate at room temperature?

Evaporation results in a + ▲ S (entropy) = more disordered = go forward Evaporation results in + ▲ H (enthalpy) = req. E to overcome IMFs = not go forward But water does evap. At room temp. therefore, + ▲ S entropy wins!!!

What makes  G negative? In general: Large positive ▲ S Large negative ▲ H Small Positive ▲ S (when temp. is high) Negative ▲ S (when temp. is low) Summary: - ▲ H = yes, + ▲ S = yes, - ▲ G = yes