Notes 19-1 Obj 19.1, 19.2

19.1 Spontaneous Processes A.) 1st Law of Thermodynamics 1.) Recall from Chapter 5 that energy cannot be created nor destroyed. 2.) Therefore, the total energy of the universe is constant. 3.) Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

B.) Spontaneous Processes 1.) Spontaneous processes are those that can proceed without any outside intervention. 2.) The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B.

3.) Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

4.) Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures. i.) Above 0 C it is spontaneous for ice to melt. ii.) Below 0 C the reverse process is spontaneous.

Sample Exercise 19.1 Identifying Spontaneous Processes Predict whether the following processes are spontaneous as described, spontaneous in the reverse direction, or in equilibrium: (a) When a piece of metal heated to 150 °C is added to water at 40 °C, the water gets hotter. (b) Water at room temperature decomposes into H2(g) and O2(g), (c) Benzene vapor, C6H6(g), at a pressure of 1 atm condenses to liquid benzene at the normal boiling point of benzene, 80.1 °C. Under 1 atm pressure CO2(s) sublimes at –78 °C. Is the transformation of CO2(s) to CO2(g) a spontaneous process at –100 °C and 1 atm pressure? Practice Exercise

C.) Reversible Processes 1.) In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

D.) Irreversible Processes 1.) Irreversible processes cannot be undone by exactly reversing the change to the system. 2.) Spontaneous processes are irreversible.

19.2 Entropy A.) Entropy (S) is a term coined by Rudolph Clausius in the 19th century. B.) Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered q T

C.) Entropy can be thought of as a measure of the randomness of a system (plenty of controversy over this point). D.) It is related to the various modes of motion in molecules. E.) Like total energy, E, and enthalpy, H, entropy is a state function. F.) Therefore, S = Sfinal  Sinitial

G.) For a process occurring at constant temperature (an isothermal process), the change in entropy is equal to the heat that would be transferred if the process were reversible divided by the temperature: S = qrev T

H.) Second Law of Thermodynamics 1.) The second law of thermodynamics states that the entropy of the universe increases for spontaneous processes, and the entropy of the universe does not change for reversible processes.

Sample Exercise 19.2 Calculating ΔS for a Phase Change The element mercury, Hg, is a silvery liquid at room temperature. The normal freezing point of mercury is –38.9 °C, and its molar enthalpy of fusion is ΔHfusion = 2.29 kJ/mol. What is the entropy change of the system when 50.0 g of Hg(l) freezes at the normal freezing point?.

Sample Exercise 19.2 Calculating ΔS for a Phase Change The normal boiling point of ethanol, C2H5OH, is 78.3 °C, and its molar enthalpy of vaporization is 38.56 kJ/mol. What is the change in entropy in the system when 68.3 g of C2H5OH(g) at 1 atm condenses to liquid at the normal boiling point? Practice Exercise

2.) In other words: For reversible processes: Suniv = Ssystem + Ssurroundings = 0 For irreversible processes: Suniv = Ssystem + Ssurroundings > 0

3.) These last truths mean that as a result of all spontaneous processes the entropy of the universe increases.