I 19 1 Spontaneous Processes Chapter 19 Chemical Thermodynamics A Spontaneous process one that proceeds on its own without any outside assistance The reverse of any spontaneous process is always a nonspontaneous one A reversible process a specific way in which a system changes its state the change occurs in such a way that the system and surroundings can be restored to their original states by exactly reversing the change In other words a reversible system can restore the system and its surroundings to its original condition with no net change to either the system or the surroundings An irreversible process one that cannot simply be reversed to restore the system and its surroundings to their original states Reversible change produce the maximum amount of work that can be done by a system Reversible processes are those that reverse direction whenever an infinitesimal change is made in some property of the system Isothermal process one that occurs at constant temperature B 19 2 Entropy and the Second Law of Thermodynamics The entropy S in a system depends only on the initial and final states of the system In an isothermal process S You must use absolute temperature and take notice of units H is in kJ mol S is in J mol Entropy is a thermodynamic function that increase with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state S K R A s J K lnW At constant T P V then E is constant Second law any irreversible process results in an increase in total entropy whereas any reversible process results in no overall change in entropy For a reversible process Suniv Ssys Ssurr 0 For an irreversible process Suniv Ssys Ssurr 0 The entropy of the universe increases in any spontaneous process 19 3 Molecular Interpretation of Entropy C The state or macrostate of a system is defined by a given set of conditions A given macrostate can exist as a result of a large number of different microstates A microstate can be thought of as a snapshot of the system at a given time and is different from moment to moment qrev T Microstates of one macrostate will have the same total energy The state with the highest entropy has the greatest dispersal of energy Entropy is a measure of how many microstates are associated with a particular macrostate Any change in the system that leads to an increase in the number of microstates W final W initial leads to a positive Svalue Entropy increases with increasing temperature Translational motion an entire molecule moving in one direction Vibrational motion the atoms in the molecule move periodically toward and away from one another Rotational motion the molecule spins about an axis Forms of motion are ways which molecules store energy collectively known as motional energy The number of microstates possible for a microstates possible for a system increases with an increase in volume and increase in temperature or an increase in number of molecules because any of these changes increase the possible positions and kinetic energies of the molecules making up the system Degrees of freedom forms of motion 19 4 Entropy Changes in Chemical Reactions Entropy increase as you go from solid to liquid or liquid to gas or as the number of gas molecules increases during a chemical reaction Third law the entropy of a pure crystalline substance at absolute zero is zero D Standard Molar Entropies SME the molar entropies for substances in their standard states Standard state pure substance at 1 atm pressure and possibly 298 K SME increases with increasing molar mass SME increases with increasing number of atoms in the formula for the substance Degrees of freedom increases with increasing number of atoms Degrees of freedom increases with increasing number of microstates E 19 5 Gibbs Free Energy Gibbs free energy G H TS If G 0 the reaction is spontaneous in the forward direction If G 0 the reaction is at equilibrium If G 0 the reaction is not spontaneous in the forward direction work must be done to make it occur but the reverse reaction is spontaneous In any spontaneous process carried out at constant temperature and pressure the free energy always decreases Standard free energies of formation the change in free energy associated with the formation of a substances Standard enthalpy of formation the change in enthalpy that accompanies the formation of one mole of a substance from its elements with all substance in their standard states F 19 6 Free Energy and Temperature 19 7 Free Energy and the Equilibrium Constant G Free energy at nonstandard conditions G G RTlnQ Under standard conditions the concentration of all reactants and products are equal to 1 Under standard conditions Q 1 and lnQ 0 At equilibrium G 0 and Q K K is the equilibrium constant if you didn t know At equilibrium G RTlnK o K e G RT If G is negative lnK must be positive which means K 1 The more negative G is the larger K is Opposite is true for a positive G