19.2 – 19.3 Student Notes

II. Spontaneous Process and Entropy

A. Spontaneous Process –

B. Entropy – S –

1. Properties of entropy

a.

b.

c.

2. Concept Check

You have a sample of 1.0 mg of solid iodine at room temperature. Later, you notice that the iodine has sublimed. What can you say about the change of entropy of the iodine?

3. 2^nd Law of Thermodynamics

a. Def –

b.

c. ΔS > q/T for a spontaneous process

4. Entropy and Molecular Disorder

Disorder

Coffee cup

5. Entropy Change for a Phase Transition

a. ΔS = q/T ( )

b. Calculating the Entropy Change for a Phase Transition

The heat of vaporization, ΔH_vap, of carbon tetrachloride, CCl₄, at 25° C is 39.4 kJ/mol.

CCl₄ (l) à CCl₄ (g); ΔH_vap = 43.0 kJ/mol

If 1 mol of liquid carbon tetrachloride at 25° C has an entropy of 216 J/K, what is the entropy of 1 mol of the vapor in equilibrium with the liquid at this temperature?

6. Criterion for a Spontaneous Reaction

ΔH – T ΔS is negative for the reaction, you would predict

ΔH – T ΔS is positive for the reaction, you would predict

C. Standard Entropies and the Third Law of Thermodynamics

1. Third Law of Thermodynamics

a. Def –

b. Note that as the temperature increases, the entropy slowly increases. But if there is a phase change, the entropy has a sharp increase

c. Standard entropy – absolute entropy - S° -

2. Entropy Change for a Reaction

a. Entropy usually increases in the following situations

1.

2.

3.

b. Predicting the Sign of the Entropy Change of a Reaction

What is the sign of ΔS° for the following reaction?

CO (g) + H₂O (g) à CO₂ (g) + H₂ (g)

3. Calculating ΔS° for a Reaction

a. ΔS° = Σ n S° (products) – Σ m S° (reactants)

b. Calculate the change of entropy, ΔS°, at 25° C for the reaction in which urea is formed from NH₃ and CO₂.

2 NH₃ (g) + CO₂ (g) à NH₂CONH₂ (aq) + H₂O (l)

The standard entropy of NH₂CONH₂ (aq) is 174 J/(mol · K).