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Affect of temperature on exothermic reactions

by ElmorshedyDr
Tags: affect, exothermic, reactions, temperature
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Chestermiller
#19
Mar25-14, 06:33 AM
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Quote Quote by MathewsMD View Post
I realize for most rxns, this is the overall trend. But before we go on, isn't this not entirely correct? I don't know of any specific examples off the top of my head, but my workbook only says "reactions tend to proceed faster at higher temperatures." I don't believe this is an absolute statement and feel like there are exceptions...
For reversible reactions, the rates of the forward and the reverse reactions always increase with temperature (for fixed values of the concentrations of the reactants and products). But, as we said, if the reaction is exothermic, the reverse reaction rate increases faster with temperature than the forward rate. So, for certain combinations of reactant and product concentrations, the net rate of the reaction slows down with temperature (because the equilibrium shifts more toward the reactants and away from the products).

Chet
Yanick
#20
Mar25-14, 02:16 PM
P: 384
It should be noted that Chestermiller's analysis is valid for 1 step reactions or elementary steps in a reaction mechanism. It goes back to what I was saying regarding the innate differences between kinetics and thermodynamics. Thermodynamics doesn't care about the path (atleast the state functions don't) while kinetics is all about the path. In a multi-step mechanism the rate law, describing the concentrations/time of reactants or products, can have no relation to the balanced chemical equation. The stoichiometric coefficients need not be equal to the order of the reactants and the rate constant need not have any relation to the equilibrium constant. Kinetic studies are by and large driven by, sort of, guess and check methods of analysis. We don't really sit down and derive stuff from first principles, we try to fit various models to whatever data we have at hand to make inferences about mechanisms and the system in general. Kinetics is also not the most exact of sciences because the data is not always uniquely determined. That is, more than 1 model may explain the data, so there are always arguments about mechanisms going between groups.

Otherwise what people have been saying is pretty much correct, keeping in mind the limitations. Looking from a classical physics perspective, we imagine that the kinetic energy of the individual molecules/atoms is a reflection of the bulk property we call temperature. As we increase the temperature, the kinetic energy of the molecules increases accordingly. In addition, reaction rates depend on collisions between molecules. Not every collision will yield a successful reaction so we are in the realm of probability. More kinetic energy means that there is more collisions per unit time which means there should be more successful collisions per unit as well. This works symmetrically for the forward and reverse reaction rate constants (see the Arrhenius Equation that Chestermiller used in his analysis), however you are shifting the equilibrium constant with temperature so the system will "come to rest" at a different point than it would at some other temperature.


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