CHM STUDY GUIDE EXAM III Chapter 17 18 and 19 CHAPTER 17 1st Law of thermodynamics states H stands for A positive H A negative H S stands for Entropy is Microstate S S The 2nd law of thermodynamics states that Suniverse relating the change in entropy to the system surrounding s Positive Suniverse Negative Suniverse How to find the S of the reaction aA bB cC dD THERMODYNAMICS cannot create or destroy energy energy can only be transferred converted Change in enthalpy heat of a reaction Endothermic reaction needs energy Exothermic reaction releases energy Entropy Measure of how spread out diverse the energy of a system is measure of disorder Possible distribution of atoms molecules or energy W a system with fewer microstates smaller W among which to spread its energy small dispersal has a lower entropy a system with more microstates larger W among which to spread its energy large dispersal has a higher entropy S k ln W W of possibilities S k ln Wf Wi Entropy of the universe increases for a spontaneous reaction entropy of the universe remains unchanged at equilibrium Suniverse Ssystem Ssurroundings 0 spontaneous if 0 then reaction is at equilibrium Disorder spontaneous More order non spontaneous aA bB cC dD S Sproducts Sreactants these values are looked up multiply the value by of moles Relationship between S and the of moles of gas in products vs reactants S surroundings this is the same for H if the reaction produces more moles of gas that it consumes more moles of gas in products vs reactants than the S is positive creating disorder if of moles of gas diminishes that S is negative creating order if of moles of gas is same on both sides of the equation than S is but the value is small S surr H sys T CHM STUDY GUIDE EXAM III in relationship to H and the temperature The third law of thermodynamics states that G stands for Gibb s free energy Gibb s free energy equation G Change in free energy equation G What happens when G 0 G 0 G 0 What is the standard free energy of reaction Go The different effects of S and H on G rxn H is S is not temp dependent H is S is not temp dependent H is S is High temp H is S is Low temp H is S is Low temp entropy of a crystalline substance is zero at a temperature of 0K Gibb s free energy tells you whether a reaction is spontaneous non spontaneous the energy available to do work G H TS S J K H kJ mol G kJ or J must convert to make all the same unit G H T S Non spontaneous reaction Equilibrium Spontaneous reaction The free energy change for a reaction when it occurs at standard state conditions get the same way you get S sum of values for products sum of reactants look up the values appendix 3 Using the equation G H T S G will be Reaction will be non spontaneous G will be Reaction will be spontaneous G will be Reaction will be spontaneous This is because this part of the equation T S will be large at a high temp subtracting this from H will produce a negative G will be Non spontaneous reaction H will be large so subtracting T S which will be small because T is low from H will produce a positive number G will be Spontaneous reaction T S will be a small positive so subtracting this from a negative number will produce a negative number Set the above equation equal to 0 so that 0 H T S Rearrange equation to set equal to T and solve for T T H S G G0 RT lnQ R 8 314 J mol K Q at non equilibrium conditions Go free energy at standard states find by looking up products reactants need to make sure all values are in same units 0 Go RT lnKeq Or CHM STUDY GUIDE EXAM III G will be Non spontaneous reaction Use G H T S If G is negative then the reaction is spontaneous H is S is High temp If a question asks you to determine if a reaction is spontaneous Question asks at what temperature does spontaneity start stop Equation relating thermodynamics to equilibrium G If reaction is at equilibrium the above equation turns into Using the equation to find the free energy above what is the equation to determine equilibrium constants CHAPTER 18 Balancing redox equations Go RT ln Keq Keq e Go RT ELECTROCHEMISTRY In acidic environmen t 1 Split reaction into two half reactions place species that look alike in the same half reaction separate by what is being reduced or oxidized 2 Make sure the half reactions are atomically balanced number of atoms 3 Add H2O to account for either side lacking oxygen 4 Balance the hydrogens by adding H to the appropriate side 5 Add up the total charge for the left and the total charge for the right side are the same of the equation 6 Balance the charges by adding a certain amount of electrons to the side that is more positive multiply by common denominator if needed to make of electrons the same 7 Combine the half reactions and cancel out any common species In basic environmen t Same steps apply After combining the half reactions after completing the above steps Add OH atoms wherever there are H atoms add same number as hydrogen CHM STUDY GUIDE EXAM III ions this changes the species to water then add the same number of OH to the opposite side of the reaction Cancel out common species Electrochemical processes oxidation reduction reactions in which the energy released by a spontaneous reaction is converted to electricity or electrical energy is used to cause a non spontaneous reaction to occur Oxidation Reduction Oxidation number Lose electrons Gain electrons The charge the atom would have in a molecule or an ionic compound if electrons were completely transferred 1 2 free elements have an ox of 0 In monatomic ions the oxidation number is equal to the charge on the ion 2 it is 1 3 The oxidation number of oxygen is usually 2 In H2O2 and O2 4 The oxidation number of hydrogen is 1 except when it is bonded to metals in binary compounds In these cases its oxidation number is 1 5 Group IA metals are 1 IIA metals are 2 and fluorine is always 1 The sum of the oxidation numbers of all the atoms in a molecule or ion the charge on the molecule or ion reducing and oxidizing agents are separated but redox chemistry is carried through conductive medium e g wire Standard reduction potential Anode oxidation Cathode reduction First is the anode The double line is the salt bridge Then next is the cathode In its simplest form is an inverted U tube containing an inert electrolyte solution such as KCl or NH4NO3 whose ions will not react with other ions in solution or with the electrodes the voltage associated with …
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