chem_kin_review

chem_kin_review - Chemical Kinetics Review Physical model...

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Chemical Kinetics Review Physical model of chemical transformations When thinking about a chemical reaction, how do we visualize it physically? C R 2 R 3 R 1 H C R 2 R 3 R 1 H C - R 2 R 3 R 1 :Base :Base + H + -Base Here, we have a proton transfer from carbon to a base catalyst, a reaction that is very common in biochemistry. Both the reactants and products are relatively stable if the carbanion formed is stabilized (e.g. triosphosphate isomerase reaction). So, we can draw potential energy wells for these states. The origin of the shape of the potential energy wells is beyond the scope of this discussion. Energy Bond Length (R) 3 C - + H + -- Base (R) 3 C -- H + :Base Transition State What happens physically is that the carbon-hydrogen bond is stretched beyond its comfortable, equilibrium position as the proton is being transferred to the base. This takes energy, just as it takes energy to stretch a spring. When we stretch the bond far enough (and make a partial bond to the base), we get to a point, the " transition state ", where it is energetically easier for the system to go towards the carbanionic product state than to continue on the potential energy diagram of the reactants. This is where the two potential energy wells cross over each
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2 other in the diagram above. There is thus an energy barrier for a chemical reaction to occur. Molecules must have enough energy to surmount this barrier for reaction to occur. At a given time, the great majority of molecules will not have enough energy to react. You might remember that the number of molecules with a given energy drops off exponentially with the value of the energy (i.e. Maxwell-Boltzman distribution). Rate equations and reaction order In kinetics, one is interested in understanding the time dependence of the disappearance of reactants and the production of products. We define the rate of a chemical reaction as the amount of product formed per unit time . The following is the simplest possible reaction: A B k (1) A is converted to B by some chemical transformation. This is a "first order" reaction (reaction order is discussed below). We know from experience that some reactions go faster than others, so we write "k" as a constant (a " rate constant ") that characterizes a particular reaction under a given set of experimental conditions. This rate constant, k, will change with conditions (temperature or ionic strength, for example). The important point is that, so far, k is an emperical constant. We just throw it in so that things make sense to us. It is important to keep the distinction between rates and rate constants clear. The units of reaction rate are generally molar/sec. The rate is not constant over time (except for zero order reactions). We therefore need to use differential equations to describe the instantaneous rate of a reaction. By inspection , we can write down a differential equation that describes the instantaneous reaction rate.
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3 d[A] dt = d[B] dt = k [A] (2) The negative sign signifies that A is being lost in the reaction. In order to get the units of rate (i.e. M/s), the rate constant
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chem_kin_review - Chemical Kinetics Review Physical model...

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