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.Machine generated alternative text: 5. + OW(aq) * H20(/) A student is asked to determine the molar enthalpy of neutralization, AH lit, for the reaction represented above. The student combines equal volumes of 1.0 M HCI and 1.0 M NaOH in an open polystyrene cup calorimeter. The heat released by the reaction is determined by using the equation q = mcAT. Assume the following. • Both solutions are at the same temperature before they are combined. The densities of all the solutions are the same as that of water. • Any heat lost to the calorimeter orto the air is negligible. The specific heat capacity of the combined solutions is the same as that of water. (a) Give appropriate units for each of the terms in the equation q = mcAT. (b) List the measurements that must be made in order to obtain the value of q . (c) Explain how to calculate each of the following. (i) The number of moles of water formed during the experiment (ii) The value of the molar enthalpy of neutralization, AH ut, for the reaction between HCl(aq) and NaOH(aq) (d) The student repeats the experiment with the same equal volumes as before, but this time uses 2.0 M HCI and 2.0M NaOH. (i) Indicate whether the value of q increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (ii) Indicate whether the value of the molar enthalpy of neutralization, AHneut , increases, decreases, or stays the same when compared to the first experiment. Justify your prediction. (e) Suppose that a significant amount of heat were lost to the air during the experiment. What effect would this have on the calculated value of the molar enthalpy of neutralization, AH ? Justify your answer.

You are doing amazing!  Congrats :)

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