Entropy and Spontaneity

• The study notes are about chemical thermodynamics, spontaneity of reactions, entropy, and chemical equilibrium.

Chemical Thermodynamics

• The branch of chemistry concerned with energy changes, focusing on transformations and heat exchanges
• Key areas include energy, work, heat, and spontaneity of processes.
• Some reactions occur spontaneously, while others require specific conditions.
• For example, sodium combines with chlorine easily
• Nitrogen and oxygen coexist without reacting.

Second Law of Thermodynamics

• Also known as the "Law of Entropy"
• States that the entropy of an isolated system, like the universe, always increases over time
• Entropy changes in the universe are never negative
• Defines the direction of spontaneous processes.

Enthalpy (H)

• Reactions are more likely to occur with a decrease in enthalpy (ΔH is negative).

Entropy (S)

• Not a form of energy but a thermodynamic property
• Measures the disorder or randomness in a system
• Higher entropy indicates a greater state of disorder
• Gases have the highest entropy, while solids have the lowest
• Measured in Joules per Kelvin (J/K)
• A change in order affects the number of ways to arrange particles, influencing spontaneity.
• Order increases from gas to liquid to solid
• Order increases as crystalline structures dissolve and form ions in solution

Entropy Change (ΔS)

• Calculated as ΔS = Sfinal - Sinitial
• A positive ΔS indicates an increase in disorder
• A negative ΔS indicates a decrease in disorder.

Determining Entropy Change

• Expect a positive ΔS (increase in entropy) when:
  • Solid reactants form liquid or gaseous products
  • Liquid reactants form gases
  • Many smaller particles combine into larger particles
• Expect a negative ΔS (decrease in entropy) when:
  • Gaseous or liquid reactants form solid products
  • Large molecules dissociate into smaller ones
  • More moles of gas are in the products than reactants

Factors Affecting Entropy

• Diffusion: Particles disperse increasing entropy
• Concentration and Pressure: Increase disorder
• Physical State: Entropy increases from solid, to liquid, to gas
• Temperature: Higher temperatures increase molecular motion and entropy
• Volume: Larger volume increases molecular motion and entropy
• Number of Particles: More particles equal higher entropy due to more arrangements
• Number of Moles: Entropy increases when the total moles of product exceed reactants
• Phase Change:
  • Increased Entropy: Melting, vaporization, sublimation
  • Decreased Entropy: Freezing, condensation, deposition

Spontaneity

• Refers to a reaction's ability to occur without continuous external influence.
• Spontaneous Reactions: occur under specific conditions without external intervention, characterized by:
  • Increased entropy
  • A decrease in Gibbs-free energy (ΔG < 0)
• Non-Spontaneous Reactions: Require external energy or intervention, often with increased Gibbs-free energy (ΔG > 0)

Spontaneity in Reactions

• Some processes occur without additional energy input.
• Decreased energy in a system explains why some processes occur naturally like a ball rolling downhill
• Reactions with large numbers of exothermic reactions are spontaneuous

Examples of Entropy Change

• Increase: Evaporation of water, bomb explosion
• Decrease: Liquefying nitrogen gas, increasing pressure of a confined gas, freezing water

Examples of Spontaneity of Reactions

• Spontaneous: Sugar dissolving, ice melts above 0°C
• Non-Spontaneous: Reformation of dissolved sugars, rust decaying into iron and oxygen

Calculating Entropy Change in a Reaction

• ΔS° = ∑ S° (products) - ∑ S° (reactants)
• Values are based on standard molar entropies

Entropy and Spontaneity of a Reaction

• The second law of thermodynamics states that the entropy of the universe increases in spontaneous processes and stays constant in an equilibrium process.
• ASuniv = ASsys + ASsurr > 0 (spontaneous)
• ASuniv = ASsys + ASsurr = 0 (equilibrium).
• The second law of thermodynamics states the ASuniv must be > 0
• Reactions that form simpler molecules from simpler ones decreases entropy
• Solid converted to gaseous product increases entropy

Chemical Equilibrium

• A dynamic state of reactions where the rates of forward and reverse reactions are equal
• Optimizing reactions involves adjusting temperature, pressure, and reactant concentrations
• In equilibrium, the reaction may favor the products or reactants.

Reversible Reactions

• Chemical reactions occur in both directions, not going to completion.
• Reactants aren't fully converted to products

Equilibrium Constants

• At equilibrium, the forward and reverse reaction rates are equal
• K = [AB]/[A][B]
• Even with multistep reactions, the equilibrium constant remains consistent.
• The equilibrium constant expression is written with product concentrations in the numerator and reactant concentrations in the denominator
• K = ([C]^c [D]^d)/([A]^a [B]^b)
• Pure liquids and solids aren't included

Numerical Values for K

• Determined from experiments or thermodynamic data
• Large K means more products, while small K means more reactants

Thermodynamic Equilibrium Constant

• Activities are ratios of concentrations/pressures to standard states that cancel out units
• Activities for pure liquids and solids = 1
• Activities for gases = ratio of partial pressure to standard pressure

Magnitude of K

• Indicates reaction extent. and is constant at a given temperature but changes with temperature
• K is always the same at a constant temperature and does not depened on intial concentrations

Equilibrium Constant (Keq)

• Describes equilibrium, may be expressed in partial pressures (Kp) or molarities (Kc).
• Expressing Reactions:
• If gases are present, the partial pressures will affect the equilibrium
• Kp applicable only to gaseous reactions
• No gases present, Kp does not apply and we have only Kc.

Reaction Quotient (Q)

• For non-equilibrium reactions, the reaction quotient (Qc) replaces the equilibrium constant using initial concentrations
• Q = ([C]^x[D]^y)/([A]^m[B]^n)

Relating K and Q

• Qc < Kc: Reaction proceeds from left to right.
• Qc = Kc: The system is at equilibrium.
• Qc > Kc: Reaction proceeds from right to left.

Homogenous vs Heterogenous Equilibria

• Homogenous: Equilibria involving all species in a single phase.
• Heterogeneous: Equilibria involving species in more than one phase.

Reaction Quotient

• Mass action expression under any conditions and the magnitude determines the direction and if it occurs

Le Chatelier's

• Mechanical and chemical equilibriums are similar becuase both respond to stresses by adjusting until new equilibriums are reached

Le Chatelier's Principle

• Helps predict the shift in an equilibrium reaction due to changes in conditions such as concentration, pressure, volume, or temperature
• System counteracts the effects of stress to restore equilibrium.

Disturbing Equilibrium

• Reacting system results in changes in concentration/pressure/temperature/volume
• Three types of change can disturb equilibrium
• Changes in concentrations
• Changes in pressure or volume that involve gases
• Changes in temperature

Changes in Concentration

• System shifts to relieve stress and restore equilibrium by either generating products or reactants
• The stress caused by the added substance will be relieved by shifting the equilibriums

Pressure/Volume Change

• Does not affect condensed phases due to incompressibility
• Affects gases
• Increasing pressure favors fewer moles of gas; decreasing pressure favors more.
• Pressure (or volume) doesn't affect reactions when gas moles are unchanged.

Changes In Temperature

• Only changes in temperature can alter equilibrium constant values
• Treat heat like a chemical
• Increasing temperature favors endothermic reactions; decreasing temperature favors exothermic reactions.

Catalysts

• Enhances the reaction rate and lowers activation enery
• Adding catalyst accelerates equilibrium.
• A catalyst doesn't shift system position; only helps it reach sooner

This explores how changes in volume, phase, and molecular complexity influence entropy. Questions cover entropy's relationship with temperature, spontaneous reactions, and the conditions for endothermic processes to occur spontaneously.