Chemical Equilibrium and Thermodynamics: Key Concepts

Posted on Jan 17, 2025 in Chemistry

States of Aggregation of Matter

  • In liquids, as in gases, there is no internal order.
  • The change from liquid to gas phase involves a reduction of entropy.
  • For a gas mixture, Dalton’s Law states: PA = Pº WxH.
  • Avogadro’s Law states that V1N2 = V2N1.

Internal Energy and Enthalpy

  • The internal energy change along a constant pressure process is equal to the heat of reaction.
  • The variation of enthalpy in a constant pressure process is equal to the heat of reaction at constant volume over P)V.
  • The heat brought into play in a constant volume process is independent of the path taken.
  • In general, the heat brought into play in an isothermal process behaves like a state function.

Free Energy and Entropy

  • Stating the condition of spontaneity as ΔGsystem < 0 or ΔSuniverse > 0 is equivalent.
  • A process where ΔHsystem < 0 and ΔSsystem > 0 will always be spontaneous.
  • A process where ΔHsystem < 0 cannot be spontaneous if ΔSsystem < 0.
  • For a spontaneous process, the universe must become more disordered.

Mixtures

  • A solution of glucose in water has a lower vapor pressure than pure water.
  • A solution presents exothermic negative deviations from Raoult’s Law.
  • A solution where the interaction forces between solute and solvent are lower than those in these components separately is exothermic.
  • Ideal solutions meet Raoult’s Law endothermically.

Balancing Chemical Equations

For the reaction 2A + 3B ⇌ 1/2 C, the following holds true:

  • The equilibrium constant is: Kc = ([A]2 [C]1/2) / [B]3.
  • If the equation is reversed, the constant becomes: K’c = [C]1/2 / ([A]2 [B]3).
  • If the coefficients are divided by 2, the constant is: K”c = ([A] [B]3/2) / [C].
  • If the equilibrium is coupled with D + 2E ⇌ 1/2 C, the global constant is: K”’c = ([D] [E]2) / ([A]2 [B]3).

Equilibrium Reactions

  • 2 A (g) + B (s) ⇌ C (g). If the total pressure increases, the equilibrium shifts to the right.
  • A (s) + B (g) + 2C (g) ⇌ 1/2 D (g). If the partial pressure of D increases (at constant total pressure), Qc > Kc and the equilibrium shifts to the left.
  • A (l) ⇌ B (l) + C (l); ΔHdir > 0. If the temperature increases, a greater proportion of A is formed.
  • A (s) + B (s) ⇌ 2C (g). If the concentration of A increases, Qc remains unchanged.

Relationship Between P, V, and T for Gases

  • (P1/V1) = (P2/V2)
  • V1T2 = V2T1
  • V = a / P
  • (V1/n2) = (V2/n1)

Phase Changes

  • A change to a less ordered phase implies that the system absorbs energy as heat.
  • A change to a more disordered phase than the previous one implies that the system absorbs energy as heat.
  • During the phase change, the temperature remains constant.
  • In the dynamic equilibrium liquid-gas, the higher the temperature, the higher the vapor pressure.

First Law of Thermodynamics and Internal Energy

  • ΔE of the system in a given process is the same as that experienced by the surroundings but with the opposite sign.
  • If we have a closed system, ΔE > 0.
  • The internal energy change in a process at constant pressure is equal to the heat of reaction at constant pressure.
  • The internal energy change of a process is independent of the number of stages in the process.

Enthalpy

  • It is a state function only in the constant pressure process.
  • It only depends on the state of the system.
  • In an exothermic process at constant pressure, the enthalpy of the final state is lower than the initial state.
  • For an isothermal process that involves an ideal gas, it is related to the constant volume heat through the expression: qp = qv + RTΔn.

Spontaneity of Processes

  • In an irreversible process, the entropy of the system always increases.
  • In a process in equilibrium, the entropy of the system is equal to that of the surroundings.
  • The entropy of a process equals the sum of the entropies of the sub-processes.
  • For a system to undergo a spontaneous endothermic process, it must be fulfilled that: ΔSsystem > 0.