# 4.04 – Entropy

Entropy refers to the order within a system.  As the universe tends towards chaos, we see positive entropy changes as typically being spontaneous in nature.  This topic will help us to analyze a situation to determine whether entropy increases or decreases based upon states of matter and the stoichiometry of the reaction.

Lesson Objectives

• Explain the meaning of spontaneous process, reversible process, irreversible process, and isothermal process. (19.1)
• Define entropy and state the second law of thermodynamics. (19.2)
• Explain how the entropy of a system is related to the number of possible microstates. (19.3)
• Describe the kinds of molecular motion that a molecule can possess. (19.3)
• Predict the sign of ΔS for physical and chemical processes. (19.3)
• State the third law of thermodynamics.(19.3)
• Calculate standard entropy changes for a system from standard molar entropies. (19.4)
• Calculate entropy changes in the surroundings for isothermal processes. (19.4)

In addition to the class discussion, you should familiarize yourself with the key ideas of this topic.  These slides relate to sections 19.1-19.4 of your textbook.

Now that you are familiar with these concepts, you should put them into practice.

When you have finished working out the practice problems, take a picture of your work and add it to your OneNote under section 4.

Complete the problems below, then take a picture of your work and post it in OneNote.  Be prepared to present these problems in class.

Mastery Problems

19.41  Predict the sign of the entropy change of the system for each of the following reactions:

• 2 SO2(g) + O2(g) → 2 SO3(g)
• Ba(OH)2(s) → BaO(s) + H2O(g)
• CO(g) + H2(g) → CH3OH(l)
• FeCl2(s) + H2(g) → Fe(s) + 2 HCl(g)

19.42  Predict the sign of ΔSsys for each of the following processes:

• Gaseous Ar is liquefied at 80K
• Gaseous N2O4 dissociates to form gaseous NO2
• Solid potassium reacts with gaseous O2 to form solid potassium superoxide, KO2
• Lead bromide precipitates upon mixing Pb(NO3)2(aq) and KBr(aq)

19.49  Using S° value from Appendix C, calculate ΔS° values for the following reactions.  In each case, account for the sign of ΔS°.

• C2H4(g) + H2(g) → C2H6(g)
• N2O4(g) → 2 NO2(g)
• Be(OH)2(s) → BeO(s) + H2O(g)
• 2 CH3OH(g) + 3 O2(g) → 2 CO2(g) + 4 H2O(g)

19.50  Calculate ΔS° values for the following reactions by using tabulated S° values from Appendix C.  In each case, explain the sign of ΔS°.

• N2H4(g) + H2(g) → 2 NH3(g)
• K(s) + O2(g) → KO2(s)
• Mg(OH)2(s) + 2 HCl(g) → MgCl2(s) + 2 H2O(l)
• CO(g) + 2 H2(g) → CH3OH(g)

2002 FRQ #8

Answers to these mastery problems can be found in the Content Library within OneNote.

You are doing amazing!  Congrats :)