Enthalpy, entropy and Gibbs free energy

Introduction to thermodynamics

In our journey through chemistry, it is important to understand the energy changes that occur during chemical reactions. Thermodynamics is the branch of physical chemistry that deals with these energy changes. The three fundamental concepts in thermodynamics are enthalpy, entropy, and Gibbs free energy. These parameters help predict whether a reaction will be spontaneous or not and how energy-efficient it will be.

Enthalpy (H)

Enthalpy is a measure of the total energy of a thermodynamic system, including internal energy and the energy required to displace it to its environment. It is often thought of as the "heat content" of a system. Enthalpy, represented by H, is important in chemical thermodynamics.

Understanding enthalpy

In its simplest form, the change in enthalpy (ΔH) of a system can be represented as the change when a reaction occurs at constant pressure. The change is often associated with the heat absorbed or released:

ΔH = H_{products} - H_{reactants}

If ΔH is negative, the reaction is exothermic, meaning it releases heat. Conversely, if ΔH is positive, the reaction is endothermic and absorbs heat.

Examples of enthalpy change

Let us consider a simple reaction: the combustion of methane:

CH_4(g) + 2O_2(g) → CO_2(g) + 2H_2O(l)

ΔH of this reaction is about -890 kJ/mol, which means that it releases 890 kJ of energy per mole of methane burned. This exothermic reaction is why methane is such an effective fuel source.

Entropy (S)

Entropy is a measure of the disorder or randomness of a system. It provides information about the number of configurations a system can have. Unlike enthalpy, entropy takes into account not only energy but also the distribution of that energy.

Understanding entropy

Entropy, denoted by S, can often be associated with the degree of disorder or randomness:

ΔS = S_{products} - S_{reactants}

A positive change in entropy (ΔS > 0) indicates an increase in disorder, while a negative change (ΔS < 0) indicates a decrease.

Visual example of entropy

The left box represents a more ordered system in which the particles are packed closer together. The right box represents a disordered system in which the particles are spread out, indicating an increase in entropy.

Examples of entropy changes

Consider the dissolution of salt in water:

NaCl(s) → Na^+(aq) + Cl^-(aq)

As the solid salt dissolves, the ordered structure of the crystalline solid breaks down into more randomly mobile ions in solution, resulting in an increase in entropy.

Gibbs free energy (G)

Gibbs free energy is a thermodynamic potential that assesses the spontaneity of a process. It is a balance between enthalpy and entropy changes. Gibbs free energy is represented by G

Gibbs free energy formula

The most important equation relating to Gibbs free energy is:

ΔG = ΔH - TΔS

Here, T is the temperature in Kelvin.

If the Gibbs free energy change (ΔG) for a reaction is negative, then the reaction is spontaneous. If ΔG is positive, then the reaction is non-spontaneous.

Examples of Gibbs free energy calculations

Let us consider the combustion of methane again:

CH_4(g) + 2O_2(g) → CO_2(g) + 2H_2O(l)

Under standard conditions, let's assume:

• ΔH = -890 kJ/mol
• ΔS = +0.242 kJ/mol·K
• T = 298 K (room temperature)

Calculate ΔG:

ΔG = -890 kJ - (298 K) × 0.242 kJ/mol·K
ΔG = -890 kJ - 72 kJ
ΔG = -962 kJ/mol

With a large negative ΔG, the combustion is highly spontaneous.

Visualization of Gibbs free energy

Here, the blue circle represents the reactants, and the red circle represents the products. The downward curve in green indicates a reaction with a negative ΔG, indicating spontaneity.

Combination of concepts

By combining enthalpy, entropy, and Gibbs free energy, chemists can determine the feasibility and spontaneity of reactions. A negative ΔG suggests that the reaction can proceed without energy input, while understanding ΔH and ΔS provides additional information about the nature of the reaction, such as exothermic vs. endothermic or order/disorder transitions.

Conclusion

In summary, enthalpy, entropy, and Gibbs free energy are important concepts in understanding thermodynamics in chemistry. They allow us to predict what reactions are likely to occur and how energy is transformed in chemical processes. These parameters are constantly analyzed in laboratories and industries to optimize chemical reactions and energy efficiency.

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