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PDF Atomic/Molecular Encounters

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In solution, we cannot so easily speak of collisions, because the relative migration of the species is diffusion, and the solvent prevents their free escape. Therefore, the rate of change of the number of A molecules per unit volume, ZAB multiplied by the fraction of collisions occurring with a. Collision with sufficient energy is not the only criterion for reaction, and another factor, such as the relative orientation of the colliding species, is taken into account.

Since the molecule moves only slowly into the region of a possible reaction partner, it also moves away from it only slowly. The activation energy of a reaction is a much more complicated quantity in solution than in gas: the happy couple is surrounded by. A dating couple can break up without reaction, or they can react and give products.

21B.1. Rate at which the Molecules Diffuse Together

A molecule waits. [과장]

Reacted

The concentration profile for a reaction in solution when molecule B diffuses toward another reactant molecule and. The reaction rate is equal to the average flow of B molecules to all A molecules in the sample.

Transition State of Reaction: A pair of reactants has been

21C. Activated Complex Theory

Transition State Theory

The reaction coordinate and the transition state

AB) ‡ in equilibrium with (AB)

21C. Formation and Decay of the Activated Complex

Pre-Equilibrium

Assume (AB) ‡

Once the activated complex has been found, movement along the reaction coordinate corresponds to a distortion of some relevant bond.

21C.1(c). The Concentration of the Activated Complex

Sec. 21C.1(b)

No dip

Saddle Point

How to use the Eyring equation

The real difficulty

21D. Collision of Structureless Particles

RTRE

21C.2. Thermodynamic Aspects

Reaction-Rate Constant

21D.3. Potential Energy Surfaces

Potential energy surface for the reaction H + H2 when the atoms are constrained to be collinear. B corresponds to a path in which the RBC is elongated at an early stage during HA approach. If the HB-HC bond length is constant during the initial approach of HA, the potential energy of the H3 group will increase along the path marked A.

Path C: path of minimum potential energy RBCs lengthen as HA approaches and begins to form a bond with HB. Battery, Water Separation, CO2 Reduction, Fuel Cells, Quantum Rod Solar Cells, Sensors, Quantum Rod Solar Cells, Sensors,.

Electro-Analysis, Electro-Synthesis, Electrodeposition, Corrosion,

21E. Electrochemistry

Dynamics of Electron Transfer

21E.1. Electron Transfer in Homogeneous System

21E.(a) The rate of electron tunneling

Gibbs energy surfaces. of the DA and D+A− complexes, which participate in the electron transfer process. characteristic of harmonic oscillators, where the displacement coordinate q corresponds to the change. On the graph and are the values ​​of q at which the minima of the reactant and. The graphs also show the Gibbs energy of activation, the standard reaction Gibbs energy, °, and.

Correspondence between electronic energy levels (shown on the left) and nuclear energy levels (shown on the right) for DA and D+A− complexes involved in an electron transfer process. a). In the nuclear configuration marked with , the electron to be transferred to DA is in an occupied energy level and the lowest unoccupied energy level of D+A− is too high in energy to be a good electron acceptor . As nuclei rearrange into a. the configuration represented by q*, DA and D+A−. become degenerate and electron transfer occurs by tunneling through the barrier of height V and width r, the edge-to-edge distance between the donor and.

21E.1. The expression for the rate of electron transfer

21E. Experimental results

Helmholtz Layer

21F. Electron Transfer Processes at the Electrodes

The Gouy-Chapman model of the diffuse double layer The disordering effect of thermal motion is taken into account in

The Gouy-Chapman model of the electric double layer treats the outer region as an atmosphere of. The plot of electric potential versus distance from the electrode surface shows the importance of the diffuse double layer.

Diffuse Layer (~100 Å)

The Stern model

The above two models are combined in the Stern model, where the ions closest to the electrode are confined to a rigid. Helmholtz plane, while the ions outside that plane are scattered as in the Gouy-Chapman model. The current density j: the electric current flowing an electrode divided by the area of ​​the electrode.

RTfF

This equation shows that the current density is proportional to the overvoltage, so at low overvoltages the interface behaves like a conductor obeying Ohm's law. The net current density at the electrode is the difference between the current density resulting from the reduction of Ox and the oxidation of Red. The current density j resulting from redox processes is the rate multiplied by the charge transferred per mole of reaction (F).

The net current density is defined as the difference between the cathodic and . a) When yes>jc the net current is anodic and there is a net oxidation of the species in solution. The potential φ varies between two plane parallel layers of charge, and its effect on the Gibbs energy of the transition state depends on the degree to which the latter resembles the species on the inner or outer face. If the transition state of the activated complex is product-like. the activation of Gibbs energy in the absence of a potential difference across the double layer.

When the transition state resembles a species that has undergone reduction, the Gibbs activation energy for the anodic current is almost unchanged, but the full effect applies to the cathodic current. When the transition state resembles a species that has undergone oxidation, the Gibbs activation energy for the cathodic current is almost unchanged, but the Gibbs activation energy for the anodic current is strongly affected. a) Zero potential difference; (b) nonzero potential difference. Red throws an electron to the electrode, so the additional work is zero if the transition state is similar to the reactant.

If the cell is balanced against an external source, the Galvani potential difference can be identified as the (zero current) electrode potential E.

Problems from Chap. 21

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