Entropy

• What is entropy (S)?

A measure of the disorder or randomness of a system.

State the Second Law of Thermodynamics.

• The total entropy of the universe must increase for a process to be feasible (spontaneous).

Which has the highest entropy: solid, liquid, or gas? Why?

• Gas. The particles are far apart and move randomly in all directions, representing the highest state of disorder.

How does a change in temperature affect entropy?

Increasing temperature increases entropy.

• Reason: Particles gain kinetic energy, move more vigorously, and occupy a greater variety of energy states, increasing disorder.

How does the number of particles, especially in gases, affect the entropy of a system?

An increase in the number of moles of gas leads to a large increase in entropy.

• Reason: More gas molecules mean more ways to distribute energy and more possible microstates, greatly increasing disorder.

• Example: The reaction 2H₂(g) + O₂(g) → 2H₂O(l) has a negative ΔS because 3 moles of gas become 2 moles of liquid.

How do you calculate the entropy change of the system, ΔSₛyₛₜₑₘ?

ΔS_system^ꝋ = ΣΔS_products^ꝋ - ΣΔS_reactants^ꝋ

CaCO₃(s) → CaO(s) + CO₂(g)

In this reaction, how does entropy increase?

• In this reaction, Calcium Carbonate decomposes into CaO and CO₂ gas, which is more disordered than the reactant as it constantly moves around

• Hence the system is more disordered resulting in an increase in entropy.

What is a feasible (spontaneous) reaction?

• A reaction that can proceed on its own, without continuous external energy input, once started.

What is the difference between a feasible reaction and a spontaneous reaction?

• Feasible tends to be used to describe reactions which are energetically favourable, so reactions that should go

• Spontaneous tends to be used to describe reactions that go of their own accord

What is the fundamental condition for a reaction to be feasible?

• The total entropy change of the universe must be positive: ΔSₜₒₜₐₗ > 0

What is standard entropy?

• Standard entropy (S^⦵) is the entropy of 1 mole of a substance under standard conditions (100 kPa pressure and 298 K temperature). Its units are J K^-1 mol^-1.

What is the Gibbs Free Energy equation?

• ΔG = ΔH - TΔS

Where:

• ΔG = Change in Gibbs Free Energy (kJ mol⁻¹)

• ΔH = Enthalpy Change (kJ mol⁻¹)

• T = Temperature (K)

• ΔSₛyₛₜₑₘ = Entropy Change of the System (kJ K⁻¹ mol⁻¹) - Note the unit change from J to kJ!

What is the condition for a reaction to be feasible in terms of ΔG?

• ΔG ≤ 0 (ΔG must be negative or zero).