The Second Law of Thermodynamics

The Old Guitarist, Pablo Picasso, 1904

1^st Law of Thermodynamics

Energy cannot be created or destroyed.

Reminder

Which arrow shows what happens “spontaneously”

to a bedroom over time?

Which arrow shows what happens “spontaneously”

to a bedroom over time?

Occurs spontaneously

Requires Energy

Which of the following processes occur spontaneously?

1. a) a piece of iron rusting, or �b) a piece of rusted metal unrusting?

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• a) a heavy weight falling to the ground, or �b) a heavy weight rising from the ground?

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• a) glucose reacting with oxygen to form carbon dioxide and water, or�b) carbon dioxide reacting with water to form glucose and oxygen?

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• a) ice melting, or �b) water freezing

Tricky! Depends on the temperature!

Thermodynamic Favorability

A physical or chemical process is spontaneous (thermodynamically favorable) if it can proceed on its own without any outside energy input in a given set of conditions.

Spontaneous Process

=

Thermodynamically Favorable Process

spontaneous

above 0^oC

spontaneous

below 0^oC

Spontaneity does not refer to kinetics

�Important note: the spontaneity of a chemical reaction (thermodynamics) is not related to the speed (kinetics) of the chemical reaction.

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Example:

C(diamond) + O₂(g) 🡪 CO₂(g)

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slow (almost impossibly slow) reaction

but

thermodynamically favorable

kinetics

thermo

Determinants of Spontaneity

Two factors determine whether a reaction is spontaneous:

• Enthalpy change (∆H): most exothermic reactions are spontaneous
• Entropy change (∆S): most reactions tend to increase the overall entropy of the system

?!?

What the heck is

Entropy

Examples of increasing entropy:

• your room over time
• melting water
• dissolving salt in water
• burning coal to produce CO₂(g) + H₂O(g)
• a sample of oxygen gas mixing with a sample of neon gas

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What do these have in common?

Entropy

• Entropy is a measure of the disorder (randomness) of a system.
• (technical: a thermodynamic function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state.)
• Increasing entropy means a system has increased its disorder
• Decreasing entropy means a system has decreased its disorder (or alternatively, increased its order.)

Entropy

What factors could be changed for a system that would increase the entropy of the system?

• increase temperature (more random movement)

Entropy

What factors could be changed for a system that would increase the entropy of the system?

• mix substances (more different arrangements of the particles)

Entropy

What factors could be changed for a system that would increase the entropy of the system?

• change of state from solid to liquid or liquid to gas (more random movement, less organization)

Entropy

What factors could be changed for a system that would increase the entropy of the system?

• break bonds (more particles = more arrangements)

The 2^nd and 3^rd Laws of Thermodynamics

The laws of thermodynamics address entropy:

3^rd Law of Thermodynamics

The entropy of a perfect crystal is 0 J/K at absolute zero.

2^nd Law of Thermodynamics

For any spontaneous process, the entropy of the universe increase.

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∆S_universe > 0

(This is a big one!)

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∆S_univ = S_sys + S_surr

Hess’s Law and ∆S_sys

Remember Hess’s Law?

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∆S_sys = ∑nS_products - ∑nS_reactants

Hess’s Law and ∆S_sys

Try this:

What is ∆S^o_rxn for the reaction shown below?

2 H₂(g) + O₂(g) 🡪 2 H₂O(g)

From experimental data:

S^o_H2(g) = 130.7 J/molK

S^o_O2(g) = 205.2 J/molK

S^o_H2O(g) = 188.8 J/molK

∆S_sys = ∑nS_products - ∑nS_reactants

∆S^o_rxn = (2molx188.8 J/molK) – ((2molx130.7 J/molK) +

(1molx205.2 J/molK))

∆S^o_rxn = -89.00 J/K

Did we just break the law?

The previous example is a thermodynamically favorable reaction. But its ∆S^o_rxn was -89 J/K.

• Does this violate the 2^nd Law of Thermo?
• No! The ∆S^o_univ must increase (and it does...more below.)
• How is it thermodynamically favorable?
• It is exothermic (∆H^o_rxn > 0)! And that’s a factor, too. (This is why ∆S^o_univ increases...the energy released by the system increases the entropy of the surroundings.)