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Entropy and Spontaneous Processes in Chemistry

SECTION 16 - Spontaneity Entropy and Free Energy


What is "entropy" ?
What is "spontaneity" ?
What the heck is a "spontaneous process" ?

All important questions that we're going to answer here today.

But before we do, it's helpful to first go (back) over the 1st Law of Thermodynamics...

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First Law of Thermodynamics

First Law of Thermodynamics  =  tells us that energy can neither be created nor destroyed.

In other words, the total energy of the universe is constant.

Although the total energy remains constant, it can be interchanged between various forms (potential, kinetic).

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ex:  Raising a book (potential energy) and then dropping the book (kinetic energy).

The same quantity of energy exists before and after.

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ex:  CH4(g)  +  2O2(g)  ➞  CO2(g)  +  2H2O(g)  + Energy

➞  This is a combustion reaction that releases energy in the form of heat.

➞  The energy flow results from the lowering of potential energy in the bonds of the reactants.

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NOTE:   The First Law of Thermodynamics tells us nothing about whether a process is likely to occur or not (spontaneity).

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Spontaneous Processes and Entropy

A process (reaction) is spontaneous if it occurs naturally, all on its own, without outside help or intervention.

Spontaneous processes may be fast or slow.

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A Few Spontaneous Process Examples:

  • rusting of steel
  • diamond 🡢 graphite
  • heat flows from hot objects to cold objects
  • ice melting above 0°C

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What Makes a Process Spontaneous?

Each spontaneous process occurs with an increase in entropy (S).

Entropy (S) - a measure of the randomness or disorder of a process.

The natural progression of things is from order to disorder, from lower to higher entropy.

In general, entropy increases in going from a solid, to a liquid, to a gas:

Entropy of Solids, Liquids, and Gases
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ex:  Determine which sample of molecules has the higher entropy in each pair (i.e. more disorder).

a.  CO2(s)  vs.  CO2(g)

b.  N2(g) at 1atm  vs.  N2(g) at 0.01atm
_________
answer:

➞  part (a) answerCO2(g)

➞  part (b) answer:  N2(g) at 0.01atm

Lower pressure corresponds to a larger volume (PV = k), which means N2(g) has "more space" to move around in.

All of this -- causes more disorder (higher entropy).

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ex:  Predict the sign of the entropy change (ΔS) when solid sugar is added to water to form a solution.

Predict the Sign of the Entropy Change
_________
answer:

➞  Sugar molecules are getting dispersed into a larger medium and a larger volume.

➞  Disorder increases, so ΔS > 0

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ex:  Predict the sign of the entropy change (ΔS) when iodine vapor condenses on a cold surface.
_________
answer:

➞  iodine vapor is going directly from a gas to a solid (often called "deposition").

➞  So, gas 🡢 solid.  Disorder decreases, so  ΔS < 0

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In the next video blog entry covering SECTION 16 - Spontaneity, Entropy, and Free Energy,

We'll discuss the Change in Entropy and Gibbs Free Energy.