Thermodynamics · Thermodynamics The Three Laws of Thermodynamics ( 18.1) Spontaneous Processes...

Thermodynamics

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)

General Chemistry I – Concepts

Representations of matter (1.3-1.4) Formula calculations and stoichiometry

(3.6-3.9) The concepts of the first law of

thermodynamics and enthalpy (6.1-6.6)

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?

p. 214 of LA book

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”?

p. 214-215 of LA book


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?

p. 215 of LA book

18.2 Spontaneous Processes Which process would be more likely to

occur, an exothermic or endothermic reaction?◦ Will this reaction always happen?◦ Will the reverse reaction happen?

What are some examples of exothermic and endothermic reactions that occur?◦ Oxidation of iron◦ Solution of ammonium nitrate◦ Combustion of octane



Can we assign spontaneity based on enthalpy of reaction?

If not, what else should we consider?

Figure 18.1, p. 632

p. 217 of LA book


Let’s simplify this to a process for which change in enthalpy is zero…

p. 217 of LA book

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?


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?

p. 221 of LA book

18.3 Entropy

Figure 18.3, p. 636

p. 221 of LA book

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

p. 222 of LA book

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 statethe entropy of the universe increases in a spontaneous process and remains constant (unchanged) in an equilibrium process

What does this mean in terms:◦ ΔSuniverse◦ ΔSsystem and ΔSsurroundings

What do we know about entropy (is it a state function, etc)?

How do we calculate ΔSsystem? What are the rules for calculating ΔSsystem?


18.4 The Second Law of Thermodynamics

What is the second law of thermodynamics?

How do we calculate ΔSsystem?

Practice:What is the standard change in entropy at 25oC for the reaction of nitrogen and hydrogen to produce ammonia?

p. 224 of LA book

18.4 The Second Law of Thermodynamics Consider the differences

in standard (absolute) entropy values at 25oC (Table 18.1, p. 634) –what appears to make the greatest difference between entropy values of the same substance?

Can we predict ΔSsystem?

p. 224 of LA book

Table 18.1, p. 637


Practice:Is change in entropy positive or negative for:1. PCl5(g) PCl3(g) + Cl2(g)2. 2NO(g) + O2(g) 2NO2(g)3. MgCO3(s) MgO(s) + CO2(g)4. N2(g) + O2(g) 2NO(g)

p. 225 of LA book


What is the second law of thermodynamics?

How do we calculate ΔSuniverse?◦ How do we calculate ΔSsurroundings?

What if we consider this in terms of exothermic/endothermic processes?◦ What is ΔSsurroundings in terms of ΔHsystem?◦ What is the effect of temperature?



Figure 18.5, p. 641

p. 226 of LA book


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?



Figure 18.6, p. 643

p. 227 of LA book

18.5 Gibbs Free Energy

In terms of entropy, when will a process be spontaneous?◦ What is the 2nd 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?



What is Gibbs free energy? How is Gibbs free energy calculated?

Practice:What is the standard Gibbs free energy at 25oC for the combustion of 1 mol of nitric oxide (NO(g)) to form nitrogen dioxide (NO2(g))?

p. 233 of LA book


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?



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 (NO2(g)) be spontaneous at 25oC? Is this true for all temperatures?

p. 234 of LA book


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.)

p. 235 of LA book


Consider the similar system from the text (p. 645):

Figure 18.8, p. 648

p. 235 of LA book


How does temperature affect spontaneity?

Practice:At what temperature will the decomposition of ammonium chloride become spontaneous? What does this mean in terms of PN2, PH2, PCl2 near this temperature?

p. 235 of LA book

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

p. 236 of LA book


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. 651


18.6 Free Energy and Chemical Equilibrium Case 1:

Figure 18.9, p. 651

p. 237 of LA book


Figure 18.9, p. 651

p. 237 of LA book


Practice:What is ΔGo and KP at 25oC?

What is ΔG if PNO = 1.0 atm, PO2 = 1.0 atmand PNO2 = 0.0030 atm?

2 212

NO g O g NO g

p. 238 of LA book


Figure 18.9, p. 651

p. 238 of LA book


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 Br2?

p. 238 of LA book


Practice:In Chapter 15, we discussed how K changes with temperature. Show this using the relationships between K, H, S, and G.

If KP for the combustion of nitric oxide at 25oC is 1.3x106, what is KP at 75oC?

2 212

NO g O g NO g

p. 239 of LA book

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


18.7 Thermodynamics in Living Systems

Figure 18.11, p. 655


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Thermodynamics · Thermodynamics The Three Laws of Thermodynamics ( 18.1) Spontaneous Processes...

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