5.02 Spontaneous vs. Nonspontaneous

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Из курса от партнера University of Kentucky

Advanced Chemistry

415 оценок

University of Kentucky

Advanced Chemistry

415 оценок

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.

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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.

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

In this module we are going to look at the difference between spontaneous

and non spontaneous processes. Our objective is to understand the definitions about spontaneous

And non spontaneous and to be able to predict the spontaneity of simple processes.

Some topics that we previously

learned they were going to bring back into the thermodynamics chapter are kinetics,

talking about how fast or slow a reaction goes. Equilibrium

how far the reaction proceeds for the formation a products and enthalpy,

the measure up the change in heat or energy

that our system has. It is important to understand

the relationships between these items and thermo dynamic values

but it's also too important to understand when they are not related.

A common misconception is that spontaneity and speed are related

however this is not true. Spontaneity tells us

whether reaction will happen or not with outside intervention.

Speed tells us how fast or slow

a reaction will actually happen. These two things are not related to one another.

We can have a spontaneous reaction that happens very quickly

or spontaneous reaction that happens very slowly.

Likewise for non spontaneous process.

So the question we asked in the previous module was

will enthalpy predict dick spontaneity? To determine this

we have to answer the questions: Are all exothermic reactions spontaneous?

The answer is no. There are exothermic reactions that are non spontaneous.

The next question is are all endothermic reactions not spontaneous?

This is also no there are endothermic reactions that are spontaneous.

Therefore enthalpy alone does not predict spontaneity.

We need some other value some other information that will allow us to

predict the spontaneity of a particular reaction.

So what do we mean by spontaneous?

This means the reaction happens alone with outside intervention.

If I take a car and leave it outside for many years I'm going to see that the car is going to rust.

This is a spontaneous process, it may not happen very quickly

but it will happen. It is spontaneous. The reverse of this reaction is not spontaneous.

A car will not go from being rusty to being in perfect condition, but it will happen. It is spontaneous. The reverse of this reaction is not spontaneous.

A car will not go from being rusty to being in perfect condition,

That is a non spontaneous process. We can look at other examples by looking for

We can look at other examples, by looking for example, at the ball at the top of the slide.

If I like your best it going to roll down the slide. This is a spontaneous process.

No outside intervention is needed however If I like your best it going to roll down the slide. This is a spontaneous process.

No outside intervention is needed however

if I wanna go from the bottom of the slide to the top of the slide.

That will be an on spontaneous process. Some outside intervention will have to

happen in order to get the ball from the bottom

up to the top at the slide. The driving force in this process is entropy.

Entropy is a measure up the disorder

or we could you think about it as a number of ways

to arrange particles. If I look at the potential energy

of the ball at the top at the slide. We see the potential energy is stored there

based on the position.

If I roll the ball down the slide my potential energy is converted to kinetic energy.

I lose a little bit through heat from friction and as a result I'm dispersing that energy.

Therefore it is entropy that drives this reaction

for the ball going at the top of the slide to the bottom.

If you are lifting a block we see that it has potential energy based only on its position.

If I were to let go of the rope or cut the rope, or cut the rope. If you are lifting a block we see that it has potential energy based only on its position.

If I were to let go of the rope or cut the rope, or cut the rope.

What we're going to see is that the block will fall, that's the direction

of spontaneous change. Because I'm dispersing energy

As that potential energy is converted to kinetic energy

and heat through any friction that may occur. A similar argument can be made

for a chemical compound it has chemical potential.

The bonds that are holding the atoms together have potential energy stored in them.

As we had this KBr salt to water what we see is that there's a spontaneous change.

We go from the molecule from the ions being attracted to one another in this compound.

To having I am separated by one another there are more ways to arrange the ions

in the aqueous solution then there are and a solid crystal.

Therefore we're increasing the entropy where increasing the disorder

and that is the direction of spontaneous change going from the ionic salt

to the aqueous dissolved ions.

So as the commercial goes diamonds are forever

but are they really? Well not exactly.

The process to convert carbon in the diamond form to a carbon in the graphite form

is a spontaneous process it will happen with outside intervention.

Thankfully for the jewelry industry that is it happens at a very slow rate.

Such that, that ring passed down from generation to still going to be

around for several more generations.

We are not going to notice an appreciable change in a diamond over our lifetime.

When we look at reversibility what we see is that spontaneous processes

are irreversible. It only going to proceed in one direction.

The direction in which it is spontaneous. However,

if a process is spontaneous in one direction. It must be non spontaneous in the opposite direction.

This is similar to what we look at for enthalpy. If we had a possible value of

enthalpy in the forward direction.

Then we knew that the enthalpy in the reverse direction

would be negative or less than 0.

We looked at many reversible processes when we talked about the chapters on equilibrium.

A reversible process will proceed back and forth so from the forward

a reversible process.

We've looked at many reversible processes when we talked about equilibrium.

A reversible process will perceived back and forth so in the forward direction and in the reverse rate.

And reversible process

is at equilibrium. Let's look at an example.

Given the reaction A in equilibrium with B which one of the following is true?

A is the only possible answer. If the forward reaction a spontaneous the reverse reaction is

non spontaneous. If we look at B A is the only possible answer. If the forward reaction a spontaneous the reverse reaction is

non spontaneous. If we look at B

it says the forward a spontaneous and the reverse is spontaneous.

This can't be true because if a reaction a is spontaneous than one direction it says the forward a spontaneous and the reverse is spontaneous.

This can't be true because if a reaction a is spontaneous than one direction

it must be non-spontaneous in the reverse direction. If the forward

reaction is spontaneous

the spontaneity of the reverse reaction is unknown this is also untrue.

We know that if a reaction a spontaneous in the forward direction

it must be non spontaneous in the reverse direction.

Next we'll look at the second law of thermodynamics.

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