CHEM 2087: Analysis of Kinetic Data - Enthalpic and Entropic Components

Flashcards covering the enthalpic and entropic components of activation energy, illustrated with specific chemical reactions and kinetic data from the CHEM 2087 lecture.

Free Energy of Activation ($\Delta G^{\ddagger}$)

The energy barrier defined by the equation $\Delta G^{\ddagger} = \Delta H^{\ddagger} - T\Delta S^{\ddagger}$.

Enthalpy of Activation ($\Delta H^{\ddagger}$)

The enthalpic component of the free energy of activation, calculated as the difference between the sum of dissociation energies of bonds broken and bonds formed: $\Delta H^{\ddagger} = \sum D(\text{bonds broken}) - \sum D(\text{bonds formed})$.

Enthalpy Controlled Reaction

A reaction where the enthalpic component ($\Delta H^{\ddagger}$) is the primary contributor to the total free energy of activation ($\Delta G^{\ddagger}$).

Eyring Equation

The fundamental equation in transition state theory relating reaction rate to activation parameters: $k = \frac{k_B T}{h} e^{-\frac{\Delta G^{\ddagger}}{RT}}$.

Gas Phase Dissociation of Ethane ($H_3C-CH_3$)

A reaction involving homolytic cleavage with a high activation enthalpy of $\Delta H^{\ddagger} = +361\,kJ\,mol^{-1}$ due to the lack of compensating solvent interactions.

Concerted Reaction ($S_N2$)

A reaction where bond formation and bond cleavage occur simultaneously in the transition state, resulting in a lower $\Delta H^{\ddagger}$ (e.g., $+71\,kJ\,mol^{-1}$ for ethyl bromide and chloride ion).

Asynchronous Reaction

A reaction such as an $S_N2$ where bond cleavage is slightly ahead of bond formation, leading to a slight positive $\Delta H^{\ddagger}$.

Solvation Effects in $S_N1/E_1$

The phenomenon where the enthalpy of activation is lowered because the carbocation transition state is better solvated than the neutral reactant in polar solvents like methanoic acid.

Entropy of Activation ($\Delta S^{\ddagger}$)

The entropic component of activation energy, calculated as $\Delta S^{\ddagger} = S(\text{solvated activated complex}) - \sum S(\text{solvated reactants})$.

Entropy Unit (e.u.)

An alternative unit for measurements of entropy, equivalent to $cal\,K^{-1}\,mol^{-1}$.

Total Entropy ($S_T$)

The sum of the translational, rotational, and internal entropies of a molecule or activated complex: $S_T = S_{trans} + S_{rot} + S_{int}$.

Acid-catalysed hydrolysis of ethyl acetate ($\Delta S^{\ddagger}$)

A reaction with a very negative entropy of activation ($\Delta S^{\ddagger} = -104\,J\,K^{-1}\,mol^{-1}$ or $-26\,e.u.$) involving a termolecular activated complex.

Bimolecular $S_N2$ Entropy

Characterized by negative values (e.g., $-124\,J\,K^{-1}\,mol^{-1}$ for methyliodide and pyridine) because two molecules come together in the rate-determining step.

Dissociative $S_N1/E_1$ Entropy

Characterized by a positive entropy of activation (e.g., $+40\,J\,K^{-1}\,mol^{-1}$ for tert-butyl chloride in water) because one molecule dissociates into two in the rate-determining step.

Cycloaddition (Diels-Alder) Entropy

Typically results in very negative entropies of activation (e.g., $-144\,J\,K^{-1}\,mol^{-1}$) due to the highly ordered activated complex and specific geometry requirements.

Entropy Controlled Reaction

A reaction where the entropic term ($\Delta S^{\ddagger}$) is the main contributor to the free energy of activation ($\Delta G^{\ddagger}$).