General Chemistry I Concepts

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1 Thermodynamics

2 Thermodynamics The Three Laws of Thermodynamics (18.1) Spontaneous Processes (18.2) Entropy (18.3) The Second Law of Thermodynamics (18.4) Gibbs Free Energy (18.5) Free Energy and Chemical Equilibrium (18.6) Thermodynamics and Living Systems (18.7)

3 General Chemistry I Concepts Representations of matter ( ) Formula calculations and stoichiometry ( ) The concepts of the first law of thermodynamics and enthalpy ( )

4 18.1 The Three Laws of Thermodynamics Review: What is the first law of thermodynamics? In terms of the conservation of energy? In terms of the energy of the system and the surroundings? In terms of the energy of the universe? What does the first law tell us?

5 18.2 Spontaneous Processes What does it mean for a reaction to occur or be spontaneous? Consider: Heat flows from a hotter body to a colder body Will the opposite happen? Methane combusts in oxygen to produce carbon dioxide and water Will the opposite happen? If something is spontaneous ( happens ) does that tell us how long it will take?

6 18.2 Spontaneous Processes What is a spontaneous reaction? A reaction that does occur under the given set of conditions. What does it mean for a reaction to occur or be spontaneous? If something is spontaneous ( happens ) does that tell us how long it will take?

7 18.2 Spontaneous Processes Review: What is an exothermic process? Review: What is an endothermic process? In terms of the sign on enthalpy In terms of the energy diagram In terms of energy entering or leaving the system In terms of energy entering or leaving the surroundings

8 18.2 Spontaneous Processes Review: What is an exothermic process? Review: What is an endothermic process? Which process would be more likely to occur? What are some examples of exothermic and endothermic reactions that occur?

9 18.2 Spontaneous Processes Can we assign spontaneity based on enthalpy of reaction? If not, what else should we consider? Figure 18.1, p. 632

10 18.2 Spontaneous Processes Let s simplify this to a process for which change in enthalpy is zero

11 18.3 Entropy What is disorder? Let s consider three scenarios: a deck of cards in terms of ordered and disordered states How does going to a more disordered state explain spontaneity? flipping coins and what is the most probable outcome gas particles and the probability of the distribution between two containers What is most probable? What will happen or is spontaneous?

12 18.3 Entropy What is entropy? a measure of how spread out or dispersed the energy of a system is among the different possible ways that a system can contain energy How does entropy relate to individual configurations of disorder? What are these called? How does an increase in microstates justify an increase in entropy and a spontaneous process?

13 18.3 Entropy Figure 18.3, p. 636

14 18.3 Entropy What are standard molar entropies? How are these related for: The same substance in different states Similar substances with increasing complexity Similar substances with different masses Table 18.1, p. 637

15 18.4 The Second Law of Thermodynamics What is the second law of thermodynamics? for a spontaneous process to occur, the configuration of the universe goes from a less probable to a more probable state the entropy of the universe increases in a spontaneous process and remains constant (unchanged) in an equilibrium process What does this mean in terms: ΔS universe ΔS system and ΔS surroundings What do we know about entropy (is it a state function, etc)? How do we calculate ΔS system? What are the rules for calculating ΔS system?

16 18.4 The Second Law of Thermodynamics What is the second law of thermodynamics? How do we calculate ΔS system? Practice: What is the standard change in entropy at 25 o C for the reaction of nitrogen and hydrogen to produce ammonia?

17 18.4 The Second Law of Thermodynamics What is the second law of thermodynamics? How do we calculate ΔS system? Can we predict ΔS system? Consider the differences in standard (absolute) entropy values at 25 o C (Table 18.1, p. 634)

18 18.4 The Second Law of Thermodynamics What is the second law of thermodynamics? Can we predict ΔS system? Practice: Is change in entropy positive or negative for: 1. PCl 5 (g) PCl 3 (g) + Cl 2 (g) 2. 2NO(g) + O 2 (g) 2NO 2 (g) 3. MgCO 3 (s) MgO(s) + CO 2 (g) 4. N 2 (g) + O 2 (g) 2NO(g)

19 18.4 The Second Law of Thermodynamics What is the second law of thermodynamics? How do we calculate ΔS universe? How do we calculate ΔS surroundings? What if we consider this in terms of exothermic/endothermic processes? What is ΔS surroundings in terms of ΔH system? What is the effect of temperature?

