Thermodynamics and Chemical Equilibrium: Key Concepts

Posted on Apr 12, 2025 in Geology

Gas Laws and Properties

  • For a gas with no internal order: ls-in líki2 k in ls as there is no ordn gass intrno.
  • Phase change from liquid to gas involves an increase in entropy.
  • Dalton’s Law: pA = p0 xA
  • Avogadro’s Law: V1N2 = V2N1

Internal Energy and Enthalpy

  • At constant pressure, the change in internal energy equals the heat exchanged.
  • At constant pressure, the enthalpy change equals the heat exchanged plus pΔV.
  • Heat involved in a constant volume process is calculated as follows.
  • In general, the heat involved in an isothermal process is state-dependent.

Free Energy and Entropy

  • Spontaneity condition: ΔGsystem < 0 or ΔSuniverse > 0.
  • A process with ΔSsystem < 0 and ΔSsurroundings > 0 will always be spontaneous.
  • A process with ΔSsystem < 0 cannot be spontaneous if ΔSsurroundings < 0.
  • For a spontaneous process, the universe tends towards disorder.

Mixtures and Solutions

  • A solution of glucose in water increases the vapor pressure compared to pure water.
  • Exothermic dissolution shows negative deviations from Raoult’s Law.
  • In a solution where solute-solvent interactions are weaker than solute-solute or solvent-solvent interactions, dissolution is exothermic.
  • Endothermic ideal solutions comply with Raoult’s Law.

Chemical Equilibrium

Example Equilibrium: 2A + 3B ⇌ 1/2 C

  • Equilibrium constant: Kc = ([A]2 [C]1/2) / [B]3
  • Reversed equilibrium constant: K’c = [C]1/2 / ([A]2 [B]3)
  • If coefficients are divided by 2: K”c = ([A] [B]3/2) / [C]
  • Coupled equilibrium: ½ C ⇌ D + 2E, the global constant is K = ([D] [E]2) / ([A]2 [B]3)

Shifting Equilibrium

  • 2A(g) + B(s) ⇌ C(g). Increasing total pressure shifts the equilibrium to the right.
  • A(s) + B(g) + 2C(g) ⇌ 1/2 D(g). Increasing partial pressure of D (constant total pressure) shifts the equilibrium to the left (Qc > Kc).
  • A(l) ⇌ B(l) + C(l), ΔH > 0. Increasing temperature favors the forward reaction.
  • A(s) + B(s) ⇌ 2C(g). Increasing concentration of C leads to Qc < Kc.

Gas Relationships

  • (p1/v1) = (p2/v2)
  • V1t2 = v2t1
  • V = a / p
  • (v1/n2) = (v2/n1)

Phase Changes

  • A phase change to a more ordered state implies the system releases heat.
  • A phase change to a more disordered state implies the system absorbs heat.
  • Temperature varies during a phase change.
  • In liquid-gas dynamic equilibrium, higher temperature corresponds to higher vapor pressure.

First Law of Thermodynamics and Internal Energy

  • In a determined process, the system’s change is equal to the surroundings’ change with the opposite sign.
  • If the system is isolated, ΔE > 0.
  • At constant pressure, the change in internal energy equals the heat exchanged at constant pressure.
  • Changes in internal energy are independent of the path taken.

Enthalpy

  • Enthalpy is a state function at constant pressure.
  • Enthalpy depends only on the state of the system.
  • In an exothermic process at constant pressure, the final state has lower enthalpy than the initial state.
  • For an isothermal process involving ideal gases: qp = qv + RTΔn.

Spontaneous Processes

  • In an irreversible process, the system’s entropy always increases.
  • In a process at equilibrium, the system’s entropy change equals the surroundings’ entropy change.
  • The entropy change of a process is the sum of the entropy changes of its partial processes.
  • A system can undergo a spontaneous endothermic process if ΔSsystem > 0.