Presentation on theme: "Spontaneous Processes"— Presentation transcript:
1Spontaneous Processes Spontaneous: process that does occur under a specific set of conditionsNonspontaneous: process that does not occur under a specific set of conditions
2Spontaneous Physical and Chemical Processes A waterfall runs downhillA lump of sugar dissolves in a cup of coffeeAt 1 atm, water freezes below 0 0C and ice melts above 0 0CHeat flows from a hotter object to a colder objectA gas expands in an evacuated bulbIron exposed to oxygen and water forms rustspontaneousnonspontaneous
4Spontaneous expansion of a gas stopcock closed1 atmevacuatedstopcock opened0.5 atm
5Spontaneous Processes Often spontaneous processes are exothermic, but not always….Methane gas burns spontaneously and is exothermicIce melts spontaneously but this is an endothermic process…There is another quantity!
12MicrostatesBoltzmann - entropy of a system is related to the natural log of the number of microstates (W)S = k ln Wk = Boltzmann constant (1.38 x 1023J/K)
13Microstates Entropy is a state function just as enthalpy S = Sfinal SinitialRewrite:S = k ln Wfinal k ln WinitialWf > Wi then S > 0 entropy increases
14Standard EntropyStandard entropy: absolute entropy of a substance at 1 atm (typically at 25C)Complete list in Appendix 2 of textWhat do you notice about entropy values for elements and compounds?Units: J/K·mol
16Entropy (S) is a measure of the randomness or disorder of a system. DS = Sf - SiIf the change from initial to final results in an increase in randomnessSf > SiDS > 0For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas stateSsolid < Sliquid << SgasH2O (s) H2O (l)DS > 0
181. Temperature changesThe increase in entropy from solid to liquid to gas.S0 increases as the temperature rises.
192. Physical states and phase changes The entropy change accompanying the dissolution of a salt.pure solidsolutionMIXpure liquidS0 increases as a more ordered phase changes to a less ordered phase.
20The small increase in entropy when ethanol dissolves in water. 3. Dissolution of a solid or liquidThe small increase in entropy when ethanol dissolves in water.EthanolSolution of ethanol and waterWaterS0 of a dissolved solid or liquid is usually greater than the S0 of the pure solute. However, the extent depends upon the nature of the solute and solvent.
214. Dissolution of a gasThe large decrease in entropy when a gas dissolves in a liquid.O2 gasO2 gas in H2OA gas becomes more ordered when it dissolves in a liquid or solid.
225. Atomic size or molecular complexity Entropy and vibrational motion.N2O4NONO2In similar substances, increases in mass relate directly to entropy.In allotropic substances, increases in complexity (e.g. bond flexibility) relate directly to entropy.
23Processes that lead to an increase in entropy (DS > 0)
24The Second and Third Laws of Thermodynamics System: the reactionSurroundings: everything elseBoth undergo changes in entropy during physical and chemical processes
25Second Law of Thermodynamics Entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.Equilibrium process: caused to occur by adding or removing energy from a system that is at equilibrium
26Second Law of Thermodynamics Mathematically speaking:Spontaneous process:Suniverse = Ssystem + Ssurroundings > 0Equilibrium process:Suniverse = Ssystem + Ssurroundings = 0
27Entropy Changes in the System Entropy can be calculated from the table of standard values just as enthalpy change was calculated.Srxn = nS products mS reactants
28Entropy Changes in the Surroundings Change in entropy of surroundings is directly proportional to the enthalpy of the system.Ssurroundings HsystemNotice: exothermic process corresponds to positive entropy change in surroundings
29Entropy Changes in the Surroundings Change in entropy of surroundings is inversely proportional to temperatureSsurroundings 1 / TCombining the two expressions:
30Entropy ChangesIf the entropy change for a system is known to be 187.5 J/Kmol and the enthalpy change for a system is known to be 35.8 kJ/mol, is the reaction spontaneous?Spontaneous if: Suniv= Ssys + Ssurr > 0
31Entropy Changes Is the reaction spontaneous? Suniv= < 0 so the reaction is non-spontaneous
32Third Law of Thermodynamics Entropy of a perfect crystalline substance is zero at absolute zero.Importance of this law: it allows us to calculate absolute entropies for substances
33Gibbs Free Energy G = H – T S The Gibbs free energy, expressed in terms of enthalpy and entropy, refers only to the system, yet can be used to predict spontaneity.
