Energy Changes During a Molecular Collision

3 Sketch and/or interpret an energy-reaction graph. Identify the (a) activated complex region, (b) activation energy, and (c) AE for the reaction. 

The change from kinetic energy to potential energy and then back to kinetic energy for a bouncing ball is an example of the Law of Conservation of Energy (Section 2.9). 

Activation energy is similar to the escape energy in the evaporation of a liquid, as described in Section 15.4. Only molecules with more than a certain minimum kinetic energy are able to tear away from the bulk of the liquid and change to the vapor state. 

The vapor pressure of a liquid rises as temperature increases. The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure above the liquid. A higher pressure over the liquid requires a higher vapor pressure-and therefore a higher temperature-to boil the liquid. See Section 15.5. 

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The Collision Theory of Chemical Reactions Energy Changes During a Molecular Collision Conditions That Affect the Rate of a Chemical Reaction The Development of a Chemical Equilibrium Le Chatelier s Principle The Equilibrium Constant The Significance of the Value of K... 

Section 18.3 Energy Changes During a Molecular Collision... 

Conservation laws at a microscopic level of molecular interactions play an important role. In particular, energy as a conserved variable plays a central role in statistical mechanics. Another important concept for equilibrium systems is the law of detailed balance. Molecular motion can be viewed as a sequence of collisions, each of which is akin to a reaction. Most often it is the momentum, energy and angrilar momentum of each of the constituents that is changed during a collision if the molecular structure is altered, one has a chemical reaction. The law of detailed balance implies that, in equilibrium, the number of each reaction in the forward direction is the same as that in the reverse direction i.e. each microscopic reaction is in equilibrium. This is a consequence of the time reversal syimnetry of mechanics. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

In 1951, Kantrowitz and Grey recommended using a supersonic jet as a molecular beam source [13, 14]. The beam source and orifice design were changed to D Aq thus, the gas molecules have a strong collision at the orifice and downstream. During the expansion process, heat energy of the random movement... 

Now we turn to vibrational Raman spectroscopy, in which the incident photon leaves some of its energy in the vibrational modes of the molecule it strikes or collects additional energy from a vibration that has already been excited. The gross selection rule for vibrational Raman transitions is that the molecular polarizability must change as the molecule vibrates. The polarizability plays a role in vibrational Raman spectroscopy because the molecule must be squeezed and stretched by the incident radiation in order that a vibrational excitation may occur during the photon-molecule collision. Both homonuclear and heteronuclear diatomic molecules swell and contract during a vibration, and the control of the nuclei over the electrons, and hence the molecular polarizability, changes too. Both types of diatomic molecule are therefore vibrationally Raman active. It follows that the information available from vibrational Raman spectra adds to that from infrared spectroscopy. 

When an atom hits a surface, the initial kinetic energy of the atom can not only be transfered to the substrate. If the surface is corrugated, i.e. if the atom-surface interaction varies as a function of lateral coordinates of the atom, then the impinging atom can also change its lateral component of the initial velocity upon the collision. In the case of molecules, there are also the internal degrees of molecular vibration and rotation that can be excited (or de-excited) during the collision with the surface. 

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