Chapter 17 THERMODYNAMICS

What is Thermodynamics? Thermodynamics is the study of energy changes that accompany physical and chemical processes. Word origin: “Thermo”, from temperature, meaning heat “Dynamics”, which means motion (under the action of forces)  What determines the direction of spontaneous chemical reactions? Chemical thermodynamics answers the questions:  How much heat is evolved during a chemical reaction ("thermochemistry")?

Spontaneous Processes A spontaneous process occurs by itself without any ongoing outside intervention  Reaction continues until equilibrium is reached Examples: A cube of ice melting in water; a steel pipe rusting; Spontaneous chemical reactions behave the same way  If a reaction is spontaneous in one direction, it is not spontaneous in the other  Spontaneity has nothing to do with rate/speed of reaction

C Diamond Spontaneous; 1 atm, 25 0 C Spontaneity versus Speed/Rate Spontaneous = Thermodynamically favorable; Large K eq ) C Graphite Extremely slow = Kinetically unfavorable; Small rate) Spontaneous ≠ Instantaneous (Fast) Nonspontaneous; 1 atm, 25 0 C BUT…

Spontaneous Processes If a reaction is spontaneous in one direction, it is not spontaneous in the other  Diamond to graphite: Spontaneous (but extremely slow) at room T In the examples above energy is conserved (1 st law). Yet one process occurs while the other does not.  Melting of ice cube at room T is spontaneous, but freezing of water at room T is not spontaneous.  Graphite to diamond: Nonspontaneous at room T (otherwise we’ll all be rich!)

Spontaneous Processes (Cont.) Spontaneous processes may need a little “push” to get started Example: Hydrogen and oxygen gases burn spontaneously only after being ignited by a spark 2H 2 (g) + O 2 (g) 2H 2 O (l)  The reverse reaction is nonspontaneous (i.e. Water does not simply decompose into H 2 and O 2 gases)

Nonspontaneous reactions Can we make nonspontaneous reactions happen?  Yes. It happens everyday. HOW?  By supplying energy Examples: Photosynthesis (CO 2 (g) + H 2 O (l) = Carbohydrates) takes place upon absorption of solar energy 2H 2 O (l) 2H 2 (g) + O 2 (g) The nonspontaneous reaction happens if we pass electricity through water (electrolysis)

Factors Affecting Spontaneity 1. Changes in enthalpy (ΔH) a) Exothermic reactions ( - ΔH) → tend to be thermodynamically favorable [ large K eq ] Example: All combustion reactions (such as the combustion of natural gas or methane) are spontaneous and exothermic. b) Endothermic reactions ( + ΔH) → tend to be unfavorable CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) ΔH rxn 0 = kJ Methane  Spontaneous and  Exothermic (- ΔH)

Factors Affecting Spontaneity – Cont. NOTE: Not all exothermic reactions are spontaneous Examples of endothermic reactions that are spontaneous  Vaporization of water (+ΔH) at ordinary T and P  Dissolving of NaCl in water (+ΔH) Thus, the sign of ΔH does not always predict spontaneous change Q. What other factor affects direction of spontaneous reactions?

Factors Affecting Spontaneity – Cont. 2. Changes in entropy (ΔS) or degree of “disorder” a) Increase in entropy ( + ΔS) or disorder → tend to be favorable [ large K eq ] Recall: Entropy (S) is a measure of the dispersal of energy as a function of temperature b) Decrease in entropy ( - ΔS) or more order → tend to be unfavorable  Closely associated with randomness or disorder

The “Randomness” Factor  Randomness also predicts spontaneous processes In general, nature tends to move spontaneously from a more ordered state to more random (less ordered) state. molecularmachines.html An example of a + ΔS What? Are you expecting your room to become more ordered in time?

Change to more disordered (more random) state = Increase in entropy (+ ΔS) Q. Which of the two images below has a + ΔS? Is spontaneous above 0 0 C? Image source: Demo/scripts/demo_descript.idc? DemoID=1114 MELTING  More disorder = higher entropy (+ ΔS) Factors Affecting Spontaneity – Cont.  Spontaneous when T > 0 0 C

Entropy (S) is a State Function Recall that entropy is a state function: ΔS = S final - S initial  Depends only on the initial and final states of the system (Not on path taken from one state to another) ΔS rxn = ∑ n x ΔS products - ∑ n x ΔS reactants For a chemical reaction, entropy change is expressed as: Most disordered Most ordered (Least disorder) In general:ΔS gas > ΔS liquid > ΔS solid

Entropy (S) and Phase Changes Exercise: Predict the sign of ΔS in each of the following processes. Freezing Vaporization Condensation - ΔS + ΔS (becomes more disordered) - ΔS Reactions that produce gas (+ ΔS) tend to be favorable

