JP

Chapter 17 - Spontaneity, Entropy, and Free Energy

17.1 Spontaneous Processes and Entropy

  • Thermodynamics can tell us the direction in which a process will occur but can say nothing about the speed of the process.
    • Thermodynamics considers only the initial and final states and does not require knowledge of the pathway between reactants and products
    • It lets us predict whether a process will occur but gives no information about the amount of time required for the process
  • The driving force for a spontaneous process is an increase in the entropy of the universe
  • Entropy is a thermodynamic function that describes the number of arrangements that are available to a system existing in a given state.
    • Entropy can be viewed as a measure of molecular randomness or disorder.
  • Nature spontaneously proceeds toward the states that have the highest probabilities of existing
  • Solids are more ordered than liquids or gases and thus have lower entropy
  • The tendency to mix is due to the increased volume available to the particles of each component of the mixture
    • For example, when two liquids are mixed, the molecules of each liquid have more available volume and thus more available positions

17.2 Entropy and the Second Law of Thermodynamics

  • The total energy of the universe is constant, but the entropy is increasing
  • Using entropy, thermodynamics can predict the direction in which a process will occur spontaneously
  • For a spontaneous process, ▵Suniv must be positive
    • For a process at constant temperature and pressure:  ▵Ssys is dominated by “positional” entropy
    • For a chemical reaction, ▵Ssys is dominated by changes in the number of gaseous molecules
  • ▵Ssurr is positive for an exothermic process (▵H is negative)
  • Because ▵Ssurr depends inversely on T, exothermicity becomes a more important driving force at low temperatures
  • Thermodynamics cannot predict the rate at which a system will spontaneously change; the principles of kinetics are necessary to do this

17.3 The Effect on Temperature on Spontaneity

  • The central idea is that the entropy changes in the surroundings are primarily determined by heat flow
  • In an endothermic process, heat flows from the surroundings into the system. In an exothermic process, heat flows into the surroundings from the system.
  • The significance of exothermicity as a driving force depends on the temperature at which the process occurs
  • In a process occurring at a constant temperature, the tendency for the system to lower its energy results from the positive value of ▵Ssurr
  • The transfer of a given quantity of energy as heat produces a much greater percent change in the randomness of the surroundings at a low temperature than it does at a high temperature
  • When no subscript is present, the quantity (for example, ▵H) refers to the system
  • The minus sign changes the point of view from the system to the surroundings
  • Exothermicity is most important as a driving force at low temperatures

17.4 Free Energy

  • Free energy is a state function
  • A process occurring at constant temperature and pressure is spontaneous in the direction in which its free energy decreases
  • For a reaction, the standard free energy change is the change in free energy that occurs when reactants in their standard states are converted to products in their standard states
  • N any real process, w < wmax
  • When energy is used to do work in a real process, the energy of the universe remains constant but the usefulness of the energy decreases
  • Concentrated energy is spread out in the surroundings as thermal energy

17.5 Entropy Changes in Chemical Reactions

  • ****Fewer molecules mean fewer possible configurations
  • When a reaction involves gaseous molecules, the change in positional entropy is dominated by the relative numbers of molecules of gaseous reactants and products.
  • If the number of molecules of the gaseous products is greater than the number of molecules of the gaseous reactants, positional entropy typically increases, and ▵S will be positive for the reaction.
  • Every particle must be in its place
  • The standard entropy values represent the increase in entropy that occurs when a substance is heated from 0 K to 298 K at 1 atm pressure
  • Since entropy is a state function of the system, the entropy change for a given chemical reaction can be calculated by taking the difference between the standard entropy values of products and those of the reactants
  • The more complex the molecule, the higher the standard entropy value

17.6 Free Energy and Chemical Reactions

  • ****The standard free energy of formation of a substance is defined as the change in free energy that accompanies the formation of 1 mole of that substance from its constituent elements with all reactants and products in their standard states
    • The standard free energy change for a reaction is not measured directly
  • The standard free energy of formation of an element in its standard state is zero
  • The number of moles of each reactant (nr) and product (np) must be used when calculating ▵G° for a reaction.
  • To get an accurate comparison of reaction tendencies, we must compare all reactions under the same pressure or concentration conditions

17.7 The Dependence of Free Energy on Pressure

  • The free energy of a reaction system changes as the reaction proceeds because free energy is dependent on the pressure of a gas or on the concentration of species in the solution
  • To understand the pressure dependence of free energy, we need to know how pressure affects the thermodynamic functions that comprise free energy, that is, enthalpy and entropy
  • Q is the reaction quotient (from the law of mass action)
  • T is the temperature (K)
  • R is the gas law constant and is equal to 8.3145 J/K x mol
  • ▵G° is the free energy change for the reaction with all reactants and products at a pressure of 1 atm

17.8 Free Energy and Equilibrium

  • Equilibrium point occurs at the lowest value of free energy available to the reaction system
    • When substances undergo a chemical reaction, the reaction proceeds to the minimum free energy

17.9 Free Energy and Work

  • ****The change in free energy is important quantitatively because it can tell us how much work can be done with a given process
  • In fact, the maximum possible useful work obtainable from a process at constant temperature and pressure is equal to the change in free energy
  • ▵G for a spontaneous process represents the energy that is free to do useful work
  • Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway. Any real pathway wastes energy.
  • In any real cyclic process in the system, work is changed to heat in the surroundings and the entropy of the universe increases
    • This is another way of stating the second law of thermodynamics
  • When energy is used to do work, it becomes less organized and less concentrated and thus less useful
    • Thus the crux of the energy problem is that we are rapidly consuming the concentrated energy found in fossil fuels.
    • We must use these energy sources as wisely as possible