34Gibbs Free EnergyIf G < 0,negative, the forward reaction is spontaneous.If G = 0, the reaction is at equilibrium.If G > 0, positive, the forward reaction is nonspontaneous
36Predicting Temperature from Gibbs Equation Set G = 0 (equilibrium condition)0 = H – T SRearrange equation to solve for T- watch for units!This equation will also be useful to calculate temperature of a phase change.
37Gibbs Free Energy (G)G, the change in the free energy of a system, is a measure of the spontaneity of the process and of the useful energy available from it.DG0system = DH0system - TDS0systemG < 0 for a spontaneous processG > 0 for a nonspontaneous processG = 0 for a process at equilibriumDrG0 = S mDG0products - S nDG0reactants
38DG0 of any element in its stable form is zero. The standard free-energy of reaction (DrxnG0 ) is the free-energy change for a reaction when it occurs under standard-state conditions.aA + bB cC + dDD G0rxndD G0 (D)fcD G0 (C)=[+]-bD G0 (B)aD G0 (A)D G0nD G0 (products)SmD G0 (reactants)Standard free energy of formation (D G0) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states.fmDG0 of any element in its stable form is zero.fm
392C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l) What is the standard free-energy change for the following reaction at 25 0C?2C6H6 (l) + 15O2 (g) CO2 (g) + 6H2O (l)DG0rxnnDG0 (products)f=SmDG0 (reactants)-DG0rxn6DG0 (H2O)f12DG0 (CO2)=[+]-2DG0 (C6H6)DG0rxn=[ 12x– x–237.2 ] – [ 2x124.5 ] = kJIs the reaction spontaneous at 25 0C?DG0 = kJ< 0spontaneous
40Sample Problem 2.5:Determining the Effect of Temperature on DG0PROBLEM:An important reaction in the production of sulfuric acid is the oxidation of SO2(g) to SO3(g):2SO2(g) + O2(g) SO3(g)At 298K, DG0 = kJ; DH0 = kJ; and DS0 = J/K(a) Use the data to decide if this reaction is spontaneous at 250C, and predict how DG0 will change with increasing T.(b) Assuming DH0 and DS0 are constant with increasing T, is the reaction spontaneous at 900.0C?PLAN:The sign of DG0 tells us whether the reaction is spontaneous and the signs of DH0 and DS0 will be indicative of the T effect. Use the Gibbs free energy equation for part (b).SOLUTION:(a) The reaction is spontaneous at 250C because DG0 is (-). Since DH0 is (-) but DS0 is also (-), DG0 will become less spontaneous as the temperature increases.
41Sample Problem 2.5:Determining the Effect of Temperature on DG0continued(b) DG0rxn = DH0rxn - T DS0rxnDG0rxn = kJ - (1173K)(-187.9J/mol*K)(kJ/103J)DG0rxn = 22.0 kJ; the reaction will be nonspontaneous at 900.0C
42DG and the Work a System Can Do For a spontaneous process, DG is the maximum work obtainable from the system as the process takes place: DG = workmaxFor a nonspontaneous process, DG is the minimum work that must be done to the system as the process takes place: DG = workmaxAn example
43Reaction Spontaneity and the Signs of DH0, DS0, and DG0 -TDS0DG0Description-+Spontaneous at all T+-Nonspontaneous at all T+-+ or -Spontaneous at higher T;nonspontaneous at lower T-++ or -Spontaneous at lower T;nonspontaneous at higher T