Genchem

• Topics in Chemical Thermodynamics


  • Spontaneous processes
  • Entropy
  • The Second Law of Thermodynamics
  • Gibbs Free Energy and Chemical Equilibrium

  
• The First Law of Thermodynamics


  "Energy of the universe is constant."
• The Law of Conservation of Energy


  "Energy can be converted from one form to another but cannot be created or destroyed."
• Parts of the universe of interest for thermodynamic studies

  • The system (part we are investigating)
  • The surroundings (everything else)
• Change in temperature


  ΔT = Tf - Ti
• Change in temperature


  • Hot coffee cools from 55 oC to 28 oC, ΔT = 28 oC - 55 oC = -27 oC
• Change in height


  • Climbing up a ladder from 5ft to 12ft, Δh = 12ft - 5ft = +7ft
• State Functions


  Properties that can be expressed as (final - initial) and we write with Δ such as ΔE = Ef - Ei

  
• Important State Functions


  ΔT, ΔH, ΔE, ΔS, and ΔG
• Path Functions


  Properties that you cannot calculate by just knowing final and initial states but must know how process occurred
• Important Path Functions


  q and w
• Spontaneous Process


  A physical or chemical change that occurs by itself, without requiring an outside force, and continues until equilibrium is reached
• Examples of Spontaneous Processes

  • Heat flows from a hotter object to a colder one
  • An iron object rusts in moist air
  • Sugar dissolves in a cup of coffee
• Spontaneity of Chemical Reactions


  For a chemical reaction to be spontaneous, it should proceed as written (from left to right), without an input of energy
• Entropy, S


  A thermodynamic quantity that is a measure of how spread out or dispersed the energy of a system is among the different possible ways that system can contain energy

  
• The SI unit of entropy is joules per Kelvin (J/K) and, like enthalpy, is a state function
• Entropy changes


  Changes from more order to more random, the spreading out of more concentrated molecules and energy

  
• Entropy change examples


  • Molecules of gas at high pressure spread to lower pressure regions
  • Gas in balloon spreads out into room and deflates
  • Heat always goes from high temperature into cooler regions
  • Hot coffee in a room gets cooler and the heat spreads out into the room
• Entropy changes with phase changes


  At high enough temperature, the spontaneous change is from Solid->Liquid->Gas; gas is more random than liquid and liquid is more random than solid. There is an increase in entropy (S) of the system by going from solid to liquid to gas.

  
• Second Law of Thermodynamics


  The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process

  
• Calculating Entropy Changes in the System: Standard Entropy of Reaction, ΔS°rxn


  1. Step 1: Identify the reactants and products

  2. Step 2: Using the equation for the standard entropy of reaction: ΔS°rxn = Σ(n*S°products) - Σ(n*S°reactants)

  
• General rules for predicting entropy change of the system

  • If the reaction produces more gas molecules than it consumes, ΔS° is positive
  • If the total number of gas molecules diminishes, ΔS° is negative
  • If there is no net change in the total number of gas molecules, ΔS° may be positive or negative, but will be relatively small numerically