Chapter 17 Free Energy and Thermodynamics Georgia Gwinnett College Chem 1212K Fall 2013 B Shepler Chapter 17 1 Thermodynamics Kinetics and Spontaneity Chemical Reactions Thermodynamics predicts direction Chemical Kinetics predicts speed rate and pathway from reactants to products Spontaneous Processes Occur without outside intervention Spontaneous does not mean fast Nonspontaneous does not mean impossible It just requires outside intervention Chapter 17 2 System and Surroundings When discussing chemical reactions and thermodynamics we typically divide the universe into two parts System Surroundings Universe Surroundings System We can exchange energy between the system and surroundings through heat or work Chapter 6 3 The First Law of Thermodynamics The First Law of Thermodynamics The energy of the universe is a constant The Law of Conservation of energy Energy can t be created or destroyed it can only be transformed from one form to another Internal Energy E The total energy of a system Energy is transferred as heat or work Esys q w Euniv Esys Esurr 0 Chapter 6 4 Enthalpy Change in enthalpy H a measure of the heat absorbed or given off by a chemical reaction Exothermic System gives off heat to the surroundings H 0 Endothermic System absorbs heat from the surroundings H 0 Chapter 6 5 Entropy Entropy is a thermodynamic function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state The change in entropy of the universe for a given process is a measure of the driving force behind that process spontaneity More equivalent arrangements higher probability of that state occurring Nature tends towards states that have greater probabilities Chapter 17 6 Entropy Probability and Microstates Consider the expansion of a gas into a vacuum We know this process is spontaneous but why Chapter 17 7 Entropy Probability and Microstates Microstates Chapter 17 8 Entropy Conceptual Understanding Entropy S is related to probability and disorder The state with the greatest probability most microstates has the highest entropy Higher disorder generally correlates to greater probability Greater disorder greater entropy Nature tends to move to states of higher disorder Entropy is also related to the dispersal of energy Greater entropy correlates with greater dispersal of energy Chapter 17 9 Example Predict the sign on S for each of the following processes H2O s H2O l Dissolving NaCl in water 2NO2 g 2NO g O2 g Chapter 17 10 The Second Law of Thermodynamics First Law of Thermodynamics The law of conservation of energy Euniverse Esystem Esurroundings 0 Second Law of Thermodynamics In a real spontaneous process the Entropy of the universe meaning the system plus its surroundings must increase Ssystem Ssurroundings Suniverse 0 Chapter 17 11 The Second Law of Thermodynamics In a real spontaneous process the Entropy of the universe meaning the system plus its surroundings must increase DSuniverse DSsystem DSsurroundings 0 if DSuniverse 0 then the FORWARD reaction is spontaneous if DSuniverse 0 then the REVERSE reaction is spontaneous if DSuniverse 0 then the reaction is at EQUILIBRIUM Chapter 17 12 Ssys and Ssurr To determine the spontaneity of a reaction we need to determine the sign and magnitude of Ssys and Ssurr Ssys determined by increase decrease in disorder of the system Ssurr determined primarily by the flow of heat into or out of the system If the system is exothermic then heat is released to the surroundings Surroundings are more disordered If the system is endothermic then heat is absorbed from the surroundings Surroundings are less disordered Ssurr proportional to Hsys Chapter 17 13 The Effect of Temperature on Spontaneity Consider the melting of water H2O s H2O l For the system s l so Ssystem 0 Reaction is endothermic so Ssurroundings 0 Which one takes precedence Depends on Temperature Ice spontaneously melts above 0 C but water spontaneously freezes below 0 C At low temperature there is very little random motion of the surrounding s particles Heat gained or lost to the system has a large effect on Ssurroundings At high temperatures there is a lot of random motion in the surroundings particles Heat gained or lost by the system has a relatively small effect on Ssurroundings DH DSsurr sys T Chapter 17 14 DSsystem DSsurroundings DSuniverse Chapter 17 15 Example Determine the sign of Ssys and Ssurr Predict the temperatures under which the reaction will be spontaneous 2 H2 g O2 g 2 H2O g Hrxn 483 6 kJ 2 N2 g O2 g 2 N2O g Chapter 17 Hrxn 186 kJ 16 Josiah Willard Gibbs 1939 1903 Highlights Devised much of the theoretical foundation for chemical thermodynamics Established the concepts of chemical potential and free energy The first person to recognize the importance of Entropy in determining spontaneity Moments in a Life 1863 Yale awarded him the first American Ph D in engineering Book Equilibrium of Heterogeneous Substances deemed one of the greatest scientific achievements of the 19th century His work was presented in such a cryptic mathematical format that it was largely not understood and was ignored by most of his contemporaries Didn t get major recognition until the 1930s Chapter 17 17 Gibbs Free Energy How are Enthalpy and Entropy related G H T S For the change in the Gibbs Energy of system at constant Temperature and Pressure Gsys Hsys T Ssys The sign of the Gibbs Free Energy tells us about the spontaneity of the reaction A reaction is spontaneous if Suniverse 0 A reaction is spontaneous if Gsystem 0 Gives us a way to predict spontaneity by only considering the system reaction of interest We will henceforth drop the system label because we will assume we are talking about the system unless noted otherwise Chapter 17 18 Gibbs Free Energy and Spontaneity For a change at constant temperature and pressure Gsys 0 Spontaneous Gsys 0 Equilibrium Gsys 0 Non spontaneous but the reverse is spontaneous From Earlier Suniv 0 Spontaneous Suniv 0 Equilibrium Suniv 0 Non spontaneous Chapter 17 19 Temperature Dependence of Free Energy 3NO g N2O g NO2 g G H T S DG has a strong temperature dependence Chapter 17 20 Temperature Dependence of Free Energy G H T S Chapter 17 21 Example Consider the following reaction C2H4 g H2 g C2H6 g H 137 5 kJ S 120 5 J K Calculate G at 25 Celsius Does the Gibbs Free Energy become more negative or more positive as temperature increases Chapter 17 22 The Third Law of Thermodynamics The Entropy of a perfect crystal