CHEM 108 1st Edition Lecture 28 Chapter 18: ThermodynamicsTrends in Entropies:- Ssolid < Sliquid < Sgas- Entropy increases as the complexity of molecular structure increases. More bonds, more opportunities for internal motion (more microstates)- Rigid network of covalent bonds means less range of motion = less entropyo Ex: Diamonds- More flexibility and range of motion in planar rings = more entropyo Ex: Graphite Calculating change in Entropy:- Entropy change for the system (ΔSrxn °): ΔSrxn ° = ΣnproductsSproducts ° − ΣnreactantsSreactants ° o Where n = coefficients of the products/reactants in the balanced equation.o Can’t determine if reaction is spontaneous with just the entropy change for a system. Need both entropy change for the system and entropy change of the surroundings to determine if reaction is spontaneous.- Entropy change for the universe: Heat gained/lost by system affects entropy of the surroundings.o ΔSsurr ° = (−qsys / T) = (qsys = ΔHrxn °)Free energy (G): - The energy available to do useful work. - Free energy change (ΔG): The change in free energy of a process occurring at constant temperature and pressure. - For spontaneous processes, ΔG <0- ΔG = −TΔSuniverse- ΔSuniverse = ΔSsys − ΔHsys/T rearrange into −TΔSuniverse = ΔHsys − TΔSsyso = ΔHsys − TΔSsysΔG ΔH ΔS SpontaneityAlways <0 Negative Positive Always spontaneous<0 at lower temps Negative Negative Spontaneous at lower temps<0 at higher temps Positive Positive Spontaneous at higherThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.tempsAlways >0 Positive Negative Never spontaneous- Change in internal energy of system can be used to perform work: ΔE = q + w o Some energy lost as heat to the surroundings (q). o Some energy lost due to entropy (−TΔS). o Efficiency = (work done)/(energy consumed).- Free-energy change for a reaction: o ΔGrxn ° = Σ(Δnprod.ΔGf prod. °) − Σ(Δnreact.ΔGf react. °)Free Energy and Chemical Equilibrium:- If ΔG is a large negative number (more negative than about –10 kJ), the reaction is spontaneous, and the reactants transform almost entirely to products when equilibrium is reached.o When Q < K, ΔGrxn < 0 and the reaction is spontaneous- If ΔG is a large positive number (more positive than about +10 kJ), the reaction is nonspontaneous, and reactants do not give significant amounts of product at equilibriumo When Q > K, ΔGrxn >0 and the reaction is nonspontaneous- If ΔG is a small negative or positive value (less than about 10 kJ), the reaction gives an equilibrium mixture with significant amounts of both reactants and products.o When Q =K, ΔGrxn =0 and the reaction is at equilibrium- The value of ΔGrxn depends on the value of Qo ΔGrxn= ΔGrxn ° + RT ln Q- Once reaction is at equilibrium Q=K and ΔGrxn =0 ΔGrxn ° = -RT ln
View Full Document
Unlocking...