Entropy and the 2 nd Law of Thermo. Second law of thermodynamics: In a spontaneous process there is a net increase in the entropy of the universe * Layman’s term: All real processes occur spontaneously in the direction that increases disorder. * Universe = System + Surrounding Thus, ΔS universe = ΔS system + ΔS surrounding  A process (or reaction) is spontaneous if ∆S universe is (+)

Entropy and the 2 nd Law of Thermo. – Cont. ΔS universe = ΔS system + ΔS surrounding Q. When is ∆S universe positive?  (+) ∆S universe when: ΔS system + ΔS surrounding > 0 If this decreases, ∆S surr. must increase even more to offset the system’s decrease in entropy Example: During growth of a child, food molecules (carbs, proteins, etc) are broken down into CO 2, water and energy  Increases ∆S surrounding But formation of body mass decreases ∆S system Exothermic ( -∆H) Thus, ↓ ∆S system is offset by ↑ ∆S surrounding = Still obeys 2 nd law

Entropy and the 3 rd Law of Thermo. Third law of thermodynamics: The entropy of a pure crystalline substance at 0 K is zero. All motion/vibration at absolute zero has stopped  Particles in highest order  S system = 0 at T = 0 K = System Q. What happens to a system if we decrease the temperature continuously?  Entropy will decrease (more order)  Recall: ∆S (g) > ∆S (l) > ∆S (s)

Entropy and 3 rd Law - Cont. It follows that as the temperature of a system increases, its entropy also increases.  Thus, T (related to ∆H) and entropy (∆S) both influence spontaneous reactions  ∆H can be measured using a calorimeter  How is ∆S measured?

Standard Molar Entropies, S 0  Entropy (S) at standard conditions of 1 atm pressure and 25 0 C temperature for one mole of a substance Standard Molar Entropy, S 0  Unit for S 0 : J/molK Table 17.1: Standard molar entropies of substances  Elements and compounds have (+) S 0  Aqueous ions may have (+) or (-) S 0 ΔS 0 rxn = ∑ n x ΔS 0 products - ∑ n x ΔS 0 reactants  NOTE: Be able to calculate ΔS 0 rxn from given S 0 values

Gibbs Free Energy, G At a constant temperature, the free energy change, ΔG, of a system is given by the Gibbs-Helmholtz equation:  So far you’ve learned that both ∆H and ∆S affect reaction spontaneity  Is there an equation that relates these two thermodynamic quantities? ΔG = ΔH - TΔS Free energy change Enthalpy change Kelvin T * Entropy change

Standard Free Energy Change, ΔG 0 The standard free energy change, ΔG 0, applies to standards states as follows: (1) 1 atm pressure for pure liquids and solids ΔG 0 = ΔH 0 - TΔS 0 (2) 1 atm partial pressure for gases (3) 1 M concentration for solutions Mathematically: Importance of Free Energy Change:  The sign of ΔG 0 determines reaction spontaneity

Importance of ΔG 0 The sign of ΔG 0 determines reaction spontaneity (1) - ΔG 0 = spontaneous reaction* (2) + ΔG 0 = nonspontaneous reaction (3) ΔG 0 = 0 for reactions at equilibrium (occurs in either direction) * At a constant T and P, reactions go in the direction that lowers the free energy of the system.

ΔG 0 and Spontaneous Reactions Q. Just when is a reaction spontaneous by looking at ΔG 0 ? ΔG 0 = ΔH 0 - TΔS 0 Source: Principles of General Chemistry by Silberberg, © 2007.

ΔG 0 versus ΔG ΔG 0 vs. ΔG ΔG 0 = free energy of the system at standard conditions [P gas = 1 atm; [ ] = 1 M for species in solution (aq)]  Fixed during a reaction under std. conditions ΔG = free energy of the system at any given condition [i.e. at any point in the reaction]  Changes during a reaction

Importance of ΔG 0 For a given chemical reaction aA + bB → cC + dD the free energy of the reaction is given by the equation ∆G = ∆G 0 + RT ln Q Std. free energy Gas constant Kelvin temp. Reaction quotient Q = [C] c [D] d [A] a [B] b

Importance of ΔG 0 – Cont. At equilibrium, reaction seem to have stopped (rate fwd = rate rev. ) and: At equilibrium ∆G = 0 ; Q = K eq ∆G = ∆G 0 + RT ln Q Thus, 0 = ∆G 0 + RT ln K eq or at equilibrium ∆G 0 = - RT ln K eq = 0= K eq or K eq = e - (∆G 0 /RT) Importance of this equation:  Allows us to calculate ∆G 0 given K eq or K eq given ∆G 0