Spontaneous Process - Thermochemistry Video Lecture | Physical Chemistry

[Music]
all right so we're going to talk about
Santana's processes I'm new here the
word spontaneous a lot of things that
come to mind one thing might be
something that happens instantly when
you think of spontaneity or something
being spontaneous it happens instantly
that's not the case in chemistry
there's spontaneous reactions like when
you think about uranium decaying uranium
takes a long time to decay we don't do
anything to make it decay it's just the
case on its own which makes the
spontaneous but it decays over millions
and millions and millions of years or
there's a reaction of like diet coke and
mentos you drop some Mentos and Diet
Coke it explodes instantly they're both
spontaneous reactions we didn't do
anything to make that reaction occur
they both just occur on their own that's
kind of what spontaneous means in
chemistry it's a physical or chemical
change that occurs with no outside
intervention at ordinary conditions and
ordinary conditions or things like 1
atmospheric pressure and 25 degrees
Celsius everyday life that would be
ordinary conditions some energy might be
supplied initially and that's okay like
for example when you're talking about
lighting a Bunsen burner for example if
you have the methane gas running you
need it the only way it's going to
combust or create that flame and your
Bunsen burner is if you create a spark
that small spark is just initial like to
make the reaction get started but once
it started we do nothing to commit the
reaction continue that we call a
spontaneous process let's look over here
at some spontaneous reactions or not to
spontaneous reactions and we'll discuss
them so first we have methane ch4 reacts
with oxygen what's coming out of your
Bunsen burner we know that when it
reacts it'll produce carbon dioxide and
water totally okay it actually the net
difference between the reactants and
products is going to be 89 kilojoules
it's going to release 89 kilojoules of
energy so let's see if the this we
caught we know is the spontaneous we've
done it before in lab but is it reverse
reaction spontaneous if we take carbon
dioxide and water and combine them
together are we going to be able to get
oxygen the methane gas well no it takes
a lot of energy takes 89 kilojoules but
not only that we have to continuously
push the reaction along so if we
continue to force the reaction to occur
put it in some sort of like maybe you
know have catalyst or have other like
things that'll actually make the
reaction go without it
on its own we're gonna call that
non-spontaneous and I know for a fact
this is not spontaneous so we're going
to say no this is that's not let's talk
about rust we know if you leave your
bike outside in the rain or if things
are outside exposed the elements that
iron will react with the oxygen in the
air and it will form rust or iron 3
oxide we know it releases 1600 sorry
1625 kilojoules of energy so this
reaction will happen without us without
us doing anything we actually sometimes
don't want it to happen and it happens
all the time
so we definitely think this reaction is
spontaneous it will happen without us
doing a thing to it so yes this is a
spontaneous reaction however we can't
take rust and then go back and get iron
and oxygen and get our nice iron back it
takes a lot of energy to do that we're
going to push a lot of weight a lot of
things to happen to make this reaction
occur
it requires 1,600 sorry 1625 kilojoules
of energy it's not going to happen
unfortunately once your bike is rusted
its rusted so I can't do anything to get
that back unless I do some major work on
it and in the lab or something so if you
notice a correlation here and exothermic
reactions or reactions that release
energy are going to be our spontaneous
ones but our endothermic reactions are
the ones that are not spontaneous so for
a really long time we've just we've
chemists have assumed that all X with
thermic reactions are spontaneous and
I'll under the make reactions our
processes even our non spontaneous but
is that true so let's go down here what
about this process we have solid water
aka ice and liquid water water so we
know at ordinary conditions that a solid
ice will melt if you pop up an ice cube
on this table it's going to melt so
without me doing anything I can just
leave it there and it will melt so but
this actually requires 6.01 kilojoules
of energy so we just disproved our whole
theory there so spontaneity does not
just dependent on enthalpy or Delta H it
must be dependent on something else so
let's go back and also look over here
since what's a spa Genuity it depends on
a combination of enthalpy which we just
discussed or delta h or energy and
entropy entropy is a measurement of
disorder this actually
whole video on entropy alone so if you
want to learn about entropy you might
want to check out that video this the
symbol for entropy is s Delta s and so a
combination of this we like things to be
disorderly we like things to have chaos
and we also like to be less an energy so
a combination of these two guys
Gibbs came up with this combination to
tell us if something spontaneous or not
and he said Gibbs free energy the amount
of energy that is actually released in a
system is a combination of Delta H
subtracted from the temperature that
it's the reaction is occurring at times
the entropy are the disorder that's
actually happening so we like things
being disorderly and we also like things
being less energetic and a combination
of those two things are going to make
something spontaneous or not
[Music]