CHEM 104 1st Edition Exam 4 Study Guide Lecture 1 April 6 Introduction to Spontaneity S and Entropy How can we tell if a reaction is spontaneous or in other words will happen without needing extra energy Absolute Molar Entropy S CAN be quantified or put into numbers Value is NEVER 0 Absolute molar entropy is ALWAYS positive while the CHANGE in entropy might not be S is a STATE FUNCTION independent of path S sum of S of products sum of S reactants Gas to liquid negative change in entropy because it LOSES disorder gains order Ions Na and Cl forming a salt NaCl negative entropy because it goes from being in a solution to a solid form loses disorder Solid to gas positive entropy If there are only gases present in the equation the side with the greatest number of moles has the greatest entropy Second Law of Thermodynamics ALL spontaneous irreversible processes INCREASE entropy of the universe S Ssystem Ssurroundings 0 If a reaction is not spontaneous in one direction the REVERSE DIRECTION will always be spontaneous EXOTHERMIC IF S IS POSITIVE AND H IS NEGATIVE FAVORABLE Endothermic S is positive in system but negative in surroundings Gibbs Free Energy Equation G H TS for a state function April 10 Lecture What is Gibb s Free Energy How can we use the equation to determine the sign of the other components Gibbs Free Energy Thermodynamic quantity considering enthalpy temperature and spontaneity Reaction is spontaneous when G is negative If G 0 reaction is in equilibrium Four Scenarios Negative H and Positive S favorable G is negative and spontaneous Positive H and Negative S not favorable G positive and not spontaneous doesn t happen Negative H and Negative S more likely to be spontaneous at lower temperatures Higher number of moles to lower number of moles negative S so needs catalyst G G reaction RT ln Q At equilibrium G 0 and Q K Equilibrium Greaction RT ln Q April 13 Lecture Introduction to Oxidation Reduction Reactions What are Re Ox reactions What are voltaic cells Oxidation Reduction Reactions transfer of electrons in a reaction For ever oxidation there is ALWAYS a reduction and vice versa Oxidation is the loss of electrons while reduction is the gain of electrons In redox reactions Oxidation numbers change The reactant getting reduced is the oxidizing agent The reactant getting oxidized is the reducing agent Oxidation Numbers Free elements are 0 Monoatomic Ions Charge oxidation number Cl 1 Oxygen in compounds number is 2 except for peroxides H2O2 where it is 1 Hydrogen In compounds 1 except for metal hydrides Halogens In compounds 1 F is always 1 and 2 Cl Br and I 1 except in a compound with F or O Molecules Sum of oxidation numbers must add up to the charge Voltaic Cell Sum of G values is ALWAYS POSITIVE in a voltaic cell Battery converting chemical energy into electrical energy spontaneous reaction Anode Oxidation side Cathode Reduction Side Cell Diagram a shorthand representation of a voltaic cell Salt Bridge Tube with past of soluble salt to balance the electron gain or loss April 15 Lecture Introduction to half reactions What are half reactions and how can we find the final G standard reduction potential voltage value for a reaction Half reactions Separate oxidation and reduction reactions Example Problem Want to know the voltage potential for Zn Zn 2 in a Standard Hydrogen Electrode Anode Oxidation Zn Zn 2 2e Potential 76 V Cathode Potential 0 2H 2e H2 Cell Voltage Potential 76 V April 17 Lecture Positive potential Greater affinity to gain electrons and be reduced Negative Potential Lower affinity to be reduced Any electrode can act as an anode or cathode and is determined by potential values For example Mn 2 2e Mn E 1 18 V Cu 2 2e Cu E 34 V Copper has the easiest time being reduced so if it was in a voltaic cell with Manganese it would be the cathode while Mn would be oxidized Relationships Between Ecell G and K G RT lnK K e G Rt Ecell kT nf ln K Ecell 0 0592V n logK G nFEcell April 20 Lecture AT 25 DEGREES CELCIUS Ecell E cell 0 0257V n ln Q What is electrolysis and why is it important How is it different from a voltaic cell The Ecell of a voltaic cell had to be positive but now with electrolysis it can be negative or positive Electrolysis Converting electrical energy to chemical energy forcing the reaction to happen whether its spontaneous or not April 22 Lecture Example Problem Want Na and Cl2 from NaCl Na and Cl 1 Cl2 2e 2Cl E 1 36 V Cl is all we have this reaction must be flipped and the sign for E must change so 1 36V 2 2Na 2e 2Na E 2 71 V 3 Add reactions together 2Na 2Cl 2NaCl2 E 4 07 V Coulomb Unit of charge 96500 C mol 1 mol of e 1 F charge of 96500 C Current I I amp 1C s