Entropy is the disorder or chaos associated with a system’s inability to convert thermal energy into mechanical work.
Concept #1: The 2nd and 3rd Laws of Thermodynamics
The 2nd Law of Thermodynamics states systems (chemical reactions) spontaneously move to state of disorder.
Concept #2: Entropy & Phase Changes
During a phase change as our molecules grow farther apart then entropy will increase.
During a phase change as our molecules grow closer together then entropy will decrease.
Example #1: Which should have the highest molar entropy at 25oC?
Example #2: Which substance has greater molar entropy.
When comparing the entropy of different compounds then we must follow a set of guidelines in the following order.
Example #3: Arrange the following substances in the order of increasing entropy at 25oC.
Example #4: Containers A and B have two different gases that are allowed to enter Container C. Based on the image of Container C, what is the sign of entropy, ΔS°.
Practice: An ideal gas is allowed to expand at constant temperature. What are the signs of ∆H, ∆S & ∆G.
Whenever a phase change occurs, first determine if bonds are broken or formed to figure out the signs of enthalpy and entropy. Afterwards, determine if the reaction is spontaneous to determine the sign of Gibbs Free energy.
Example #5: Consider the spontaneous fusion of ice at room temperature. For this process what are the signs for ΔH, ΔS and ΔG?
Practice: Consider the freezing of liquid water at 30°C. For this process what are the signs for ∆H, ∆S, and ∆G?
If bonds are broken then the entropy of a reaction increases, but if bonds are formed then the entropy of a reaction decreases.
Practice: Predict the sign of ∆S in the system for each of the following processes:
a) Ag+ (aq) + Br - (aq) → AgBr (s)
b) CI2 (g) → 2 CI - (g)
c) CaCO3 (s) → CaO (s) + CO2 (g)
d) Pb (s) at 50°C → Pb (s) at 70°C
Practice: For each of the following reactions state the signs of ∆H (enthalpy) and ∆S (entropy):
a) Fusion of ice.
b) Sublimation of CO2
c) Vaporization of aqueous water.
d) Deposition of chlorine gas.
e) Condensation of water vapor.
The 2nd Law of Thermodynamics states that the entropy of the universe is always increasing and so it must always be greater than zero.
Concept #3: The Universe & Entropy
The entropy of the universe takes the look at the entropy of our system (the chemical reaction) and of the universe.
Concept #4: Total Entropy & Spontaneity
If the entropy is greater than zero then we classify the process as spontaneous.
Concept #5: The Entropy of a Reaction
The entropy, enthalpy and Gibbs Free energy of a reaction is equal to products minus reactants.
Example #6: The oxidation of iron metal is given by the following reaction:
Practice: Diethyl ether (C4H10O2, MW = 90.1 g/mol) has a boiling point of 35.6oC and heat of vaporization of 26.7 kJ/mol. What is the change in entropy (in kJ/K) when 3.2 g of diethyl ether at 35.6oC vaporizes at its boiling point?
Concept #6: Trouton's Rule & Boltzmann's Equation