Entropy of Phases and Physical Changes

Entropy of Physical Changes

Comparing the freedom of the particles​

Multiple Choice

True or False?

The particles of a liquid have more freedom of movement than the particles of a solid.

• True

• False

Particle Motion of the 3 States of Matter

• Solids can only vibrate. They cannot flow. They hold their shape

• Liquids can flow, but are still tightly packed.

• Gases flow and have lots of freedom of motion. Fills the entire container and feels little attraction to other particles.

​FYI: Terms "state" and "phase" are often used interchangeably

Entropy, S

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The disorder, (randomness), of a system.

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In chemistry, the amount of entropy is based on particle freedom.

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IMPORTANT COMPARISONS

Solids < Liquids < Gases

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Solids < Aqueous (dissolved)

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Tie breaker can be # of moles if matter being compared is in same state​

Multiple Choice

Which of the listed choices has the greatest (highest) entropy?

• solid sodium

• liquid mercury

• liquid bromine

• gaseous fluorine

Entropy Comparison question for formulas

• Answer is choice 4

• A sample of gas will have more randomness to its particle arrangement than liquid or solid.

• What if question asked which had the least entropy?

• Then, it would have been choice 1, the solid

Phase Changes

• Since the states have different entropy levels, changes from one state to another either increases entropy or decreases entropy.

• When heat is absorbed (ex: melting), entropy increases

• When heat is released (ex: condensing), entropy decreases

Multiple Choice

Which of the listed phase changes results in an increase in entropy? (ΔS= +)

• precipitation of iodine

• condensation of methane

• vaporization of bromine

• freezing of water

Phase Change Question

• Answer Choice 3 (vaporization)

• It is the only phase change in which the arrangement of particles becomes less ordered

• ​

• (i.e. more random, more dispersed)

• (All other changes result in greater order, thus less entropy)

When a substance dissolves, the entropy of the system increases

Examples:

Dissolving salt in water

​ NaCl (s) --> Na⁺ (aq) + Cl^- (aq)

or

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dissolving solid sugar in water

​ C₆H₁₂O₆ (s) --> C₆H₁₂O₆ (aq)

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aqueous > solid

in terms of entropy​

Systems in nature tend toward lower energy and higher entropy

(It takes work to undo chaos)

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Changes (chemical and physical) that result in greater entropy tend to be spontaneous.

Greater freedom of particle motion = greater entropy

Multiple Choice

Exit Question 1:

Which sample of water has the greatest entropy?

• H₂O _(g)

• H₂O _(s)

• H₂O _(l)

Multiple Choice

Which phase change involves a decrease in entropy?

• melting

  (solid to liquid)

• vaporizing

  (liquid to gas)

• freezing

  (liquid to solid)

• sublimation

  (solid to gas)