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Entropy

Mr. Shields Regents Chemistry U13 L08

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EntropyEntropy

One thing you need to understand is thatthe Universe tends toward disorder.

For example … If I drop a box of crayons organized by color in a boxThey don’t stay organized as they fall and hit the floor.

New born babies grow old as time passes as cellular defectsContinue to pile up. Old people do not grow young.

And chemical reactions don’t suddenly Undo what has already taken place

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-Here’s some more examples…

- If I drop a glass on the floor it shatters!

- And what if I gathered up all those pieces and re-dropped them on the floor again? Do they recombine to form the original glass?

Of course not! These are examples of the universe tending Towards disorder

EntropyEntropy

neat one day and messy the next!

Your room !

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EntropyEntropyEntropy (S) is the second thermodynamic term we’ll discuss.

This term is a measure of the DISORDER of a system

This will happen

But this won’t

Greater disorder

Less order

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Entropy of statesEntropy of states

When matter changes states Entropy either increasesOr Decreases

Consider for example the following changes of states:

ICE Water STEAM

Has Entropy increased or decreases?

Sure, entropy has increased (i.e ∆S increases, and when Entropy increases the change in entropy is positive, +∆S).

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Entropy of StatesEntropy of StatesSo we now know that when we change phase Entropy increases in this order:

Solids < Liquids < Gas

What about the entropy of a dissolved solid ??? What has Greater entropy, the solidor the dissolved solid?

The solid exists as acrystalline substance. Thesolvated ions have lessorder and thus have moreentropy

∆S

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Entropy ValuesEntropy ValuesThe larger the entropy value (i.e. the more positive) thegreater the disorder

Just as the change in Enthalpy is H = Hproduct – H reactant

The change in Entropy is S = Sproduct – Sreactant

We can however measure H directly. For instance, the heatLost or gained can be measuredin a CALORIMETER

But …

S can not be measured directly.

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Entropy ValuesEntropy ValuesWe will not focus on how S can be measured Instead we will simply focus on attributes that will enableUs to determine whether S is + or –

If S is +

- Entropy has increased- Products have more disorder- And Sproducts > Sreactants

For example: Propane (l) Propane (g)

The entropy change would be positive (+S) becauseliquid Propane is more ordered than Propane gas.

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Entropy ValuesEntropy ValuesIf S is (–)

- Entropy has decreased- Products have less disorder- And Sproducts < Sreactants

For example: Water(l) water(s)

Entropy is decreasing for this phase change since a solid is more ordered than a liquid (∆S is negative)

Another example could be …

I2(g) I2 (s) why?

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Increasing EntropyIncreasing Entropy

Summary of Positive changes in ENTROPY (+S):

- Solid liquid Liquid Gas Solid Solution Liquid Solution

- Entropy also increases if the Temp. of the system increases

Example: Hg (l, 20 deg C) Hg (l, 40 deg C)Higher ∆S … Why?

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Positive EntropyPositive Entropy

Entropy also increases if there are more molecules of product than reactant (+∆S)

examples: 2NH3(g) N2(g) + 3H2(g)

HgO Hg + O2

Problem: What’s the sign of S in the following reaction

NH3(g) + HCl(g) NH4CL (s)

Negative (-S) since order is increasing because we areGoing from more particles to fewer particles

(+∆S)

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SpontaneitySpontaneity

Since the Universe favors disorder…

In any SPONTANEOUS process, the overall

ENTROPY of the Universe will always increases (+S)

Therefore, if the S of the rxn is +

Then the process will tend to be spontaneous

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SpontaneitySpontaneity

- We’ve discussed in this unit two thermodynamic terms

- Enthalpy and Entropy; Keep in mind…

- A change in Enthalpy (Hrxn) can be (+) or (-)

- If it is (-) the reaction is exothermic- Exothermic reactions tend to be spontaneous

- A change in Entropy (Srxn) can be (+) or (-)

- If Srxn is (+) the rxn tends towards greater disorder- Increasing disorder favors rxn spontaneity

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Predicting SpontaneityPredicting Spontaneity

So how can we predict if a reaction is spontaneous or not?

??(+)(+)favorablefavorable

(+)(+)unfavorableunfavorable

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??(-)(-)unfavorableunfavorable

(-)(-)favorablefavorable

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NONO(-)(-)unfavorableunfavorable

(+)(+)unfavorableunfavorable

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YESYES(+)(+)favorablefavorable

(-)(-)favorablefavorable

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Spontaneous Spontaneous Rxn?Rxn?

SSRxnRxn

HHRxnRxn

CaseCase

Know case 1 & 2

Case 1: Spontaneous reaction

Case 2: Non-spontaneous reaction

Case 3: ? Depends upon the valueof S (is it smaller than H)

Case 4: ? Depends upon the valueof H (is it smaller than S)

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PROBLEM: What is the sign of ∆H and ∆S for each reaction and Predict if the rxn is Spontaneous, notspontaneous or not predictable:

a) C6H12O6(s) + 6O2(g) 6CO2(g)+ 6H2O(g) + Heat

b) 2CO(g) + O2(g) CO2(g) + Heat

c) 2C(s) + 2H2(g) + heat C2H4(g)

a) -∆H, +∆S, Spontaneousb)-∆H, -∆S, Not Predictablec) +∆H, -∆S, Not Spontaneous