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Day14 - CE 561 Lecture Notes Fall 2009 Day 14 Reactions in...

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CE 561 Lecture Notes Fall 2009 p. 1 of 7 Day 14: Reactions in Liquids For the past couple of weeks, we have been discussing theories of chemical kinetics for reactions occurring in the gas phase. In this set of lecture notes, we will turn our attention to reactions in solution (in the liquid phase). There are substantial, qualitative differences in the dynamics of molecules in the liquid phase compared to the gas phase, and liquid phase kinetics are not understood at the same level as gas phase kinetics. Models of reactions in solution will therefore be simpler and more approximate than those used in the gas phase. In the gas phase, collisions between reactants are rare, and the interval between collisions is random. The collision process can be considered Markovian , meaning that the occurrence of a collision does not affect the likelihood of having another collision. In contrast, molecules in solution interact continuously with the solvent, and in order to move long distances they must squeeze past closely-spaced solvent molecules. As a result, when two solute molecules (reactants) collide, they are likely to undergo multiple collisions while they are held together by the surrounding solvent molecules, before diffusing apart. In the approach we will take here, each of these sets of collisions is called an encounter between the reactants, and while the reactants are trapped close together, we refer to them as an encounter pair or an encounter complex. This difference in collision distributions between the gas and liquid phase is illustrated schematically below. The gas phase system and solution system shown above have the same overall collision rate between reactants, but the distribution of those collisions in time is dramatically different, and this changes the way in which reactions occur. In addition to changing the pattern of collisions, the solvent also can change the energetics of the reaction. For example, formation of ions is often stabilized by solvation, and as a result, NaCl Na + + Cl - is highly endothermic in the gas Gas Phase Collisions time Solute-Solute Collisions time ‘encounters’
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CE 561 Lecture Notes Fall 2009 p. 2 of 7 phase, but exothermic in solution. A third important effect of the solvent is rapid energy transfer to reactants and products, so that thermal equilibrium is maintained. We will now consider a simple phenomenological model of reaction in solution. In order to react, the molecules must first diffuse together, then react before they diffuse back apart. We can think of this as Where A and B are the reactants, and {AB} is the encounter complex , the pair of reactants trapped close together and undergoing multiple collisions. k 1 is the rate constant for the reactants to diffuse together and form the encounter complex, k -1 is the rate constant for breakup of the encounter complex (rate constant for the reactants to diffuse apart), and k 2 is the rate constant for the reaction of the encounter complex to form products. We may apply the pseudo-steady-state approximation to the collision complex {AB}.
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