Chapter 19: Free Energy and Thermodynamics (Exam 3) Flashcards
Spontanteous Process
- One that occurs without ongoing outside intervention
Spontaneity
- Direction in which and to what extent a chemical reaction proceeds (in thermodynamics)
Speed
- How fast a reaction takes to complete (kinetics)
Entropy + Equation
- (S)
- Thermodynamic function that increases with number of energetically equivalent way to arrange components of a system to achieve a particular state
- S = klnW
- k = Boltzmann’s constant
- W = number energetically equivalent ways to arrange components of the system
Macrostate
- Given set of conditions (P, V, T)
Microstate
- Exact internal energy distribution among particles at any one instant
- State in which given amount of energy is more highly dispersed has more entropy
Second Law of Thermodynamics
- For any spontaneous process, the entropy of the universe increases
- Entropy is a state function and therefore its value is dependent upon the state of the system
Units of Enropy
- Joules / Kelvin
- Measure of energy dispersal (joules) per unit temperature (Kelvin)
State Changes and Energy (in terms of what causes an increase in entropy)
Increase in Entropy:
- Solid –> Gas
- Solid –> Liquid
- Liquid –> Gas
- Increase in number of moles of a gas during a chemical reaction (gas has more microstates)
Translational Energy
- Straight-line motions of molecules
Rotational Energy
- Rotations of the molecules
Entropy (∆S˚rxn) Standards
- Gas: Standard is at 1atm
- Liquid or Solid: Most stable format 1 atm and temperature (usually 25˚C)
- Substance in solution: 1M
Standard Entropy Change For Reaction
∆S˚rxn = S˚(products) - S˚(reactants)
- Standard molar entropies are used to calculate S˚rxn
Third Law of Thermodynamics
- Entropy of a perfect crystal at absolute zero is zero (only one way to arrange components)
Effect of:
- Allotropes
- Molar mass
- Molecular complexity
- Dissolution
on entropy
- Allotrope: Some elements that exist in two or more forms with different structures; more stable have less entropy
- Entropy increases with molar mass
- Entropy increases with increasing molecular complexity
- Dissolution of crystalline solid into solution results in increase in entropy
Standard Entropy Change for Reaction (equation)
∆S˚rxn = ∑npS˚(products) - ∑nrS˚(reactants)
Spontaneity and
- Exothermic process
- Endothermic process
- Increases entropy of surroundings (Qsys = - | ∆Ssurr = +)
- Decreases entropy of surroundings
Entropy Changes in Surroundings
∆Ssurr = -Hsys/T
(assuming constant P and T)
Gibbs Free Energy
∆G = ∆H - T∆S
∆G < 0 : Spontaneous
∆G > 0 : Nonspontaneous
+ H˚ and + S˚
- Low temperatures: Spontaneous
- High temperatures: Spontaneous
+ H˚ and - S˚
- Low temperatures: Nonspontaneous
- High temperatures: Nonspontaneous
- H˚ and - S˚
- Low temperatures: Spontaneous
- High temperatures: Nonspontaneous
+ H˚ and + S˚
- Low temperatures: Nonspontaneous
- High temperatures: Spontaneous
Free Energy of Formation + Equation
- ∆G˚f
- Change in free energy when 1 mol of compound forms from its constituent elements in their standard states
- ∆G˚rxn = ∑np∆G˚f(products) - ∑nr∆G˚f(reactants)
Free Energy in Stepwise Reactions (similar to enthalpy)
1) If chemical equation is multiplied by some factor, ∆Grxn is multiplied by same factor.
2) If chemical equation is reversed, ∆Grxn changes sign.
3) If chemical equation can be expressed as sum of series of steps, ∆Grxn for overall equation is sum of free energies of reactions for each step.
Reversible Reactions
- Reaction that achieves the theoretical limit of available free energy
- Occurs infinitesimally slowly; free energy can only be drawn out in same infinitesimally small increments
Irreversible Reactions
Change in free energy = positive signifies what?
- All real reactions
- Do not achieve theoretical limit of available free energy
- ∆G˚rxn represents minimum amount of energy required to make reaction occur
Free Energy Change of Reaction under Nonstandard Conditions
∆Grxn = ∆G˚rxn + RTlnQ
- Q: Reaction quotient
- T: Temperature in Kelvins
- R: Gas constant (8.314 J/mol•K)
Relationship between ∆G˚rxn and K
∆G˚rxn = -RTlnk
1) K < 1 : lnK is negative; ∆G˚rxn is positive
2) K > 1 : lnK is positive; ∆G˚rxn is negative
3) K = 1 : lnK = 0; ∆G˚rxn = 0
Temperature Dependence + Equilibrium Constant
lnK = -∆H˚rxn/R x (1/T) + ∆S˚rxn/R
- Form of a straight line; use when two different temperatures are known and equilibrium constant is given at those temperatures
