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• Enthalpy


  A thermodynamic quantity used to describe heat changes taking place at constant pressure
• Enthalpy of reaction (ΔHrxn)

  The difference between the enthalpies of the products and the enthalpies of reactants
• Enthalpy calculation


  1. n = number of mol

  2. Hproducts = value of enthalpy of each product

  3. Hreactants = value of enthalpy of each reactant
• Enthalpy calculation examples


  • Example 1
  • Example 2
• Spontaneous process


  A type of process which does not need the application of energy to take place. It is a physical or chemical change that occur by itself.
• Spontaneity


  The property of a process to occur without an external input of energy. It can be spontaneous or nonspontaneous process.
• If a process is spontaneous at a given temperature, its reverse process is nonspontaneous. Both processes are possible but only the spontaneous one will occur without intervention.
• Nonspontaneous process


  A reaction that does not favor the formation of products at the given set of conditions
• Examples of spontaneous processes

  • Melting of ice at 25°C
  • Sublimation of dry ice (CO2) at 25°C
  • Vaporization of water at 100°C
  • Dissolution of food dye in water
  • Diffusion of gases
• Melting


  Process in which a solid is transformed to a liquid. Solids have a more ordered structure than liquids.
• Vaporization


  Process in which a liquid is transformed to a gas.
• Sublimation


  Process in which a solid is transformed to a gas.
• Dissolution


  Process in which a solid is transformed to mobile ions/particles in which a solid is transformed to a gas.
• Rusting of iron is an example of a spontaneous process.
• Conversion of water to hydrogen and oxygen gas is an example of a nonspontaneous process.
• Reassembling of broken glass is an example of a nonspontaneous process.
• Heat flows from a colder object to a hotter one is an example of a spontaneous process.
• Substances ordered by increasing entropy

  • ice
  • water
  • steam
• Entropy


  A thermodynamic property that describes the way that the energy of a system is distributed among its available microscopic energy levels. It 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.
• Microstate


  A specific arrangement of the particles (atoms, ions, or molecules) in a system.
• The more disordered a system is, the more microstates it possesses, and therefore, the higher is its entropy.
• Boltzmann equation for entropy


  S = kB * ln(W), where S is the entropy, kB is the Boltzmann constant, and W is the number of microstates.
• The Boltzmann equation is named after Ludwig Eduard Boltzmann, an Austrian physicist and philosopher.
• Calculating entropy using Boltzmann equation


  1. Given: kB = 1.38 x 10^-23 J/K, W = 36 possibilities

  2. Solution: S = 1.38 x 10^-23 J/K * ln(36) = 4.95 x 10^-23 J/K
• Second law of thermodynamics

  The entropy change in the universe (ΔSuniv) for any process is the sum of the entropy changes in the system (ΔSsys) and in the surroundings (ΔSsur). This law states that the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.
• Possible entropy change scenarios

  • ΔSuniv > 0: Process is spontaneous
  • ΔSuniv = 0: Process tends not to occur; equilibrium
  • ΔSuniv < 0: Reverse process occurs spontaneously
• Entropy of reaction (ΔS°rxn)

  ΔS° = Σn S°(products) - Σm S°(reactants), where n and m are the stoichiometric coefficients.
• 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.
• Calculating entropy of reaction (sample problem)


  Step 1: Write the standard entropy of each formula (H2(g) = 130.6 J/K·mol, I2(s) = 116.7 J/K·mol)
• Equilibrium


  ΔSuniv = ΔSsys + ΔSsur < 0
• Reverse process


  Occurs spontaneously
• Entropy of reaction ΔS°

  ΣnS°(products) - ΣmS°(reactants)
• Standard entropy values of compounds have been measured in J/K mol
• n and m are the stoichiometric coefficients in the reaction
• Entropy of reaction


  • 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
• Sample Problem C1: Calculate ΔS° for the reaction H2(g) + I2(s) → 2HI(g)

  1. Step 1. Write the standard entropy of each formula

  2. Step 2: Using the equation for the standard entropy of reaction ΔS° = ΣnS°(products) - ΣmS°(reactants)

  3. Step 3: Substitute the entropy values
• Gibbs free energy, G


  Thermodynamic function used to express the spontaneity of a reaction more directly
• Change in free energy, ΔG

  Equal to the sum of the enthalpy plus the product of the temperature and entropy of the system