professor Dave here, let's learn the laws of thermodynamics
the laws of thermodynamics help us understand why energy flows in certain directions and
in certain ways. a lot of the concepts described by thermodynamics seem like
common sense but there is a layer of math beneath the intuitive level that
makes them very powerful at describing systems and making predictions. we won't
get into the math but we should be able to describe these laws conceptually. the
first law described in the most basic way highlights conservation of energy
energy is not created or destroyed it only changes forms, from potential energy
to kinetic energy to heat energy, etc. while we have found this to be untrue on
the quantum level, for chemists it does just fine. however there seems to be a
preferred direction in which energy flows from one form to another. in order
to understand why we look at the 2nd law. the 2nd law introduces a new concept:
entropy. entropy is quite difficult to understand but we can most easily
describe entropy as disorder, and the 2nd law states that the sum of the
entropies of a system and its surroundings must always increase. in
other words the entropy or the disorder of the universe is always increasing
within a system there is also a tendency to go towards higher entropy. the classic
analogy is that your bedroom will over time become messy but it won't suddenly
become neat. another way to look at this is to say that entropy is a measure of
how dispersed the energy of the system is amongst the ways that system can
contain energy. yet another way is to analogize entropic states to computer
code. let's take for example an ionic solid compared to the same substance as
a liquid. clearly the solid state is more ordered and the liquid state is more
disordered, or higher in entropy. to describe the solid state using computer
code you would need to include terms
that describe the geometry of the lattice, the intermolecular distances, the
precise configuration of every molecule and many other things. but to describe
the liquid state you would need to simply describe the volume of liquid and
the shape of the vessel because the motion and configuration of the
molecules are random. that's far less information that needs encoding which is
a way of rationalizing why increasing the entropy of a system is
thermodynamically favorable. we can look at all kinds of processes to highlight
entropic influence. heat will flow from a hot coffee cup into the table or your
hand because the heat energy will be more disordered if more dispersed. this
is why heat spontaneously flows from hot to cold and not the other way around.
entropy. the third law states that a perfectly crystalline solid at absolute
zero has an entropy of zero as this is the most ordered state the substance can
be in. entropy is measured in joules per kelvin. note that entropy is not a
measure of energy itself but of how energy is distributed within a system. it
is enthalpy, the thermodynamic quantity we learned about before that is more
accurately describing the energy of a system. as we will see
enthalpy and entropy intricately relate to tell us something about the Gibbs
free energy of a system. G, or Gibbs free energy tells us whether a process will
be spontaneous or not
meaning if it will simply happen on its own
change in Gibbs free energy is given by this equation which includes change in
enthalpy, change in entropy and temperature. if delta G is negative the
process is spontaneous, if positive it is nonspontaneous. so we can use this
equation to see how a spontaneous process can be either enthalpically or
entropically favorable or both but not neither. for example if delta H is
negative which means exothermic
and energetically favorable, and delta S is positive which means an increase in
entropy which is also favorable, a negative minus a positive will always be
negative or spontaneous. if the opposite is true and both are unfavorable we have
a positive minus a negative which will always be positive or nonspontaneous. if
only one of the two is favorable we have to do some math. if delta H is positive
or endothermic, that energetic unfavorability could be outweighed by
the other term if the process is entropically favorable, and since T is here
this factor will increase with a larger T so entropically favorable processes
are more likely to be spontaneous at higher temperatures. conversely if it
is energetically favorable but entropically unfavorable the entropic
unfavorability will be minimized at lower temperatures. this is a very
important equation to understand because it describes all of the spontaneous
processes in the universe
there are those who incorrectly use entropy and the second law of
thermodynamics to imply that order
can't happen spontaneously, but we just showed that entropically unfavorable
processes can be spontaneous at lower temperatures if they are
energetically favorable. an example of this is soap. you need soap to wash
nonpolar dirt and grime off your hands because they are immiscible with polar
water molecules, but soap molecules have polar heads and long nonpolar
tails which allows them to spontaneously form structures called micelles. these
are spheres where the soap molecules orient themselves with the polar heads
facing out in order to maximize ion-dipole interactions with water molecules
that bring the system to a lower energy and the nonpolar tails will all face in
trapping the dirt by making a network of van der Waals interactions. the dirt
trapped in the micelles washes away because the micelle as a whole is
water-soluble, due to the polar heads facing out. that's how soap works and
that's also how highly ordered structures can form spontaneously if by
enthalpically favorable or energy storing processes. in this way
systems can defy entropy on the small scale but the 2nd law does hold true in
that the entropy of the universe is always increasing.
let's check comprehension
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