Chemistry 103 Spring Announcements 1. Ch. 16 OWL homework is active. 2. Next midterm exam on May 17 or 19.

Transcription

1 Today 1. More on entropy. Announcements 1. Ch. 16 OWL homework is active. 2. Next midterm exam on May 17 or CSULA closure on May 21 (furlough). 4. CSULA closure on May 31 (holiday). 5. Bring textbook and calculator to each lecture.

2 Review entropy: a measure of the extent to which energy is dispersed throughout a system; a quantitative (numerical) measure of disorder at the nanoscale; given the symbol S. In brief, processes that increase entropy (ΔS > 0) create more disorder and are favored, while processes that decrease entropy (ΔS < 0) create more order and are not favored. Examples: 1. Things fall apart over time. (constant energy input is required to maintain things) 2. Spilling water from a glass vs. spilled water moving back into the glass. 3. Thermal energy disperses from hot objects to cold objects, not from cold to hot. 4. A gas will expand to fill its container, not concentrate itself in one part of the container. 2

3 Energy disperses (spreads out) over a larger number of particles. Ex: exothermic reaction, hot object losing thermal energy to cold object. Energy disperses over a larger space (volume) by particles moving to occupy more space. Ex: water spilling, gas expanding. Consider gas, liquid, and solid, Fig. 17.2, p

4 Example: Predict whether the entropy increases, decreases, or stays about the same for the process: 2 CO 2 (g) 2 CO(g) + O 2 (g). Practice: Predict whether ΔS > 0, ΔS < 0, or ΔS 0 for: NaCl(s) NaCl(aq) Guidelines on pp summarize some important factors when considering entropy. 4

5 Measuring and calculating entropy At absolute zero 0 K ( C), all substances have zero entropy (S = 0). At 0 K, no motion and no energy dispersal occur. Instead, a perfect crystal exists with particles locked into lattice positions (no wiggling, no vibrations) and with S = 0. This idea is known as the third law of thermodynamics. To measure entropy, measure the energy dispersed at a specific temperature. Typically, use a calorimeter (Chapter 6) to measure thermal energy transferred (q). 5

6 Mathematically, keep the temperature value constant by assuming only very small changes in the conditions of the process. ΔS = q rev / T (assumes T changes very little)* *Note: Integration (calculus) is actually used to calculate total entropy change as a function of T. q rev = thermal energy transferred for a reversible process, meaning a small change will reverse the direction of the process. Example: Measure the energy for melting ice by melting ice in a calorimeter containing water barely above 0 C. If the temperature was barely below 0 C, then the reverse process would occur (the water would freeze). 6

7 Example: P-S Example 17.1, p Calculate the entropy of any substance at any temperature by integration using ΔS = q rev / T over the range from 0 K (where S = 0) up to the desired temperature T. The result is a table of standard molar entropy (S ) values, similar to standard molar enthalpy values of formation (ΔH f ) from Chapter 6. Table 17.1, p

8 S = enthalpy value for one mole of substance at standard conditions (25 C, 1 atm). ΔH f = enthalpy value to form one mole of substance at standard conditions (25 C, 1 atm). Similar to Hess s Law calculations (Chapter 6) for enthalpy changes (ΔH rxn ), we can calculate entropy changes (ΔS rxn ) for a process. ΔS rxn = ΣnS products - ΣnS reactants Example: Calculate the entropy change for MgCO 3 (s) MgO(s) + CO 2 (g). 8

9 Practice: Calculate the entropy change for 2 CO 2 (g) 2 CO(g) + O 2 (g) NaCl(s) NaCl(aq) 9

10 Why doesn t entropy control everything? 1. NaCl(s) NaCl(aq) K sp = very large Entropy favors products. Equilibrium constant shows products favored. 2. CaCO 3 (s) CaCO 3 (aq) K sp = 8.7 x 10-9 Entropy favors products. Equilibrium constant shows reactants favored. Why the difference? Remember that physical and chemical processes also have an enthalpy change (energy change). Next time we will look at how enthalpy changes play a role in favoring products or reactants. 10

11 Announcements 1. Ch OWL homework is active. 2. Next midterm exam on May 17 or CSULA closure on May 21 (furlough). 4. CSULA closure on May 31 (holiday). 5. Bring textbook and calculator to each lecture. Extra Chapter 16 Problems: 1-9, 12-37, 40-53, 61-73, 75, 77, 83. Before next class, 1. Study Chapter Work on OWL HW. 3. Read