Thermodynamics: Entropy

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1 Name: Band: Date: Thermodynamics: Entropy Big Idea: Entropy When we were studying enthalpy, we made a generalization: most spontaneous processes are exothermic. This is a decent assumption to make because exothermic processes result in products that are lower in energy and therefore more stable than the reactants and that s a good thing! So how is it possible that some endothermic processes are spontaneous? For example, consider ice. The melting of ice is endothermic, but ice will melt spontaneously above 0 C. What gives? Clearly, enthalpy must not be the only factor to consider when determining the spontaneity of a process. Enter entropy, S. Entropy is more complicated than most students realize. You ve probably heard that entropy is a measure of disorder, which isn t wrong, but it isn t perfectly accurate, either. Entropy measures not only extent of disorder of matter (atoms and molecules) but also the dispersal of energy throughout those atoms and molecules. What we discover is that if energy and matter are both dispersed during a process, it is spontaneous. In other words, if entropy increases (ΔS is positive) then the process is spontaneous. For a deeper, more complex explanation, come chat with me or visit Kotz 19.3! Ultimately, though, you can think of entropy as disorder. More entropy, means more disorder. That ll at least get you through the AP exam. 1. Consider enthalpy. a. What is the definition of enthalpy? b. What letter is used to symbolize enthalpy? c. What type of enthalpy change positive or negative is usually associated with spontaneous reactions? 2. Consider entropy. a. What is the definition of entropy? b. What letter is used to symbolize entropy? c. What type of entropy change positive or negative is usually associated with spontaneous reactions?

2 Big Idea: Entropy Values To get a sense of entropy values, consider a perfect crystal at 0 K. At 0 K, also known as absolute zero, none of the atoms are moving not even vibrating and all atoms are in an exact lattice structure. The entropy of this crystal is S = 0. To determine the entropy of an element or compound above absolute zero, just measure the heat required to raise the temperature of the substance from 0 K. Use the equation: S = q rev T The term q rev refers to the heat (q) absorbed by the substance as heat is added slowly and in very small increments, which approximates a reversible (rev) process. Notice that all entropies are positive; negative values of entropy cannot occur. The units of entropy are the units of q (J/mol) divided by the units of T (K): S will be in J/mol K. 3. Define absolute zero. 4. What is the entropy of any substance at 0 K? 5. What is the relationship between the temperature of a substance and the substance s entropy? Big Idea: Standard Entropy The standard entropy, S, of a substance is the entropy gained by converting the substance from a perfect crystal at 0 K to a standard state condition. Remember: standard state is the state of a substance at 1 atm and a specified temperature. A list of standard entropies is given below.

3 6. What does the degree symbol ( ) mean when placed enthalpy or entropy symbols? 7. When comparing the same or similar substances, are the entropies of gases greater than, equal to, or less than those for liquids? What about liquids vs. gases? 8. As a general rule, larger molecules have greater entropy than smaller molecules, and more complex structures have greater entropies than simpler molecules. This is because there are more ways for large and complex molecules to twist, rotate, and vibrate, which means there are more ways for energy to be distributed. Use specific examples from the table to support this generalization. 9. Which substance is expected to have higher entropy under standard conditions: NO 2 (g) or N 2 O 4 (g)? Explain. Big Idea: Entropy Changes for Processes When looking at the change in entropy over the course of a chemical or physical change, you need to compare the entropies of the reactants and the entropies of the products. If the products are more disordered, then the change in entropy, ΔS, is positive. If the products are less disordered, then the change in entropy, ΔS, is negative. Notice that delta S can be negative, but S for a substance cannot be negative. To calculate ΔS, you can use a modified version of the equation in the previous section. Notice how q and ΔH both refer to heat content and are therefore basically interchangeable. For processes, we generally measure enthalpy changes, so ΔH is used. ΔS = -ΔH 10. For each process described below, indicate if entropy increases (ΔS is positive) or decreases (ΔS is negative). Think qualitatively, not quantitatively, for this group. a. Iodine vapor condenses on a cold surface to form crystals. b. The volume of a container containing a gas is increased. T c. In a reaction, four moles of gaseous reactants are converted to two moles of gaseous products. d. Solid sugar is added to water to form a solution.

4 11. Iron is used to recover antimony from sulfide ores according to the following equation: Sb 2 S 3 (s) + 3Fe(s) 2Sb(s) + 3FeS(s) ΔH = -125 kj Calculate the change in entropy, in J/K for this recovery at 25 C and 1 atm. Big Idea: Calculating Entropy Changes The equation for calculating the change in entropy of a process or reaction is the same equation used for calculating changes in enthalpy: ΔS rxn = Σ[S (products)] Σ[S (reactants)] Remember to reflect stoichiometry in your calculations! 12. Calculate the ΔS rxn for the oxidation of nitrogen monoxide: 2NO(g) + O 2 (g) 2NO 2 (g) The S of NO is J/mol K. The S of NO 2 is J/K mol. Use the table from a previous section for additional standard molar entropy values. Big Idea: Entropy, Enthalpy, and Spontaneity So far, we ve concluded that processes that are exothermic and result in increased entropy are spontaneous, but we also know that there are exceptions to these rules. The table below summarizes all possibilities of enthalpy and entropy for different processes and how these different conditions affect spontaneity. YOU MUST KNOW THIS FOR THE AP EXAM! EASY MULTIPLE CHOICE POINTS! ΔH ΔS Spontaneous? ΔH < 0 (exothermic) ΔS > 0 (more disorder) Always! ΔH < 0 (exothermic) ΔS < 0 (less disorder) Only at low temperatures ΔH > 0 (endothermic) ΔS > 0 (more disorder) Only at high temperatures ΔH > 0 (endothermic) ΔS < 0 (less disorder) Never!

5 13. For each of the following reactions, indicate if the reaction is spontaneous. If spontaneity is conditional, indicate the temperature conditions necessary for spontaneity. a. b. c. d.