5.03 The Second Law of Thermodynamics

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From the course by University of Kentucky

Advanced Chemistry

405 ratings

University of Kentucky

Advanced Chemistry

405 ratings

A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.

From the lesson

Thermodynamics

The overarching theme of thermodynamics is the prediction of whether a reaction will occur spontaneously under a certain set of conditions. Entropy and Free Energy are defined and utilized for this purpose.

Meet the Instructors

Dr. Allison Soult
Lecturer
Chemistry
Dr. Kim Woodrum
Sr. Lecturer
Chemistry

In this module we're going to talk about the second law of thermodynamics

Our objective at this module is to understand the 2nd law

and to see how it applies to chemical systems.

One of the things we look at to determine spontaneity, is a measure of the disorder in the system.

here are some examples of some spontaneous endothermic processes.

This means that our enthalpy values are greater than 0

and we're having to put energy and to make them happen. But this can still be a

spontaneous process.

Above your degree Celsius ice melts.

To go from the solid to the liquid phase I have to put energy in.

However also see an increase in the disorder

those molecules as I melt the ice. Evaporation up liquid water to gas-phase water

is a spontaneous but endothermic process. As is the dissolution of sodium chloride

or other ionics salt in water. When we dissolve those salts

we're spreading those ions around. There are more ways for them to be arranged

we're increasing the disorder

therefore we're increasing the entropy of the system.

If I go from the liquid to the gas phase

that is also a spontaneous endothermic process. Because I'm taking water molecules that have some order

about them. albeit less than a solid, but I'm taking them to the gas phase where there is

virtually no order.

There are more ways for those molecules to be arranged therefore I'm increasing

the entropy of the system.

As the KBr dissolves in water the ions spread out

increasing their disorder and increasing their entropy. There are more ways for

those ions to be arranged.

So let's backtrack a little bit and talk about the first law of thermodynamics

which says energy cannot be created or destroyed. We know from this that the

total energy at the university is

constant. Now we can transfer energy and converted from one type to the other

but ultimately the sum of all those energies must be equal to the same constant value.

The second law of thermodynamics is a little bit different

instead of talking about energy we're talking about spontaneity,

Specifically talking about entropy and what we know is that for any spontaneous process.

The intro bf the universe increases. Processes that happen spontaneously

are those that increase the entropy of the universe. So although parts of a

process may not be spontaneous.

When we look at the entire system and its surroundings

what we find is that overall there's an increase in the entropy

of the universe . And that's what makes our spontaneous reaction spontaneous.

This will depend heavily on the temperature and as we get later in this

unit and talk about Gibbs free energy will be able to quantify that.

Next we're going to a more in-depth about what entropy really is.

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