Entropy and the 2nd Law of Entropy and the 2nd Law of ThermodynamicsThermodynamics

Thermodynamic lawsThermodynamic laws

1st Law – Energy is always conserved. It 1st Law – Energy is always conserved. It cannot be created or destroyedcannot be created or destroyed

2nd Law – The Entropy, (randomness or 2nd Law – The Entropy, (randomness or disorder), of the universe is always disorder), of the universe is always increasing.increasing.

3rd Law – States that the entropy of a pure 3rd Law – States that the entropy of a pure crystal at 0 Kelvin is zero.crystal at 0 Kelvin is zero.

EntropyEntropy

Defined as randomness or disorder. Units Defined as randomness or disorder. Units are J/K moleare J/K mole

The more disorganization in the molecules The more disorganization in the molecules or atoms means higher entropy.or atoms means higher entropy.

The entropy of a gas is much greater than The entropy of a gas is much greater than a solid or liquida solid or liquid

There is a natural tendency to increasing There is a natural tendency to increasing entropy.entropy.

EntropyEntropy

So, any reaction or system in which So, any reaction or system in which disorder is increased will tend to be disorder is increased will tend to be favored.favored.Symbol for entropy is “S”, and we look at Symbol for entropy is “S”, and we look at the change in entropy which is, ∆S.the change in entropy which is, ∆S.If ∆S is (+) then entropy is increasing – If ∆S is (+) then entropy is increasing – getting more random or disorder.getting more random or disorder.If ∆S is (-) then entropy is decreasing – If ∆S is (-) then entropy is decreasing – getting more ordered, or less randomgetting more ordered, or less random

EntropyEntropy

The total entropy of the universe can be The total entropy of the universe can be described as the sum or the entropies of a described as the sum or the entropies of a system and its surroundings.system and its surroundings.

∆∆SSuniverse universe = ∆S = ∆Ssystemsystem + ∆S + ∆Ssurroundingssurroundings

∆∆SSsystemsystem depends on depends on positional entropypositional entropy, for , for

example, how many gas molecules form in a example, how many gas molecules form in a reaction. reaction. More gas molecules is more More gas molecules is more entropy.entropy.

∆ ∆SSsurroundingssurroundings depends on depends on heat.heat. Is the process Is the process

endothermic or exothermic.endothermic or exothermic.– Exothermic processExothermic process – heat is added to the – heat is added to the

surroundings so ∆Ssurroundings so ∆Ssurroundingssurroundings is (+) is (+)

– The magnitude of ∆S depends on the amount of heat The magnitude of ∆S depends on the amount of heat that flows into the surroundings.that flows into the surroundings.

– Endothermic processEndothermic process – heat is take from the – heat is take from the surroundings, so ∆Ssurroundings, so ∆Ssurroundingssurroundings is (-) negative. is (-) negative.

– Again, the magnitude of ∆S depends on the amount Again, the magnitude of ∆S depends on the amount of heat that flows away from the surroundings.of heat that flows away from the surroundings.

∆∆SSsurroundings surroundings = = -∆H-∆H

TT

The negative sign in front of ∆H is included The negative sign in front of ∆H is included because ∆H is determined from the system’s because ∆H is determined from the system’s point of view, whereas ∆S is determined from point of view, whereas ∆S is determined from the surroundings point of view.the surroundings point of view.

Reactions with a ∆Suniverse that have a (+) Reactions with a ∆Suniverse that have a (+) value , or where entropy is increasing will be value , or where entropy is increasing will be considered spontaneous. This means that will considered spontaneous. This means that will naturally proceed to make the products naturally proceed to make the products

∆∆SSsystemsystem

∆∆SSsurroundingssurroundings ∆∆SSuniverse universe Spontaneous Spontaneous

??

(+)(+) (+)(+) (+)(+) Yes, alwaysYes, always

(-)(-) (-)(-) (-)(-) No, reverse No, reverse isis

(+)(+) (-) (-) (endothermic)(endothermic)

?? Yes, at high Yes, at high temps.temps.

(-)(-) (+) (+) (exothermic)(exothermic)

?? Yes, at low Yes, at low temps.temps.

#1 For CH#1 For CH44(g) + 2O(g) + 2O22(g) (g) CO CO22(g) + 2H(g) + 2H22O(g)O(g)What will the ∆SWhat will the ∆Sunivuniv be for the above reaction and be for the above reaction and will it be spontaneous given: will it be spontaneous given: ∆∆S S syssys = -38.5 J/K mole = -38.5 J/K mole∆∆H = -802.3 kJ/mole at 298KH = -802.3 kJ/mole at 298K

∆∆SSunivuniv = ∆S = ∆Ssyssys + ∆S + ∆Ssurrsurr

∆∆SSunivuniv = ∆S = ∆Ssyssys + + -(∆H)-(∆H)

TT = (-38.5 J/Kmol)+ = (-38.5 J/Kmol)+ -(-802,300 J/mol) -(-802,300 J/mol)

298K298K = (-38.5) + (2692) = + 2654 J/Kmol= (-38.5) + (2692) = + 2654 J/Kmol

It is spontaneous because ∆Suniverse is It is spontaneous because ∆Suniverse is increasing (+).increasing (+).

2) Is entropy increasing of decreasing in the 2) Is entropy increasing of decreasing in the following reactions?following reactions?

a. 2KClOa. 2KClO33(s) (s) 2KCl(s) + 3O 2KCl(s) + 3O22(g) (g)

Increasing – gas is formed and gases have Increasing – gas is formed and gases have higher entropy (more disorder)higher entropy (more disorder)

b. 2Ag(s) + Clb. 2Ag(s) + Cl22(g) (g) 2AgCl(s) 2AgCl(s)

Decreasing – product is all solid – and Decreasing – product is all solid – and solids are more ordered and organized.solids are more ordered and organized.

c. c. 2C2C88HH1818(l) + 25O(l) + 25O22(g) (g) 16CO 16CO22 (g) + 18H (g) + 18H22O(g)O(g)

Increasing – product side has more gas Increasing – product side has more gas molecules (25 vs. 34 moles)molecules (25 vs. 34 moles)

**More complex molecules will have more **More complex molecules will have more entropy. Aqueous solutions with more entropy. Aqueous solutions with more ions also have more entropyions also have more entropy

Calculating Entropy from a Calculating Entropy from a ReactionReaction..

We can calculate entropy like we calculate We can calculate entropy like we calculate the enthalpy of a reaction, using the the enthalpy of a reaction, using the standard entropies from the table in the standard entropies from the table in the appendixappendix

∆∆S = ∑ S S = ∑ S productsproducts - ∑ S - ∑ S reactants reactants