20 18.4 The Second Law of Thermodynamics Figure 18.5, p. 641

21 18.4 The Second Law of Thermodynamics What is the third law of thermodynamics? the entropy of a perfect crystalline substance is zero at the absolute zero of temperature What does this mean in terms of the number of configurations? Can we measure absolute entropy?

22 18.4 The Second Law of Thermodynamics Figure 18.6, p. 643

23 18.5 Gibbs Free Energy In terms of entropy, when will a process be spontaneous? What is the 2 nd Law of Thermodynamics? Incorporating enthalpy, when will a process be spontaneous? What is Gibbs free energy? the energy available to do work How is Gibbs free energy calculated? What are the rules for calculating Gibbs free energy?

24 18.5 Gibbs Free Energy What is Gibbs free energy? How is Gibbs free energy calculated? Practice: What is the standard Gibbs free energy at 25 o C for the combustion of 1 mol of nitric oxide (NO(g)) to form nitrogen dioxide (NO 2 (g))?

25 18.5 Gibbs Free Energy What is Gibbs free energy? Using Gibbs free energy, when is a reaction spontaneous? How does this relate to Enthalpy Entropy Can a reaction be spontaneous at one temperature and not spontaneous at another temperature?

26 18.5 Gibbs Free Energy Can a reaction be spontaneous at one temperature and not spontaneous at another temperature? Practice: Will the combustion of 1 mol of nitric oxide (NO(g)) to form nitrogen dioxide (NO 2 (g)) be spontaneous at 25 o C? Is this true for all temperatures?

27 18.5 Gibbs Free Energy How does temperature affect spontaneity? Practice: At what temperature will the decomposition of ammonium chloride become spontaneous? (Assume standard enthalpy and entropy do not change with temperature.)

28 18.5 Gibbs Free Energy Consider the similar system from the text (p. 645): Figure 18.8, p. 648

29 18.5 Gibbs Free Energy How does temperature affect spontaneity? Practice: At what temperature will the decomposition of ammonium chloride become spontaneous? What does this mean in terms of P N2, P H2, P Cl2 near this temperature?

30 18.6 Free Energy and Chemical Equilibrium As a reaction approaches a change in spontaneity what does this mean in terms of K? Figure 18.8, p. 648

31 18.6 Free Energy and Chemical Equilibrium How are Gibbs free energy and equilibrium related? What if the system is not at standard conditions? Consider the two cases presented in Figure 18.9, p. 648 (described, p. 647)

32 18.6 Free Energy and Chemical Equilibrium Case 1: Figure 18.9, p. 651

33 18.6 Free Energy and Chemical Equilibrium Case 2: Figure 18.9, p. 651

34 18.6 Free Energy and Chemical Equilibrium Practice: What is ΔG o and K P at 25 o C? 1 NO g O2 g NO2 g 2 What is ΔG if P NO = 1.0 atm, P O2 = 1.0 atm and P NO2 = atm?

35 18.6 Free Energy and Chemical Equilibrium Case 1: Figure 18.9, p. 651

36 18.6 Free Energy and Chemical Equilibrium Practice: At the boiling point of a substance, its liquid and gas are in equilibrium. Assuming standard enthalpy and entropy do not change with temperature, what is the boiling point of Br 2?

37 18.6 Free Energy and Chemical Equilibrium Practice: In Chapter 15, we discussed how K changes with temperature. Show this using the relationships between K, H, S, and G. If K P for the combustion of nitric oxide at 25 o C is 1.3x10 6, what is K P at 75 o C? 1 NO g O2 g NO2 g 2

38 18.7 Thermodynamics in Living Systems If a reaction is not spontaneous under standard conditions, are there other conditions that can change this? Can two reactions be coupled to make a nonspontaneous process spontaneous? Consider two examples: Refinement of metals Biological processes

39 18.7 Thermodynamics in Living Systems Figure 18.11, p. 655

In previous chapters we have studied: Why does a change occur in the first place? Methane burns but not the reverse CH 4 + 2O 2 CO 2 + 2H 2 O

Chapter 19. Spontaneous Change: Entropy and Free Energy In previous chapters we have studied: How fast does the change occur How is rate affected by concentration and temperature How much product will