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I. Direction of Processes and Reactions Examples:

I. Direction of Processes and Reactions Examples: Ball at the top of a hill Ball at the bottom of a hill Steel + H 2 O + O 2 Rust Gas in one part of a container Gas filling a container Ice at 5 o C Water at 5 o C

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I. Direction of Processes and Reactions Examples:

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  1. I. Direction of Processes and Reactions • Examples: • Ball at the top of a hill Ball at the bottom of a hill • Steel + H2O + O2 Rust • Gas in one part of a container Gas filling a container • Ice at 5 oC Water at 5 oC • What is in common? • Exothermic? Not ice melting or gas expanding • Increased Disorder = Increased Entropy = +DS • Entropy = S = driving force of spontaneous reactions = disorder or random • Probability (likelihood): there are many ways for objects/molecules to be disordered, but only a few to be ordered • Nature proceeds towards the most likely state = state with greatest number of energetically equivalent arrangements • Expansion of a Gas

  2. 5) Example: Predict the sign of the entropy change for a) Dissolving solid sugar into water b) Iodine vapor condensing to crystals on a surface 6) Second Law of Thermodynamics = in any spontaneous process, there is always an increase in the entropy of the universe • Energy is conserved = constant (First Law of Thermodynamics) • Entropy is always increasing • DSuniverse = DSsystem + DSsurroundings • For a given process: if DSuniverse = + the process is spontaneous if DSuniverse = - the process is not spontaneous 7) DSsurroundings depends on heat flow • Exothermic reactions usually favors spontaneity • Spontaneity usually lowers the energy of the starting material as it becomes product • The difference of these energies = heat released to surroundings

  3. II. Free Energy • Free Energy = G = H – TS • DGprocess = DH – TDS • Divide by –T • A process is spontaneous if DG = - • Chemists use DG rather than DS because we only need to know system • Example: Predicting Spontaneity using DG 1) H2O(s) H2O(l) DHo = 6030 J/mol, DSo = 22.1 J/K mol

  4. Classifying Processes/Reactions based on DH and DS • CaCl2(s) -------> Ca2+(aq) + 2Cl-(aq) • Perform Reaction and record observations (including DT) • + DT = - DH (exothermic); - DT = + DH (endothermic) • Won’t be able to determine DS from observations • Decide Enthalpy Controlled or Entropy Controlled from observations • DX = ΣProducts - ΣReactants (Use values from Text Appendix)

  5. Experimental Details • Goals • Determine whether the process is spontaneous or non-spontaneous • Determine if enthalpy (DH), entropy (DS), or both are controlling • Procedure • Skip Reaction # 5 (CH3COOH + NaOH) • Clean glassware between reactions. Contaminants will effect results! • Perform each reaction. • Observe: color change, dissolution, precipitation, gas formation • Record change in Temperature (calculate DT for each reaction) • Observable Change = Spontaneous Reaction • Lack of Observable Change ≠ Non-Spontaneous Reaction • Answer questions about each reaction • Calculate DHo, DSo, and DGo for each reaction from tables in App. II • Barium and Lead Waste in Waste Beaker GIVE THEM THE REACTIONS

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