Unit 9: Applications of Thermodynamics

Entropy

• Entropy, S, is the amount of disorder or chaos in a system. More disorder, greater S value.
• Standard entropy is S° and measured at 25 celsius
• Standard entropy change ∆S° is measure at the end of a reaction
  • ∆S° = (sum of ∆S° products) - (sum of ∆S° reactants)
• If a reaction goes from less moles to more moles (such as 2 moles on the reactant side to 3 moles on the product side) there is more disorder and a positive ∆S
• If a reaction goes from a gas to liquid, liquid to solid, or gas to solid, the reaction has a negative ∆S
  • If bonds are broken and phase change becomes more disordered, the ∆S is positive

Gibbs Free Energy

• ∆G is Gibbs Free Energy which determines if a process is thermodynamically favored or unfavored, also known as spontaneous or nonspontaneous

Free Energy Change

• Standard free energy change, ∆G°, is calculated the same as ∆S°
  • ∆G° = (sum of ∆G° products) - (sum of ∆G° reactants)
• For a reaction,
  • If ∆G is negative, it is TFP (thermodynamically favored process)
  • If ∆G is positive, it is not TFP
  • If ∆G is 0, it is at equilibrium

∆G, ∆H, and ∆S

• TFP must result in decreasing enthalpy, increasing entropy, or both
  • ∆G° = ∆H° - T∆S°
  • T = temperature in Kelvin
  • ∆S° is usually given in j/mol*K and must be converted to kj/mol*K
  • Gibbs Free Energy is usually kj/mol*K

Standard Free Energy Change and the Equilibrium Constant

• Gibbs free energy can be calculated if equilibrium constant is known
  • ∆G° = -RT(ln K)
  • R = gas constant (8.31 j/mol*k)
  • T = kelvin temperature
  • K = equilibrium constant
• If ∆G° is negative, K is greater than 1, the products are favored at equilibrium
• If ∆G° is positive, K must be less than 1, the reactants are favored at equilibrium

Reduction Potentials

• Every half reaction has electric potential. Potentials are given as reduction half-reactions. If the reaction is reversed, flip the sign to get the oxidation potential

Galvanic Cells

• Galvanic cells (voltaic cell) use favored redox reactions to generate current
• Two half-reactions take place in separate chambers and the electrons from the oxidation pass to the reduction reaction which creates the current
  • Current is defined as the flow of electrons from one place to another
• Oxidation takes place at the anode electrode and reduction takes place at the cathode electrode
• The salt bridge keeps electrical neutrality. Without the salt bridge the voltage would be zero. The potassium ion flows to the cathode and the chlorine flows to the anode.
• The cell voltage is equal to the total redox reaction voltage.

Non-Standard Conditions

• Reduction potentials are give at standard conditions, 25 celsius, 1 atm, and 1 M
• Voltaic cells are very favored with equilibrium constant greater than 1. If the Q = K however, the voltage would drop to ero.
• If the reaction quotient increased it would become close to the equilibrium constant and the voltage would decrease.

Electrolytic Cells

• Electrolytic cells use outside voltage sources to power unfavored redox reactions and mainly occur in aqueous solutions.
• The sign of total cell potential is always negative

Electroplating

• Electrolytic cells are used for electroplating.
  • I = (q/t)
  • I = Current (amperes, A)
  • q = charge (coulombs, C)
  • t = time (second, s)
  • Moles of electrons = (coulombs/ 96,500 coulombs per mol)

Voltage and Favorability

• Redox is favored if the potetial has a positive value. reaction potential can be calculate gibb’s free energy
  • ∆G° = -nFE°
  • n = number of moles of electrons exchanged in the reaction
  • F = Faraday’s constant. 96,500 coulombs/mol
  • E° = standard reaction potential (V)
  • If E° is positive, ∆G° is negative and is TFP
  • If E° is negative, ∆G° is positive and not TFP