This document outlines the fundamental concepts of Enthropy, Entropy Change (ΔS), and Gibbs Free Energy Change (ΔG). It covers how to calculate these values and determine reaction spontaneity under various conditions. Students should be prepared to apply the Gibbs Free Energy equation and interpret the signs of ΔH, ΔS, and ΔG to predict spontaneity, especially concerning temperature dependence.
Enthropy
Enthropy measures the degree of randomness of particles within a system.
개념
Enthropy
A measure of the degree of randomness or disorder of particles within a system.
Key Processes Increasing Entropy (MCQ!!)
Many common processes lead to an increase in entropy, often involving an increase in the number of particles or a change to a more disordered state.
개념
Decomposition
Entropy increases because a single compound breaks down into more moles of particles, often involving a state change from solid to gas/liquid, which increases disorder.
개념
Dissolving
Entropy increases due to a state change where a solid disperses into a liquid, leading to a more disordered arrangement of particles.
개념
Melting
Entropy increases as a substance transitions from a highly ordered solid state to a less ordered liquid state, allowing particles more freedom of movement.
개념
Combustion
Entropy increases significantly as solid or liquid reactants typically produce gaseous products, increasing the number of moles of gas and overall disorder.
개념
Boiling/Vaporization
Entropy increases substantially when a liquid changes to a gas, as gas particles have much greater freedom of movement and occupy a larger volume.
개념
Sublimation
Entropy increases dramatically when a solid directly changes to a gas, bypassing the liquid phase and resulting in a large increase in disorder.
Entropy Change (ΔS)
Entropy Change (ΔS) quantifies the difference in system entropy during a process.
개념
Entropy Change (ΔS)
The difference in the entropy of a system during a reaction or state change, indicating whether disorder increased or decreased.
공식
ΔS = ΣS_products - ΣS_reactants
Entropy change equals the sum of the standard entropies of the products minus the sum of the standard entropies of the reactants.
The sign of ΔS indicates the direction of entropy change:
If ΔS > 0, entropy has increased* (more disorder).
If ΔS < 0, entropy has decreased* (less disorder).
Gibbs Free Energy Change (ΔG)
Gibbs Free Energy Change (ΔG) determines reaction spontaneity based on enthalpy, temperature, and entropy.
개념
Gibbs Free Energy Change (ΔG)
A thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at a constant temperature (T) and pressure. It determines if a reaction is spontaneous.
공식
ΔG = ΔH - TΔS
Gibbs Free Energy Change equals Enthalpy Change minus Temperature times Entropy Change.
Spontaneity Condition
A reaction is considered spontaneous in the forward direction if ΔG < 0. If ΔG > 0, the reaction is non-spontaneous. If ΔG = 0, the system is at equilibrium.
Spontaneity based on ΔH and ΔS
| ΔH > 0 (Endothermic) | ΔH < 0 (Exothermic) | |
|---|---|---|
| ΔS > 0 (Entropy Increased) | ΔG < 0 at high temp (spontaneous) | ΔG < 0 (reaction is always spontaneous) |
| ΔS < 0 (Entropy Decreased) | ΔG > 0 (reaction is never spontaneous) | ΔG < 0 at low temp (spontaneous) |
Spontaneity of NH4Cl Decomposition
- 1
Given Values
Reaction: NH4Cl(s) → NH3(g) + HCl(g)
Temperature (T): 298.15 K
Enthalpy Change (ΔH°): 176 kJ/mol
Entropy Change (ΔS°): 0.285 kJ/(mol • K)
- 2
Calculate ΔG°
ΔG° = ΔH° - TΔS°
ΔG° = 176 kJ/mol - (298.15 K * 0.285 kJ/(mol • K))
ΔG° = 176 kJ/mol - 84.97275 kJ/mol
- 3
Determine Spontaneity
Since ΔG° is positive, the reaction is not spontaneous at this temperature.
정답
ΔG° = 91.0 kJ/mol. The reaction is not spontaneous in the forward direction at 298.15 K.
Temperature for Br2 Vaporization Spontaneity
- 1
Given Values
Reaction: Br2(l) → Br2(g)
Enthalpy Change (ΔH°): 31.0 kJ/mol
Entropy Change (ΔS°): 93.0 J/(mol • K) (convert to kJ: 0.0930 kJ/(mol • K))
- 2
Set up Inequality for Spontaneity
For spontaneity, ΔG < 0. So, ΔH - TΔS < 0.
31.0 kJ/mol - T * 0.0930 kJ/(mol • K) < 0
- 3
Solve for T
31.0 kJ/mol < T * 0.0930 kJ/(mol • K)
T > (31.0 kJ/mol) / (0.0930 kJ/(mol • K))
정답
The process Br2(l) → Br2(g) is spontaneous when T > 333K.