Chapter 17: Free Energy & Thermodynamics CHE 124: General Chemistry II Dr. Jerome Williams, Ph.D. Saint Leo University

Overview First Law of Thermodynamics Factors Affecting Spontaneity Enthalpy Entropy

First Law of Thermodynamics You can’t win! First Law of Thermodynamics: Energy cannot be created or destroyed the total energy of the universe cannot change though you can transfer it from one place to another DEuniverse = 0 = DEsystem + DEsurroundings Tro: Chemistry: A Molecular Approach, 2/e 3 3

First Law of Thermodynamics Conservation of Energy For an exothermic reaction, “lost” heat from the system goes into the surroundings Two ways energy is “lost” from a system converted to heat, q used to do work, w Energy conservation requires that the energy change in the system equal the heat released + work done DE = q + w DE = DH + PDV DE is a state function internal energy change independent of how done Tro: Chemistry: A Molecular Approach, 2/e 4 4

The Energy Tax You can’t break even! To recharge a battery with 100 kJ of useful energy will require more than 100 kJ Second Law of Thermodynamics Every energy transition results in a “loss” of energy an “Energy Tax” demanded by nature and conversion of energy to heat which is “lost” by heating up the surroundings Tro: Chemistry: A Molecular Approach, 2/e 5 5

fewer steps generally results in a lower total heat tax Tro: Chemistry: A Molecular Approach, 2/e 6 6

Thermodynamics and Spontaneity Thermodynamics predicts whether a process will occur under the given conditions processes that will occur called spontaneous nonspontaneous processes require energy input to go Spontaneity is determined by comparing the chemical potential energy of the system before the reaction with the free energy of the system after the reaction if the system after reaction has less potential energy than before the reaction, the reaction is thermodynamically favorable. Spontaneity ≠ fast or slow Tro: Chemistry: A Molecular Approach, 2/e 7 7

Comparing Potential Energy The direction of spontaneity can be determined by comparing the potential energy of the system at the start and the end Tro: Chemistry: A Molecular Approach, 2/e 8 8

Reversibility of Process Any spontaneous process is irreversible because there is a net release of energy when it proceeds in that direction it will proceed in only one direction A reversible process will proceed back and forth between the two end conditions any reversible process is at equilibrium results in no change in free energy If a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction Tro: Chemistry: A Molecular Approach, 2/e 9 9

Thermodynamics vs. Kinetics Tro: Chemistry: A Molecular Approach, 2/e 10 10

Diamond → Graphite Graphite is more stable than diamond, so the conversion of diamond into graphite is spontaneous – but don’t worry, it’s so slow that your ring won’t turn into pencil lead in your lifetime (or through many of your generations) Tro: Chemistry: A Molecular Approach, 2/e 11 11

Spontaneous Processes Spontaneous processes occur because they release energy from the system Most spontaneous processes proceed from a system of higher potential energy to a system at lower potential energy exothermic But there are some spontaneous processes that proceed from a system of lower potential energy to a system at higher potential energy endothermic How can something absorb potential energy, yet have a net release of energy? Tro: Chemistry: A Molecular Approach, 2/e 12

Melting Ice Melting is an Endothermic process, yet ice will spontaneously melt above 0 °C. When a solid melts, the particles have more freedom of movement. More freedom of motion increases the randomness of the system. When systems become more random, energy is released. We call this energy, entropy Tro: Chemistry: A Molecular Approach, 2/e 13

Factors Affecting Whether a Reaction Is Spontaneous There are two factors that determine whether a reaction is spontaneous. They are the enthalpy change and the entropy change of the system The enthalpy change, DH, is the difference in the sum of the internal energy and PV work energy of the reactants to the products The entropy change, DS, is the difference in randomness of the reactants compared to the products Tro: Chemistry: A Molecular Approach, 2/e 14 14

Enthalpy Change DH generally measured in kJ/mol Stronger bonds = more stable molecules Reaction is generally exothermic if bonds in the products are stronger than bonds in the reactants exothermic = energy released, DH is negative A reaction is generally endothermic if the bonds in the products are weaker than the bonds in the reactants endothermic = energy absorbed, DH is positive The enthalpy change is favorable for exothermic reactions and unfavorable for endothermic reactions Tro: Chemistry: A Molecular Approach, 2/e 15 15

Entropy Entropy is a thermodynamic function that increases as the number of energetically equivalent ways of arranging the components increases, S S generally J/mol S = k ln W k = Boltzmann Constant = 1.38 x 10−23 J/K W is the number of energetically equivalent ways a system can exist unitless Random systems require less energy than ordered systems Tro: Chemistry: A Molecular Approach, 2/e 16 16

W These are energetically equivalent states for the expansion of a gas. It doesn’t matter, in terms of potential energy, whether the molecules are all in one flask, or evenly distributed But one of these states is more probable than the other two Tro: Chemistry: A Molecular Approach, 2/e 17 17

Macrostates → Microstates These microstates all have the same macrostate So there are six different particle arrangements that result in the same macrostate This macrostate can be achieved through several different arrangements of the particles Tro: Chemistry: A Molecular Approach, 2/e 18 18

Macrostates and Probability There is only one possible arrangement that gives State A and one that gives State B There are six possible arrangements that give State C The macrostate with the highest entropy also has the greatest dispersal of energy Therefore State C has higher entropy than either State A or State B There is six times the probability of having the State C macrostate than either State A or State B Tro: Chemistry: A Molecular Approach, 2/e 19 19

Changes in Entropy, DS DS = Sfinal − Sinitial Entropy change is favorable when the result is a more random system DS is positive Some changes that increase the entropy are reactions whose products are in a more random state solid more ordered than liquid more ordered than gas reactions that have larger numbers of product molecules than reactant molecules increase in temperature solids dissociating into ions upon dissolving Tro: Chemistry: A Molecular Approach, 2/e 20 20

Increases in Entropy Tro: Chemistry: A Molecular Approach, 2/e 21 21

DS For a process where the final condition is more random than the initial condition, DSsystem is positive and the entropy change is favorable for the process to be spontaneous For a process where the final condition is more orderly than the initial condition, DSsystem is negative and the entropy change is unfavorable for the process to be spontaneous DSsystem = DSreaction = Sn(S°products) − Sn(S°reactants) Tro: Chemistry: A Molecular Approach, 2/e 22

Entropy Change in State Change When materials change state, the number of macrostates it can have changes as well the more degrees of freedom the molecules have, the more macrostates are possible solids have fewer macrostates than liquids, which have fewer macrostates than gases Tro: Chemistry: A Molecular Approach, 2/e 23 23

Entropy Change and State Change Tro: Chemistry: A Molecular Approach, 2/e 24 24

Practice – Predict whether DSsystem is + or − for each of the following A hot beaker burning your fingers Water vapor condensing Separation of oil and vinegar salad dressing Dissolving sugar in tea 2 PbO2(s)  2 PbO(s) + O2(g) 2 NH3(g)  N2(g) + 3 H2(g) Ag+(aq) + Cl−(aq)  AgCl(s) DS is + DS is − DS is − DS is + DS is + DS is + DS is − Tro: Chemistry: A Molecular Approach, 2/e 25