2/18/2013. Spontaneity, Entropy & Free Energy Chapter 16. Spontaneity Process and Entropy Spontaneity Process and Entropy 16.

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1 Spontaneity, Entropy & Free Energy Chapter 16 Spontaneity Process and Entropy Spontaneous happens without outside intervention Thermodynamics studies the initial and final states of a reaction Kinetics is concerned with the pathway Kinetics is concerned with the time or speed of the reaction Spontaneity Process and Entropy Entropy The driving force for spontaneous processes of the universe The measure of molecular randomness disorder Describes the number of arrangements available to a system in a given state probability Nature spontaneously moves towards the state that has the highest probability of existing. 1

2 Entropy and Microstates Microstate a configuration that gives a particular arrangement Positional probability depends on the number of configurations in space Entropy and Microstates Entropy and Microstates Positional entropy increases from solid to liquid to gas Positional entropy increases when solute and solvent are mixed - Favoring the formation of solutions 2

3 Sample Exercises & 16.2 For each of the following pairs, choose the substance with the higher positional entropy a. Solid CO 2 and gaseous CO 2 CO 2 gas b. N 2 gas at 1 atm and N 2 gas at 1.0 x 10-2 atm At 1.0 x 10-2 atm Predict the sign of the entropy change for each of the following processes a. Solid sugar is added to water to form a solution b. Iodine vapor condenses on a cold surface to form crystal Will entropy increase or decrease for the following state changes (s) to (aq) (g) to (aq) (l) to (aq) increase decrease increase * Mixtures have more entropy positive negative 16.2 Entropy and the Second Law of Thermodynamics In any spontaneous process there is an increase in the entropy of the universe 1 st Law Energy is conserved 2 nd Law Entropy is not conserved; it is increaseing S universe is always increasing If S universe is then the forward process is spontaneous If S universe is then the reverse process is spontaneous then the forward process is not spontaneous If S universe is 0 then the process has no tendency to occur or the system is at equilibrium Interplay of ΔS sys and ΔS surr in Determining the Sign of ΔS univ 3

4 16.2 Sample Exercise In a living cell, large molecules are assembled from simple ones. In this process consistent with the second law of thermodynamics? S universe must be positive for the process to be spontaneous. A process for with S system is negative can be spontaneous if the associated S surroundings is both larger and positive. If 50 J of energy are transferred to surrounding where the temperature is high there would not be a big difference in the motion of the molecules. However if 50 J of energy are transferred to surroundings where the temperature is low, there would be a bigger difference in the motion of the molecules. The impact of the transfer of a given quantity of energy as heat to or from the surroundings will be greater at lower temperatures Who would feel the greatest effect from a donation of $50? Exothermic increases random motion S surr = + Endothermic decreases random motion S surr = - Entropy of surroundings is determined by heat flow. The impact of energy transfer to or from a system is greater at lower temperatures The sign of S surr depends on the direction of the heat flow nature tends to seek the lowest energy The magnitude of S surr depends on the temperature S surr is directly proportional to the quantity of heat transferred S surr is inversely proportional to the temperature 4

5 S surr = quantity of heat (J) temperature (K) + for endothermic - for exothermic S surr = H T The negative changes the H from the system to the surroundings SURROUNDINGS T 2 ENDOTHERMIC T 2 > at the beginning Energy flows into the system T 2 as until T 2 = Decreasing the entropy of the surroundings H is positive EXOTHERMIC >T 2 at the beginning Energy flows out of the system T 2 as until T 2 = Increases the entropy of the surroundings H is negative SURROUNDINGS T 2 ENDOTHERMIC S sys + S surr H + - H T 2 H is the same so S is dependent on temperature < T 2 S sys + - S surr S universe > 0 EXOTHERMIC S sys + S surr - H + H T 2 H is the same so S is dependent on temperature T 2 < - S sys + S surr S universe > 0 5

6 Sample Exercise 16.4 In the metallurgy of Antimony, the pure metal is recovered via different reactions, depending on the composition of the ore. For example, iron is used to reduce antimony in sulfide ores: Sb 2 S 3 + 3Fe 2Sb + 3FeS H = -125 kj Carbon is used as the reducing agent for oxide ores: Sb 4 O 6 + 6C 4Sb + 6 CO H = 778 kj Calculate the S surr for each of these reactions at 25 C and 1 atm. S surr = H T Exercise #23 Choose the compound with the greatest positional probability in each case. a. 1 mol H 2 (at STP) or 1 mol H 2 (at 100 C, 0.5 atm) b. 1 mol N 2 (at STP) or 1 mol N 2 (at 100K, 2.0 atm) c. 1 mol H 2 O (s) (at 0 C) or 1 mol H 2 O (l) (at 20 C) Exercise #24 Predict the sign of S surr for the following processes. a. H 2 O (l) H 2 O (g) b. CO 2(g) CO 2(s) 6