Entropy and the 2nd Law of Thermodynamics

1. Entropy and the 2nd Law of Thermodynamics

2. Thermodynamic laws • 1st Law – Energy is always conserved. It cannot be created or destroyed • 2nd Law – The Entropy, (randomness or disorder), of the universe is always increasing. • 3rd Law – States that the entropy of a pure crystal at 0 Kelvin is zero.

3. Entropy • Defined as randomness or disorder. Units are J/K mole • The more disorganization in the molecules or atoms means higher entropy. • The entropy of a gas is much greater than a solid or liquid • There is a natural tendency to increasing entropy.

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

5. Entropy • The total entropy of the universe can be described as the sum or the entropies of a system and its surroundings. ∆Suniverse = ∆Ssystem + ∆Ssurroundings

6. ∆Ssystem depends on positional entropy, for example, how many gas molecules form in a reaction. More gas molecules is more entropy. • ∆Ssurroundings depends on heat. Is the process endothermic or exothermic. • Exothermic process – heat is added to the surroundings so ∆Ssurroundings is (+) • The magnitude of ∆S depends on the amount of heat that flows into the surroundings. • Endothermic process – heat is take from the surroundings, so ∆Ssurroundings is (-) negative. • Again, the magnitude of ∆S depends on the amount of heat that flows away from the surroundings.

7. ∆Ssurroundings = -∆H T • The negative sign in front of ∆H is included because ∆H is determined from the system’s point of view, whereas ∆S is determined from the surroundings point of view. • Reactions with a ∆Suniverse that have a (+) value , or where entropy is increasing will be considered spontaneous. This means that will naturally proceed to make the products

9. #1 For CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) • What will the ∆Suniv be for the above reaction and will it be spontaneous given: • ∆S­sys = -38.5 J/K mole • ∆H = -802.3 kJ/mole at 298K • ∆Suniv = ∆Ssys + ∆Ssurr ∆Suniv = ∆Ssys + -(∆H) T = (-38.5 J/Kmol)+ -(-802,300 J/mol) • 298K • = (-38.5) + (2692) = + 2654 J/Kmol • It is spontaneous because ∆Suniverse is increasing (+).

10. 2) Is entropy increasing of decreasing in the following reactions? a. 2KClO3(s)  2KCl(s) + 3O2(g) Increasing – gas is formed and gases have higher entropy (more disorder) b. 2Ag(s) + Cl2(g)  2AgCl(s) • Decreasing – product is all solid – and solids are more ordered and organized.

11. c. 2C8H18(l) + 25O2(g)  16CO2 (g) + 18H2O(g) • Increasing – product side has more gas molecules (25 vs. 34 moles) • **More complex molecules will have more entropy. Aqueous solutions with more ions also have more entropy

12. Calculating Entropy from a Reaction. • We can calculate entropy like we calculate the enthalpy of a reaction, using the standard entropies from the table in the appendix • ∆S = ∑ S products - ∑ S reactants