Energy Diagrams, Catalysts, and Reaction Mechanisms

professor Dave here, let’s learn about
reaction mechanisms we’ve learned a lot about chemical reactions
but what is necessary for a reaction to occur on the molecular level? collision
theory states that molecules involved in a reaction whether in the liquid or gas
phase have to collide in a particular orientation and with an amount of
kinetic energy that exceeds the activation energy of a reaction. this is
the amount of energy that has to be provided to allow for the reaction to
proceed. here we can see that the two molecules have to collide in a
particular way in order to react otherwise they will bounce right off
each other. as for energy let’s refer to an energy diagram. on these up means
higher energy and to the right means later in time. so we can see that any
reaction will have an activation energy which is the kinetic energy that the
molecules must collide with in order to reach the transition state. this is the
orientation which once reached the reaction will certainly occur. collisions
with less energy than this will not result in a reaction. from energy
diagrams we can also see delta H or the difference in energy between the
reactants and products. if the products sit lower it is an exothermic reaction
which releases energy. if the products sit higher it is an endothermic reaction and needs
to absorb energy to occur. most people would refer to a catalyst as something
that speeds up a reaction while this is often true the better
definition is that a catalyst is something that lowers the activation
energy of a reaction. because the activation energy is lower many more
collisions will result in a successful reaction which is why catalysis
typically results in a faster rate of reaction catalysts do not affect the delta H of a
reaction and they are not used up stoichiometrically in a reaction. in
other words one molecule of catalyst can facilitate many many reactions unlike
molecules of reactant which are used up in order to make products. reactions also
tend to have faster rates at higher temperatures because more heat energy
means faster moving molecules which means more kinetic energy. at higher
temperatures more individual collisions will have sufficient energy to surpass
the activation barrier so the overall reaction of many trillions of
molecules will proceed more quickly the Arrhenius equation shows how the rate
constant depends on the activation energy. in this equation A is a constant
called the frequency factor and we have the number e the activation energy and the gas
constant as well as temperature. this equation can be modified to show the
ratio of the rate constant for a reaction at two different temperatures. this is
the one that we can use to calculate the activation energy of a reaction. a
balanced chemical equation will tell you the starting materials and products of
the reaction but sometimes what is happening on the molecular level is more
complicated than this. the series of steps that actually occurs is called the
reaction mechanism. for the following reaction we can break it down into
elementary steps. the slowest step is called the rate-determining step because
the rate predominantly depends only on this step as the other step or steps are
so much faster that they are negligible in comparison. a species that is found in
elementary steps but not in the overall equation is called a reaction
intermediate. the rate law for the overall equation can be discerned
directly from the rate-determining step and by this method the exponents will
equal the stoichiometric coefficients. let’s check comprehension. thanks for watching guys, subscribe to my channel for more tutorials and as always feel free to email me

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