Entropy and Spontaneity

Tell me about Order and Disorder in the universe

• energy and matter disperse

  • the universe becomes more disordered over time

• natural process without the need to add work

• can reverse the process at the expense of energy

• we can express the degree of disorder of a system as entropy (S)

Tell me about Entropy (S)

the distribution/ dispersal of total available energy/ matter among particles in a system

• Ordered states with small energy distribution/localization have low entropy, and disordered states with more energy distribution have higher entropy

What is the 2nd Law of Thermodynamics in layman’s terms?

As time passes, matter and energy become more disordered so entropy of universe increases

increasing # of particles…

increases opportunity to create disorder

solid → liquid (∆S=?)

∆S=+

solid → gas (∆S=?)

∆S=+

liquid → gas (∆S=?)

∆S=+

liquid → solid (∆S=?)

∆S=-

gas → liquid (∆S=?)

∆S=-

gas → solid (∆S=?)

∆S=-

What is Absolute Entropy?

• entropy values tabulated under standard conditions (in data booklet)

• (S⁰)

What do you notice about the table?

• Entropy increased in gas compared to solids

• All states/commands have positive values

• perfectly ordered system would have a value of zero

What is the Equation for the Entropy of a Reaction?

∆S°_rxn = ∑S_(products)-∑S_(reactants)

What is the Equation for Total Entropy?

∆S°_total = ∑S_(system)-∑S_{(surroundings)}

What is important to remember in terms of Entropy of 0? How do you find the entropy of the surroundings? What are the units of entropy?

• you need to still consider the entropy of surroundings

• J K⁻¹ mol⁻¹

What are the conditions for a spontaneous reaction?

∆S_total = ∆S_system - ∆S_surroundings > 0

∆S_total = ∆S_system - ₍∆H ÷ T) > 0

*important to consider system and surroundings for both enthalpy and entropy when determining feasibility of a reaction

Define Gibb’s Free Energy. What should we know about it? What is its equation

• gives a measure of the quality of energy available to do useful work

• factors in entropy and enthalpy of a system to help predict spontaneity

• denoted by ∆G

• must be negative to be spontaneous

• △G_(system) = △H_(system) - T△S_(system)

For Gibb’s Free Energy, if the temp. is low then… and what does it mean?

△G_(system) = △H_(system)

exothermic reactions can occur at low temperatures

For Gibb’s Free Energy, if the temp. is high then… and what does it mean?

△G_(system) = -T△S_(system)

at high temps., even if the reaction is endothermic, it can still be spontaneous

If a ∆H is (-) and ∆S is (+), what does that mean for ∆G and Spontaneity?

reaction is spontaneous at all temps

If a ∆H is (+) and ∆S is (-), what does that mean for ∆G and Spontaneity?

reaction is never spontaneous

If a ∆H is (-) and ∆S is (-), what does that mean for ∆G and Spontaneity?

at low temps, reaction is spontaneous

at high temps, reaction is not spontaneous

If a ∆H is (+) and ∆S is (+), what does that mean for ∆G and Spontaneity?

at low temps, reaction is not spontaneous

at high temps, reaction is spontaneous

What is the relationship between total entropy change and spontaneity?