Spontaneous Processes and Entropy First Law “Energy can neither be created nor destroyed“. The energy of the universe is constant. Spontaneous Processes Processes that occur without outside intervention. Spontaneous processes may be fast or slow. –Many forms of combustion are fast –Conversion of diamond to graphite is slow

Entropy (S) A measure of the randomness or disorder.A measure of the randomness or disorder. The driving force for a spontaneous process is an increase in the entropy of the universe.The driving force for a spontaneous process is an increase in the entropy of the universe. Entropy is a thermodynamic function describing the number of arrangements that are available to a system.Entropy is a thermodynamic function describing the number of arrangements that are available to a system. Nature proceeds toward the states that have the highest probabilities of existing.Nature proceeds toward the states that have the highest probabilities of existing. The most likely is the most random.The most likely is the most random.

Positional Entropy The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved.The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved. Positional entropy increases from solid to liquid to gas.Positional entropy increases from solid to liquid to gas. S solid < S liquid << S gas

Solid state: molecules are close together with relatively few positions available.Solid state: molecules are close together with relatively few positions available. Gaseous state: molecules are far apart, with more positions available.Gaseous state: molecules are far apart, with more positions available. Liquid state is closer to the solid state than gaseous state.Liquid state is closer to the solid state than gaseous state. S solid < S liquid << S gas

Second Law of Thermodynamics  "In any spontaneous process there is always an increase in the entropy of the universe"  "The entropy of the universe is increasing"  For a given change to be spontaneous,  S universe must be positive  S univ =  S sys +  S surr

Temperature and Spontaneity Entropy changes in the surroundings are primarily determined by heat flow. Exothermic reactions in a system at constant temperature increase the entropy of surroundings. ∆S surr = positive Endothermic reactions in a system at constant temperature decrease the entropy of surroundings. ∆S surr = negative The impact of the transfer of a given quantity of energy as heat to or from the surroundings will be greater at lower temperatures.

∆S surr for a reaction under conditions of constant temperature (K) and pressure: ∆S surr = - ∆H / T

Gaseous reactions: Calculate change in the number of moles of gas, ∆n gas on going from reactants to products. ∆n gas is positive = entropy is positive 2NaHCO 3 (s) → Na 2 CO 3 (s) + CO 2 (g) + H 2 O(g) Gaseous products > Gaseous reactants Change in Entropy = ∆S = positive

Another factor that affects the sign of ∆S is degree of complexity of molecules. H 2 (g) → 2H(g) Decrease in complexity and increase in number of particles = increase in entropy

“At absolute zero the entropy of a perfectly ordered pure crystalline substance is zero.”

Since pure crystal is zero all other must be > 0. Standard Entropy of 1 mol of substance, S o is measured at 298 K (25 o C) and 1 atm. Units are J/K mol. Values published in Appendix 4 of textbook.

Calculating Entropy Change in a Reaction  Entropy is an extensive property (a function of the number of moles)  Generally, the more complex the molecule, the higher the standard entropy value