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Ch 9.3a First Law and Enthalpy | General Chemistry | YouTubeToText
YouTube Transcript: Ch 9.3a First Law and Enthalpy
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Video Summary
Summary
Core Theme
This content introduces the fundamental concepts of thermodynamics, focusing on the first law and the definition and application of enthalpy, particularly its relationship to heat transfer at constant pressure.
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hi everyone we're going to continue our
conversation with thermodynamics and
enthalpy so these are our learning
outcomes for the day um to First State
the first law of thermodynamics you
should be able to Define enthalpy and
explain why it's a state function and
we're going to write some thermochemical
equations so we're going to use some
more of our sto gometry then we should
be able to calculate enthalpy changes
for a bunch of different chemical
reactions and then explain hess's law
using and use it to uh compute
enthalpies and the last thing uh you
know throughout this uh activity we are
going to apply enthalpy to calorimetry
experiments so the first law of
thermodynamics is thinking about the total
total
energy um of a
system and this isolated system is the
energy is
constant that means the
transferred and it can't be created or
destroyed so this is one of the ways
your book uses it uh we call the energy
of the system U which is the internal
internal
energy and we can calculate the U uh the
change in internal energy as the heat
plus the work of the system and based on
the system and how the internal energy
is changing you can only put heat in or
heat can come out and you can only put
work in or work can be done by the
system so these terms are pretty
important these terms like bu on out and
in um are the kind of ways that we think
about our internal energy so this goes
back to our conservation of energy
energy can't be destroyed or created it
can only be transferred and for internal
energy it is only about heat and
work let's first think about uh U so U
is what we call a state
function and then a state function only
depends on its
so it doesn't matter the path of it so
the best way to describe this is trying
to get to a destination if you're trying
to drive to school it doesn't matter
that you you drove all through the
mountains or you drove to the shore and
then you came back it only matters that
you started at your home and you ended
up at
College um and then the big thing about
this is that the U is a state function
but q and w uh so your heat and the work does
depend on the
path so U uh q and W are not State
function but U is a state
function now the chemists think about
energy and and all these other kinds of
uh uh uh content through enthalpy so we
use enthalpy and we Define
enthalpy equals the internal energy plus
a PV kind of work so PV is our work that
we're thinking about um so we consider
PV um the negative of work equals P Delta
Delta
V so what we can have for internal
energy are U internal energy equals Q + w
w
so this Q + W can be put into our Delta
H equals Delta U plus P Delta V so we
can take this and dump this into the U
so that is now q and we are calling this constant
constant
pressure you see that the P doesn't
change over here then we can take this
W over here and we can import it into to
the P Delta V so therefore we end up
getting our Delta H equals Q at constant
pressure plus W minus W this ends up
being zero so therefore our Delta H
equals Q under constant pressure so this
is thinking back to our bomb calorimetry
uh kind of experiments and our coffee
cup calorimeter experiments so our
coffee cup calorimeters are under constant
constant
pressure and our bomb calorimeter is
actually under constant
volume so therefore we can start to
think about our Q of the reaction equals
our enthalpy change of the reaction in our
our
calorimetry so our next step is to go
ahead think about q and how we can use
enthalpy to actually
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