Introduction to Chemical Kinetics Video Lecture | Physical Chemistry

so this is going to be an introduction
to chemical kinetics and chemical
kinetics is the branch of chemistry that
is concerned with the rates of chemical
reactions so just to break down what I'm
going to do this video is I'm going to
discuss what a rate is what a reaction
rate is and I'm also going to point out
some of the equations associated with
reaction rates so first we need to get a
good understanding of what a reaction
rate is so when we talk about a rate of
a chemical reaction we're talking about
how fast the reaction occurs and as a
result we usually end up dividing by
time so these two examples here the
speed of your car and your hourly pay
those are both rates because you're
dividing a change in some quantity per
unit time in the case of the speed of
the car you have a displacement Delta X
and you're dividing that by time and in
the case of your hourly pay you're
taking the change in the amount of money
you make over a unit of time the hour
usually as well so these are just two
examples of rates to show you know what
all rates have in common reaction rates
are the same way you're allowing
something to change and you're dividing
it by a change in time and in chemical
reaction is the thing that we change is
the concentration of the reactants and
products so let's look at an arbitrary
reaction and see if we can't learn a
little bit more about the rates
okay so suppose I have the following
chemical reaction
h2 plus BR two yields HBR and I gotta
put a two here to balance it there we go
okay so on the left side on the
left-hand side of the equation you have
reactants those are getting used up and
on the right-hand side of the equation
you have products and those are becoming
formed as the reaction proceeds so let's
sort of look at the concentrations of
these things graphically so let's say I
want to look at the concentration of HBR
and the concentration of h2 over time so
I'm going to construct a concentration
versus time plot my x-axis is going to
be time I'll just say that's in seconds
and my Y axis is going to be molar
concentration so concentration in moles
per liter and the curves for the
reactants and products respectively will
look like this
now can you guess which curve
corresponds to which does the red curve
correspond to h2 or hbr the correct
answer is h2 why because like we said
earlier the concentrations of the
reactants are decreasing the reactants
are being used up as the reaction
proceeds and notice that this curve sort
of represents that over time you have a
decrease in concentration the blue curve
is for come is for the concentration of
HBR since HBR is a product it starts out
at zero concentration and increases as
the reaction proceeds so how do we find
a rate of a reaction that's the question
generally what we do is we can pick two
points so let's say I want to find out
how fast the concentration of HBR is
changing so I'm just going to pick two
points on this graph I'll pick this one
here and I'll pick this one here so
suppose this time is one second and this
time is two seconds and let's say that
this concentration is two molar and this
concentration is four molar
so find the rate all you do is you just
find the slope between these two points
the slope of this line so the slope of
this line is simply going to be Delta Y
over Delta X where Y is the
concentration and X is time so that's
really going to be Delta concentration
of hbr over delta T okay so we just
found the rate of formation of hbr using
the slope of the line between these two
points notice as you reduce the interval
between these two points as your time
interval becomes lower and lower and
lower your rate you get a more accurate
result as to the rate of your reaction
to add any specific instant so we've
just found the rate of formation of we
just found an expression for the rate of
formation of HBR but when it comes to
reaction rates we actually have to
account for the stoichiometry of the
overall reaction and there's really only
one equation that you have to know when
you consider this and that is the
following equation
you know this equation corresponds to
the following reaction scheme you have a
moles of a plus B moles of B yielding C
moles of C plus D moles of D ABC and D
the capital letters are the reactants
and products themselves and the little a
little B little C and little D those are
the stoichiometric coefficients so this
equation it says that rate is equal to
negative 1 over a Delta a concentration
of a over delta T which is equal to
negative 1 over B constant delta
concentration of b over delta T equals 1
over C times Delta a concentration of C
over delta T which is equal to 1 over D
times the Delta concentration of D over
delta T so based on this equates there's
actually a variety of ways that you can
get the rate notice that you can obtain
these are all equivalent quantities
provided that you have the balanced
chemical equation so once you have a
couple of parameters you don't really
need to know everything about all the
other reactants or products you can just
deduce the rate from 1 so in general the
three things that you need to determine
the rate of the chemical reaction are
like I said earlier the balanced
chemical equation you also need the
change in concentration for any reactant
or product doesn't matter which just one
will do and lastly you need a time
interval as you might notice the three
variables in this equation here in each
of these terms is the coefficient the
change in concentration and the change
in time and of course in order to get
that coefficient again you need the
balanced chemical equation if the
equation is not balanced then it'll be
wrong so alright I hope this video helps
and some more kinetic stuff on the way