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Unformatted text preview: 1 Kinetics Kinetics vs thermodynamics So far we have focused our attention on the thermodynamic properties of the chemical systems. Thermodynamics tell us what is possible, but does not necessary tell use when actually happens. For instance, the change in free energy for converting diamond to graphite is negative at room temperature and pressure, but diamond is incredibly stable material. In fact, all organic compounds are thermodynamically unstable with respect to combustion, and yet organic compounds are kinetically stable in air. The resolution to this apparent paradox is simple: just because reaction is thermodynamically favorable does not mean it occurs quickly. What governs the rate of reaction is different from what governs thermodynamics. We must pay attention on the mechanism , i.e. the precise series of events that together make up a chemical reaction. In the following lectures, we will investigate how the rates of reactions can be measured and interpreted. Definition of the reaction rate The rate of reaction is a function of the composition and temperature: Rate=f (composition, Temperature) The first step is to identify the stochiometry of the system. Consider a reaction: ) ( ) ( 2 4 2 2 g O N g NO → How do we define the reaction rate? The units will be in concentration (pressure, mol L-1 ) per unit of time. We have: Rate of consumption of NO 2 = [ ] dt NO d 2 − Rate of formation of N 2 O 4 = [ ] dt O N d 4 2 From the reaction we have: [ ] [ ] dt NO d dt O N d 2 4 2 2 1 − = 2 We want a definition of the rate that is independent of the reactants or products, whose concentration is being measured. Thus for a generic reaction: ... ... + + → + + dD cC bB aA [ ] [ ] [ ] [ ] ... 1 1 1 1 = − = − = = dt B d b dt A d a dt C d c dt D d d [ ] dt J d v j ν 1 − = Where ν j is the stoichiometric coefficient of the substance J. We will use the following convention ν j is negative for reactant and positive for products. Rate law It is perhaps not surprising that the rate of a reaction is a function is a function of the concentrations of some of the product and reactants, along with the concentrations of the other components of the reaction medium. Rates are generally expressed as rate laws. Every reaction has its own rate law. Note that rate laws are determined experimentally and cannot be inferred from the chemical equation. For the reaction (isomerization of the methyl isocyanate): N C C H C N C H ≡ − → ≡ − 3 3 The rate is: [ ] [ ] [ ] C N C H k dt C N C H d dt N C C H d v ≡ − = ≡ − − = ≡ − = 3 3 3 Where k is the rate constant . We say that the above reaction is first order in concentration of methyl isocyanate. At this point, we need to anticipate an important concept that we will refine later: the rate constant is temperature dependent! In several reactions become more rapid when the temperature is increased....
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