Thermodynamics - Part - 1

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27 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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Thermodynamics

REACTION
SPONTANEITY

“stuff happens”

Stuff happens


or does it? An
important area of science is
determining: will a process
happen spontaneously (or not)
and why?

For example:


Will a car rust?

Will a house burn?

Will a chocolate melt?

Will a drug inhibit a
cancerous tumor growing in a
patient’s body?

We can start this
topic of reaction
spontaneity

by thinking about
Humpty Dumpty
.

Why does Humpty fall, once he
has slipped off the wall?

You drop an egg.

The egg falling and breaking is
an example of a



___________________________

PROCESS

This is a physical or chemical
change that occurs with no
outside


_________________________.

Can you imagine the egg
spontaneously reassembling into

an unbroken egg,

and jumping back

into your hand?

This would be an

example of a





Can non
-
spontaneous

processes happen?

YES NO

Explain.


Lighting a match
and letting it burn

is a


SPONTANEOUS


NONSPONTANEOUS



process

What would the enthalpy diagram
for this process look like?

Notes:
Is this process (with the
match) endothermic or exothermic?


Processes that are


EXOTHERMIC


ENDOTHERMIC


are more likely to be

spontaneous. Explain.

EXPLAIN:

Rusting is a


SPONTANEOUS


NONSPONTANEOUS


process. It is also


EXOTHERMIC


ENDOTHERMIC







___________________________

processes involve a substance
gaining energy and going to a
higher energy state. Does
this mean that they are
always
non
-
spontaneous?

To answer this question,
consider this:


WHY DO ICE CUBES MELT?

Draw the enthalpy diagram for
an ice cube melting.

An ice cube melting is



ENDOTHERMIC EXOTHERMIC


An ice cube melting


(at room temperature) is a



SPONTANEOUS NONSPONTANEOUS



process

If all things want

to be in their

lowest possible

energy state,

how can an

endothermic

process (like an ice

cube melting) be

spontaneous?

There must be another factor
-

in addition to a lower
energy state
-

that explains
why anything happens in the
physical universe.

The other factor is that all things
want to be:




The natural “way of the
universe” is that things are
constantly


spontaneously




You must constantly


_____________________________


into repairing things


i.e., to


maintain __________________


Think about your room at home.

ORDER REQUIRES:


Another way to state this
alternative factor: all
things
want to attain maximum



________________________


______________



is defined as
“a
measure of the

disorder or

randomness

of the particles
that make up a
system.”

This is a part of the

Second Law of Thermodynamics.


“Spontaneous processes always
proceed in such a way


Another way of saying this is:

ALL THINGS TEND TO



What does this mean in our lives?

An example: you are lying down


not thinking


not moving….

Let’s say you’re in a coma. If
they don’t feed you
intravenously, what will
happen to you?


(Do you remember the Terry
Schaivo case in 2005?)



You’ll _______________ Why?


You eat to get _______________.

=
___________

Why would you need more of this if
you are doing absolutely nothing?

You need ___________ to constantly
repair your body


to maintain its

high degree of ____________.
Living systems are systems of
extremely high ___________!

A lot of what you eat

(i.e., a lot of the energy

that you get from your

food) goes simply into

maintaining that high

degree of order


i.e.,

to constantly fix things

in your body that are

breaking down
-



TO FIGHT



_________________!

Back to the ice cube


why
does an ice cube melt


even
though it has to go to a higher
energy state to do so?

Solid water (ice) is extremely _____________________.
Models of ice crystals are shown below. When ice
melts, the water molecules become more


______________________________ This increase in
entropy makes the process happen spontaneously


even though it is endothermic.

Therefore


a reaction in
which entropy (disorder) is


______________________


is more likely to be
spontaneous.
How can you
tell if the entropy is
increasing or decreasing in a
reaction?

Any process that gives the particles
more


_____________________________ is
likely to have an increased entropy.

As a substance goes from a solid
to a liquid to a gas, entropy is


INCREASED DECREASED

(and vice versa)

Usually if you dissolve a solid or a
liquid to form a solution, entropy



INCREASES DECREASES

Dissolving a gas into a liquid

INCREASES DECREASES

the entropy of the gas.

If there are fewer moles of
gases in the products than
the reactants, the entropy
INCREASES DECREASES

(and vice versa).


Example:

CO(
g
) + 3H
2
(
g
)
→ CH
4
(
g
) + H
2
O(
g
)


Will the entropy increase or decrease?

Explain.

a.
NH
4
NO
3

(s) → NO
3
1
-

(aq) + NH
4
1+

(aq)







b.

N
2

(g) + 3H
2

(g) → 2NH
3

(g)






Will the entropy increase or decrease?

Explain.


c.

H
2
O (l) → H
2
O (s)












d. O
2

(g) → O
2

(aq)








Next class, we’ll look at
another way to state the

Laws of Thermodynamics:

You can’t win.

You can’t even break even.

You can’t get out of the game!