1422-Chapt-16-Kinetics

1422-Chapt-16-Kinetics - Kinetics 1 Chapter 16 (no...

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Kinetics 1 Chapter 16 (no derivation of kinetic rate laws)
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Kinetics 2 Chemical Kinetics is the area of chemistry that is concerned with finding out how fast a chemical rxn will go and what is the mechanism by which it proceeds? The mechanism of a rxn is how the reactants react in a step-wise fashion to form the product molecules. Thermodynamics only tells us if the rxn is energetically possible ( exothermic , endothermic , spontaneous ) – nothing about the rate of the rxn or how it will proceed (i.e., the mechanism of the rxn). Kinetics provides us with the techniques for measuring the rate of a reaction and gaining information about how the reaction is proceeding. This, in turn, can give us valuable information about the mechanism of the rxn.
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Kinetics 3 Consider the very spontaneous and exothermic rxn of H 2 + O 2 to produce H 2 O : Note: most text books use E (potential energy) for the vertical axis. I will generally use Δ G The activation energy or activation barrier , when present and large enough, is what prevents a spontaneous reaction from reacting instantly !
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Kinetics 4 The activation energy arises due to kinetic effects, i.e., the rates at which molecules react together and the factors that affect those rates of reactions. Consider the rearrangement of methyl isocyanide into the more stable acetonitrile molecule: H 3 C N C N C CH 3 methyl isocyanide acetonitrile This reaction involves the migration (movement) of the methyl group ( H 3 C or CH 3 ) from the nitrogen atom over to the terminal carbon atom.
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Kinetics 5 The initial movement of the methyl group involves partial breaking of chemical bonds between the methyl group and nitrogen atom and the C-N triple bond, both of which take energy. The topmost part of the activation energy barrier is called the transition state , which represents the highest energy part of the reaction pathway from reactants to products . The structure of the molecule at the transition state is something that one can never directly observe or isolate !! We can only speculate (or calculate from fancy quantum mechanical programs) what it may look like. But once we hit the transition state and the methyl group starts moving towards the terminal carbon atom to form a new and stronger bond, the same amount of energy used to move up to the top of the activation barrier from the reactant side is released . So there is NO NET CONSUMPTION OF ENERGY from the activation barrier the total amount of energy released in the reaction is still just Δ G .
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Kinetics 6 But if the activation barrier is high enough, we may need to add some (or a lot of) heat (energy) to the reactants to give them enough energy to make it over the barrier. But we get that energy back when they go down the other side of the activation barrier to the products!!
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1422-Chapt-16-Kinetics - Kinetics 1 Chapter 16 (no...

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