What is the primary difference between thermodynamic feasibility and kinetic feasibility?

Thermodynamic feasibility ($\Delta G < 0$) determines if a reaction is energetically possible, while kinetic feasibility depends on the activation energy ($E_a$) and determines if the reaction occurs at a measurable rate.

How does the relationship between $\Delta G$ and the equilibrium constant $K$ change as $\Delta G$ becomes more negative?

As $\Delta G$ becomes more negative, the equilibrium constant $K$ increases exponentially. This means the reaction proceeds further toward completion, favoring the formation of products over reactants.

When comparing an exothermic reaction with a decrease in entropy, how does temperature affect spontaneity?

The reaction is spontaneous at low temperatures where the favorable $\Delta H$ dominates. At high temperatures, the $-T\Delta S$ term becomes a large positive value, eventually making $\Delta G$ positive and the reaction non-spontaneous.

What is the most common unit-related error when calculating $\Delta G = \Delta H - T\Delta S$?

Failing to convert the entropy change ($\Delta S$) from $J\,K^{-1}mol^{-1}$ to $kJ\,K^{-1}mol^{-1}$. Since $\Delta H$ is usually in $kJ$, $\Delta S$ must be divided by $1000$ to ensure the units are consistent.

Why is it incorrect to use $0^\circ C$ as the temperature in the Gibbs equation?

The Gibbs equation requires absolute temperature in Kelvin ($K$). Using $0$ would nullify the entropy term entirely, which is physically incorrect; $0^\circ C$ must be converted to $273.15\text{ K}$.

What error occurs if you interpret the slope of a $\Delta G$ vs. $T$ graph as the entropy change?

The slope of the graph is actually $-\Delta S$, not $\Delta S$. If the slope is positive, the entropy change of the reaction is negative (the system is becoming more ordered).

Define Gibbs Free Energy ($G$) in terms of work.

Gibbs Free Energy is the maximum amount of non-expansion work (such as electrical work) that can be extracted from a system at constant temperature and pressure.

What does the 'standard' symbol ($^\theta$) signify in $\Delta G^\theta$?

It signifies that the measurement was taken under standard conditions: $100\text{ kPa}$ pressure, a specified temperature (usually $298\text{ K}$), and all substances in their standard physical states.

State the Gibbs equation and define each variable.

$\Delta G = \Delta H - T\Delta S$. $\Delta G$ is free energy change, $\Delta H$ is enthalpy change, $T$ is absolute temperature in Kelvin, and $\Delta S$ is entropy change.

What is the formula to find the 'turning point' temperature for reaction feasibility?

The temperature is found by setting $\Delta G = 0$, resulting in $T = \frac{\Delta H}{\Delta S}$. This is the temperature at which the reaction transitions between being spontaneous and non-spontaneous.

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5. Advanced Physical Chemistry

Gibbs Free Energy

Summary

Gibbs Free Energy ( $G$ ) is a thermodynamic potential used to predict the spontaneity of chemical reactions by combining the effects of enthalpy and entropy. A reaction is considered thermodynamically feasible when the change in free energy ( $\Delta G$ ) is negative or zero, representing a balance between the system's drive toward lower energy and higher disorder.

1. Definition & Core Concepts

2. Underlying Principles

Thermodynamic Feasibility: A reaction is spontaneous or 'feasible' if $\Delta G \leq 0$ . This negative value indicates that the process leads to an overall increase in the entropy of the universe, satisfying the Second Law of Thermodynamics.
The Role of Temperature: Temperature acts as a weighting factor for the entropy term ( $-T\Delta S$ ). At higher temperatures, the influence of entropy becomes more dominant, which can override an unfavorable (endothermic) enthalpy change.
Energy Quanta and Surroundings: The concept of free energy accounts for the energy released to the surroundings. When an exothermic reaction occurs, it increases the entropy of the surroundings by dispersing energy quanta, which contributes to the overall feasibility of the process.

Graph of Gibbs Free Energy vs Temperature showing the linear relationship where the y-intercept is enthalpy change and the slope is negative entropy change.

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Connections & Extensions

Gibbs Free Energy

Q: How does the relationship between $\Delta G$ and the equilibrium constant $K$ change as $\Delta G$ becomes more negative?

As $\Delta G$ becomes more negative, the equilibrium constant $K$ increases exponentially. This means the reaction proceeds further toward completion, favoring the formation of products over reactants.

Q: When comparing an exothermic reaction with a decrease in entropy, how does temperature affect spontaneity?

The reaction is spontaneous at low temperatures where the favorable $\Delta H$ dominates. At high temperatures, the $-T\Delta S$ term becomes a large positive value, eventually making $\Delta G$ positive and the reaction non-spontaneous.

Q: What is the most common unit-related error when calculating $\Delta G = \Delta H - T\Delta S$?

Failing to convert the entropy change ($\Delta S$) from $J\,K^{-1}mol^{-1}$ to $kJ\,K^{-1}mol^{-1}$. Since $\Delta H$ is usually in $kJ$, $\Delta S$ must be divided by $1000$ to ensure the units are consistent.

Q: Why is it incorrect to use $0^\circ C$ as the temperature in the Gibbs equation?