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Enthalpy is the potential energy of a system. Changes in enthalpy within a process can be represented by ΔH.
ΔH = ΣH(products) - ΣH(reactants)
Exothermic processes (ΔH < 0): A process that decreases the potential energy of a system, releasing heat into the surrounding as a result.
Endothermic processes (ΔH > 0): A process that increases the potential energy of a system, absorbing heat from the system as a result.
Entropy is a measure of disorder within a system. Changes in entropy within a process can be represented by ΔS.
ΔS = ΣS(products) - ΣS(reactants)
Resulting in higher disorder (ΔS > 0): Process has higher probability of different arrangements within a system.
Resulting in lower disorder (ΔS < 0): Process has lower probability of different arrangements within a system.
Spontaneous processes happen on their own without outside intervention.
Examples: Melting of ice at room temperature, rusting of iron.
Gibbs Free Energy (ΔG)
The Gibbs Free Energy equation determines if a process is spontaneous (ΔG < 0) or not spontaneous (ΔG > 0).
Calculating with ΔH and ΔS:
ΔG = ΔH - T·ΔS
ΔG: Gibbs Free Energy (kJ/mol)
ΔH: Changes in enthalpy. (kJ/mol)
ΔS: Changes in entropy. (J/(mol·K))
T: Temperature (K)
Note: Divide the value of ΔS by 1000 if it comes in J/(mol·K) to convert to kJ/(mol·K).
This equation demonstrates how enthalpy, entropy, and temperature affect spontaneity.
Calculating Based on Change in Gibbs Free Energy:
ΔG° = ΣG°(products) - ΣG°(reactants)
ΔG: Change in Gibbs Free Energy.
ΣG: Sum of the standard Gibbs Free Energy values of products/reactants. (kJ/mol)
Note: The degrees symbol (°) indicates that the values are based on standard ambient temperature (25°C/298 K) and pressure (1 atm).
Calculating with R and Q:
ΔG = ΔG° + RT·ln(Q)
ΔG: Gibbs Free Energy (kJ/mol)
R: Gas Constant (8.314 J/mol·K)
T: Temperature (K)
Q: Reaction Quotient
This equation demonstrates how the ratio of products to reactants at a given time and system conditions affect spontaneity.
Calculating with R and K:
ΔG = -RT·ln(K)
ΔG: Gibbs Free Energy (kJ/mol)
R: Gas Constant (8.314 J/mol·K)
T: Temperature (K)
K: Equilibrum Constant
This equation demonstrates how the ratio of products to reactants at equilibrium indicate spontaneity.
Calculating with E°cell:
ΔG = -nFE
ΔG: Gibbs Free Energy (kJ/mol)
n: Moles of Electrons
F: Faraday's Constant (96,485 C/mol)
E: Electric Potential (V)
This equation demonstrates how the electric potential of an electrochemical cell relates to its spontaneity.
Predicting Spontaneity
The following values of Gibbs Free Energy, enthalpy, and entropy are thermodynamically favorable:
ΔG < 0 (Negative value, ΔG = -)
ΔH < 0 (Negative value, ΔH = -)
ΔS > 0 (Positive value, ΔS = +)
To predict if a process is spontaneous (ΔG < 0) or not spontaneous (ΔG > 0), we have to know the values of enthalpy and entropy of the process.
If ΔH = - and ΔS = +, ΔG = - (Spontaneous)
If ΔH = + and ΔS = -, ΔG = + (Not Spontaneous)
If ΔH = - and ΔS = -, ΔG = ± (Spontaneous at Low Temperatures Only)
If ΔH = + and ΔS = +, ΔG = ± (Spontaneous at High Temperatures Only)
At low temperatures, the value for T is small. Therefore, ΔH dominates the equation and primarily determines the outcome.
At high temperatures, the value for T is large. Therefore, T·ΔS dominates the equation and primarily determines the outcome.
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