Entropy & Spontaneity
The thermodynamic term that describes the dispersal or molecular disorder of matter and energy.
Entropy
A reaction is always thermodynamically favorable at all temperatures when ΔH and ΔS possess these respective algebraic signs.
Negative (ΔH < 0) and positive (ΔS > 0)
When a reversible reaction reaches chemical equilibrium, ΔG reaches this exact numerical value.
Zero
This process occurs when an unfavorable reaction is paired with a favorable one to drive a net spontaneous process.
A coupled reaction
The type of electrochemical cell that utilizes a thermodynamically favorable redox reaction to generate electrical energy. / The specific electrode where oxidation always occurs in any electrochemical cell.
A galvanic (or voltaic) cell / The anode
This state of matter universally has the highest absolute entropy value for any given substance.
Gas
The algebraic sign of ΔG° for a reaction that is thermodynamically unfavorable.
Positive (+)
If a reaction has an equilibrium constant K > 1, the sign of its standard free energy change (ΔG°) must be this.
Negative (-)
To find the net ΔG° for a series of coupled reactions, you apply this mathematical step to the individual ΔG° values.
Adding them together
This mandatory component of a galvanic cell maintains electrical neutrality by allowing ions to migrate between half-cells. / The algebraic sign of the standard cell potential (E°cell) for any spontaneous galvanic cell.
A salt bridge / Positive (+)
The predicted algebraic sign of ΔS° for the phase transition of water vapor condensing into liquid water.
negative (-ΔS°)
A reaction that has a highly negative ΔG° but fails to occur at a measurable rate is said to be under this type of control.
Kinetic control
The exact value of the ideal gas constant (R) used in the equation ΔG° = -RT ln K.
8.314 J/(mol·K)
The biological "energy currency" molecule whose highly exothermic hydrolysis is frequently coupled to unfavorable cellular reactions.
ATP (Adenosine Triphosphate)
Faraday’s constant (96,485) represents the total electrical charge (in Coulombs) carried by one mole of these. / According to the equation ΔG° = -nFE°, if a cell potential (E°) is positive, ΔG° must be this.
Electrons / Negative (-)
The mathematical formula used to calculate the standard entropy change (ΔS°rxn) using absolute standard entropies.
ΣS°(products) - ΣS°(reactants)
The physical property that determines whether a reaction under kinetic control can overcome its high activation energy barrier.
Temperature
When an ionic solute dissolves in water and the solution gets cold, the dissolution is endothermic, meaning the favorability is driven entirely by this factor.
Entropy (ΔS > 0)
If Reaction 1 has ΔG° = +20 kJ and Reaction 2 has ΔG° = -35 kJ, this is the net standard free energy change of the coupled system.
-15 kJ
This equation is used to calculate cell potential under nonstandard concentration or pressure conditions. / If the reactant concentrations in a galvanic cell are significantly increased above 1.0 M, the operating cell potential (Ecell) will change in this direction compared to E°cell.
The Nernst Equation / Increase (become more positive)
The law stating that a perfect crystal at absolute zero Kelvin (0 K) has an absolute entropy of zero.
The Third Law of Thermodynamics
The fundamental equation used to calculate ΔG° when the enthalpy change, entropy change, and temperature are known.
ΔG° = ΔH° - TΔS°
If a salt is highly insoluble (Ksp << 1), the sign of its standard free energy of dissolution (ΔG°soln) must be this.
Positive (+)
In industrial metallurgy, the unfavorable extraction of pure metals from ore oxides is typically coupled with the oxidation of this element.
Carbon
The name given to an electrochemical cell that uses a spontaneous redox reaction but has identical electrodes with different ion concentrations. / The specific type of cell that requires an external power source to force a thermodynamically unfavorable chemical reaction to occur.
A concentration cell / An electrolytic cell