[5.2.2] entropy

Entropy (S)

A measure of dispersal of energy in a system. Solids have the most order while gases have the most disorder.

Gibbs Free Energy (G)

Defined as a thermodynamic state function that combines enthalpy (H) and entropy (S) to determine whether a reaction is feasible (spontaneous). Units are kJ/mol.

ΔG

= ΔH - TΔS

Gas Produced

When the number of gaseous molecules increases, the entropy increases as more particles are present and so there are more ways their energy can be arranged.

Change in Entropy (ΔS)

Sum of Products - Sum of Reactants

Spontaneous Reactions

Reactions that take place on its own. They lead to lower energy and increased stability. For this to occur, entropy must generally increase.

Third Law of Thermodynamics

Law that states that for a spontaneous change to occur, the total entropy change of the universe must increase.

• ΔS (total) = ΔS (system) + ΔS (surroundings)

System

Used to describe the actual particles in a reaction or process.

Surroundings

Describes everything outside the system.

Why can freezing occur spontaneously?

• Entropy of system decreases when freezing occurs.

• Freezing is exothermic, releasing heat into the surroundings, giving the particles in the surroundings more energy (higher entropy) → overall entropy of universe increases.

What Spontaneity Depends On

• Enthalpy change of reaction

• Entropy change of reaction

• Temperature

(AKA Gibbs Free Energy)

ΔG = -ve

Reaction is spontaneous/feasible.

ΔG = 0

Reaction is at equilibrium, where feasibility changes.

ΔG = +ve

Reaction is not spontaneous/feasible.

ΔH = -ve

ΔS = +ve

Reaction is spontaneous at any temperature.

ΔH = +ve

ΔS = -ve

Reaction is spontaneous at no temperatures.

ΔH = -ve

ΔS = -ve

Reaction is spontaneous at low temperatures.

ΔH = +ve

ΔS = +ve

Reaction is spontaneous at high temperatures.

Limitations of Prediction Using ΔG

Whether a reaction occurs spontaneously also depend on kinetic factors:

• activation energy

• rate of reaction

ln(k)

y-axis

1/T

x-axis

ln(A)

y-intercept

Calculating E_a

gradient x - 8.314

k

Rate constant. Units vary depending on the overall order of the reaction.

A

Arrhenius constant (or pre-exponential factor). It has the exact same units as

E_a

Activation energy. Must always be in Joules per mole (J mol^-1)