Gibbs Free Energy (ΔG): Determines spontaneity of a reaction.
ΔG < 0: Spontaneous process
ΔG = 0: Equilibrium
ΔG > 0: Non-spontaneous process
Problem Analysis
Identifying False Statements
True Statements:
ΔG is less than zero for a spontaneous process.
ΔG equals zero at equilibrium.
Maximum work from a spontaneous process equals ΔG.
False Statement: ΔG is greater than zero for a spontaneous process, as it's actually true for a non-spontaneous process.
Calculating Gibbs Free Energy
Formula: ΔG = ΔH - TΔS
ΔH: Enthalpy change (kJ/mol)
T: Temperature in Kelvin
ΔS: Entropy change (J/mol·K; convert to kJ by dividing by 1000)
Example Calculation:
ΔH = -46.5 kJ/mol
T = 298 K (25°C + 273)
ΔS = 0.212 kJ/mol·K
ΔG = -46.5 - 298(0.212) = -109.7 kJ/mol
A negative ΔG indicates a spontaneous reaction.
Using Formation Data
Relation: ΔG for a reaction can be calculated using ΔH and ΔS of formation.
Products minus reactants method
Example: HCl formation using enthalpy and entropy values
Units and Equations
Ensure consistency in units: ΔG and ΔH in kJ/mol, ΔS in J/mol·K.
Conversion may be necessary:
1 kJ = 1000 J
Keep track of unit cancellations.
Reaction Quotient (Q) and Equilibrium
Equation: ΔG = ΔG° + RT ln Q
R: Gas constant (J/mol·K)
T: Temperature in Kelvin
Additional Problems
Calculating Boiling Point:
Use ΔG = 0 at phase transitions (e.g., equilibrium between liquid and gas).
Formula rearrangement: T = ΔH/ΔS
Predicting Reaction Shifts:
Negative ΔG: Reaction shifts to the right (spontaneous forward).
ΔG = 0: Equilibrium
Positive ΔG: Reaction shifts to the left (non-spontaneous forward).
Thermodynamic Principles
Enthalpy (ΔH) and Entropy (ΔS):
Both negative: Spontaneous at low temperatures
Both positive: Spontaneous at high temperatures
Opposite signs: Spontaneous regardless of temperature if ΔH negative and ΔS positive
Equilibrium Constant (K):
K > 1: Product favored, ΔG negative
K < 1: Reactant favored, ΔG positive
Practical Applications
Determining the spontaneity of phase changes
Calculating equilibrium constants from ΔG values
Conclusion
Understanding Gibbs free energy is crucial for predicting reaction spontaneity and calculating necessary thermodynamic properties. It provides insight into the direction and extent of chemical reactions.