Entropy, continued UNIT 4 Day 7 Demonstration Stretched vs. - PowerPoint PPT Presentation

Sparks CH301 The 2 nd Law of Thermodynamics Entropy, continued UNIT 4 Day 7

Demonstration Stretched vs. Relaxed Rubber Bands

POLL: iClicker Question For the process of releasing a stretched rubber band to a relaxed rubber band, ∆ S sys is A.> 0 B. = 0 C. < 0 D.No way to know

Entropy What affects the entropy of the system? Volume Change Mixing Phase Change Temperature Change Chemistry

Entropy and Temperature

Entropy and Temperature

Entropy for a Phase Change During a phase change, the heat is the reversible heat and the temperature is constant

Entropy for a Chemical Change During a chemical change, the heat is NOT the reversible heat and the temperature is constant Reactants Products We need to be able to find the entropy of a substance

The Third Law of Thermodynamics The entropies of all perfect crystals approach zero when the absolute temperature approaches zero. Now that we’ve defined zero entropy, we can establish other entropy values using it as a basis. We can use entropy change from temperature change and phase change to find the entropy of any substance at any temperature

Zero K Above zero K; below MP

Poll: Clicker Question • Does S° for an element in its standard state at 1 atm and 298K have a value of zero? A. Yes B. No 11

Standard Molar Entropies (Absolute Entropies) • The standard molar entropies (S ⁰ ) increase as the complexity of a substance increases. 12

Absolute Entropies NOTE: This is not the same as enthalpy of formation. All elements in their standard states have values 2H 2 (g) + O 2 (g) 2H 2 O(l)

Entropy What affects the entropy of the system? Volume Change - only qualitative Mixing - only qualitative Phase Change Δ S = q/T What is q for a phase change? Temperature Change Chemistry

• Determine if the change of entropy is negative or positive without calculating the change in entropy 1. H 2 O (s)  H 2 O (l) 2. H 2 O (g)  H 2 O (l) 3. PCl 3 (g) + Cl 2 (g)  PCl 5 (g) 4. 2 N 2 O 5 (g)  4 NO 2 (g) + O 2 (g) 5. MgSO 4 • 7H 2 O (s)  MgSO 4 (s) + 7 H 2 O (g) 6. HCl (g) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) 15

Calculate ΔS universe after the completion of the following reaction: 2NiS(s) + 3O 2 (g) → 2SO 2 (g) + 2NiO(s) at 25 ° C ΔH = -890 kJ Substance S(J/Kmole) SO 2 248 NiO 38 O 2 205 NiS 53 16

What have we learned today? • Entropy changes for chemical and physical processes – qualitative and quantitative determinations.

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