10.1 Relationship between kinetics and thermodynamics

Chemical kinetics and thermodynamics are two sides of the same coin in understanding reactions. Kinetics looks at how fast reactions happen and the steps involved, while thermodynamics focuses on energy changes and whether reactions can occur spontaneously.

These concepts work together to paint a full picture of chemical reactions. Thermodynamics tells us if a reaction is possible, while kinetics shows us how quickly it'll happen. Understanding both helps predict and control reactions in real-world applications.

Fundamentals of Kinetics and Thermodynamics

Kinetics vs thermodynamics fundamentals

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• Kinetics studies the rate and mechanism of chemical reactions, focusing on the pathway and intermediate steps involved in converting reactants into products (reaction progress over time)
• Thermodynamics is concerned with the overall energy changes and spontaneity of reactions, considering only the initial and final states of a system to determine the feasibility and direction of a reaction based on energy considerations (enthalpy, entropy, Gibbs free energy)

Thermodynamics in reaction feasibility

• Gibbs free energy ($\Delta G$) determines the spontaneity of a reaction at constant temperature and pressure, calculated using the equation $\Delta G = \Delta H - T\Delta S$ where $\Delta H$ is the change in enthalpy, $T$ is the absolute temperature, and $\Delta S$ is the change in entropy
• Spontaneity of a reaction depends on the sign of $\Delta G$:
  • $\Delta G < 0$ indicates a spontaneous and thermodynamically favorable reaction
  • $\Delta G > 0$ indicates a non-spontaneous and thermodynamically unfavorable reaction
  • $\Delta G = 0$ indicates the system is at equilibrium
• Relationship between $\Delta G$ and equilibrium constant ($K$) is given by $\Delta G^\circ = -RT \ln K$ where $R$ is the gas constant and $T$ is the absolute temperature, determining the position of the equilibrium and the extent of the reaction

Kinetics in reaction rates

• Activation energy ($E_a$) is the minimum energy required for reactants to overcome the energy barrier and form products, determining the rate of a reaction according to the Arrhenius equation $k = A e^{-E_a/RT}$ where $k$ is the rate constant, $A$ is the pre-exponential factor, $R$ is the gas constant, and $T$ is the absolute temperature
• Reaction mechanisms describe the sequence of elementary steps that a reaction undergoes, determining the overall rate law and the rate-determining step
• Catalysts are substances that lower the activation energy without being consumed in the reaction, increasing the reaction rate without affecting the thermodynamic equilibrium (enzymes, transition metal catalysts)

Kinetic and Thermodynamic Interplay

Interplay of kinetic and thermodynamic factors

• Thermodynamically favorable reactions may proceed slowly if the activation energy is high, but kinetic factors such as temperature and catalysts can be adjusted to increase the reaction rate (rusting of iron, diamond formation from graphite)
• Thermodynamically unfavorable reactions will not proceed to a significant extent regardless of kinetic factors, but may occur to a small degree due to the presence of reactants and the reversibility of reactions (decomposition of water into hydrogen and oxygen)
• Competing reactions are influenced by both thermodynamics, which determines the relative stability of products, and kinetics, which determines the relative rates of formation for each product, resulting in a product distribution that depends on both factors (selectivity in organic synthesis)