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Understanding Thermodynamics: Direction of Processes and Reactions

Learn about entropy, free energy, and spontaneity in chemical reactions through examples and experimental procedures. Explore how to predict the sign of entropy change and classify reactions based on enthalpy and entropy. Conduct experiments to determine the spontaneity and controlling factors in reactions.

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Understanding Thermodynamics: Direction of Processes and Reactions

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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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