Unit 9 Applications of Thermodynamics

Unit 9 : Applications of Thermodynamics INTRODUCTION TO ENTROPY ● Entropy (S): the measure of disorder in the dispersal of matter or energy in a sample of matter ● Changes in entropy (ΔS) measure how dispersed the matter or energy is in a particular system ● Entropy increases when matter become more dispersed, in cases where: ○ Phase changes from solid to liquid and from liquid to gas ■ Individual particles become more free to move and occupy a large volume ○ Volume of a gas increases ■ Gas molecules are able to move within a larger space at the same speed ○ Number of moles of gas products > number of moles of gas reactants ○ Temperature increases ■ The distribution of KE among gas particles broadens as the temperature increases ABSOLUTE ENTROPY AND ENTROPY CHANGE ● ΔS°reaction = ∑S°products - ∑S°reactants ○ Entropy is often measured in joules ○ Every substance has a nonzero value for absolute entropy ○ Number of moles of each substance (in the balanced equation) must be considered when calculating entropy ● The sign of the entropy change can be predicted from the states of reactants and products, as well as from the number of moles of reactants and products ○ +ΔS if ■ Solid→Liquid→Gas ■ Number of moles increases from reactants to products ○ -ΔS if ■ Gas→Liquid→Solid ■ Number of moles decreases from reactants to products GIBBS FREE ENERGY AND THERMODYNAMIC FAVORABILITY ● Chemical reactions proceed until equilibrium is reached ● Gibbs Free Energy (G) describes whether a reaction is thermodynamically favorable or unfavorable ○ Changes that are thermodynamically favorable proceed to equilibrium without external intervention ○ Just because a reaction is favorable, does not mean that it happens quickly ● ΔG°: the Gibbs free energy change for a chemical process in which all reactants and products are present in a standard state (pure substances, 1.0M solutions, gases at 1 atm) ● ΔG°reaction = ∑G°products - ∑G°reactants ○ ΔG° < 0 = thermodynamically favorable ○ ΔG° > 0 = thermodynamically unfavorable ● If enthalpy and entropy values can be predicted or determined at a given temperature, then ΔG° can be calculated directly by: ○ ΔG° = ΔH° - TΔS° ● In general, the temperature conditions necessary for a process to be thermodynamically favored (ΔG° < 0) can be predicted from the signs of ΔH° and ΔS°: THERMODYNAMIC AND KINETIC CONTROL ● Processes under “kinetic control” are thermodynamically favorable, but have a large activation energy, making the reaction extremely slow ● A catalyst may be used to decrease the activation energy and increase the reaction rate ○ Catalysts have no effect on the thermodynamic favorability of the reaction FREE ENERGY AND EQUILIBRIUM ● Thermodynamically favored = products are favored at equilibrium ○ ΔG° < 0 and K > 1 ● Thermodynamically unfavored = reactants are favored at equilibrium ○ ΔG° > 0 and K < 1 ● At equilibrium, no net change in concentration of reactants and products occurs, so free energy and the equilibrium constant have the following relationship: COUPLED REACTIONS ● A thermodynamically unfavorable reaction (+ΔG°) can be “coupled” with another favorable reaction to make the process occur ● Two reactions that share a common intermediate (a product of one reaction is a reactant in the other) can be coupled ● Hess’s Law is applied to determine the ΔG for the coupled reaction ● The sum of the reactions’ ΔG values is negative, making the overall process thermodynamically favorable GALVANIC (VOLTAIC) AND ELECTROLYTIC CELLS ● Electrochemical cell: a device that can convert energy released by a thermodynamically favorable redox reaction to electrical energy, or one that, conversely, can use electrical energy to drive a thermodynamically unfavorable redox reaction ○ Contains an anode (where oxidation occurs) and a cathode (where reduction occurs) ○ Requires ion flow in the cell for reaction to occur ■ Cations → Cathode ■ Anions → Anode ● Galvanic (voltaic) cells ○ Involve a thermodynamically favorable reaction ○ Anode and cathode in separate half cells ○ Salt bridge is needed ■ Allows movement of ions between half cells so that charge balance can be maintained ■ Necessary for current to flow in the circuit ○ Produces electrical energy ○ Positive voltage value ○ Electrons flow from anode through the wire to the cathode ● Electrolytic cells ○ ○ ○ ○ ○ ○ ○ ○ ○ Involve a thermodynamically unfavorable reaction Anode and cathode are in the same chamber No salt bridge is needed Power source (such as a battery) is needed Uses electrical energy Negative voltage values Electrons flow from the anode to the power source and from the power source to the cathode Occurs in an ionic solution or liquid Cations flow to the cathode, and anions flow to the anode to maintain charge balance CELL POTENTIAL AND FREE ENERGY ● A reaction occurring in an electrochemical cell involves an electric potential difference (voltage) between the oxidation and reduction processes occurring in the half cells ● Standard reduction potentials (E°red) give the reduction potentials for processes at standard condition (1 M solutions, 1 atm of pressure for gases, 25℃) ○ To get the oxidation potentials, the reduction half-reaction equation and the sign of the voltage are both reversed ○ Do not multiply standard reduction potentials by the stoichiometric coefficients in the equation ● More positive the E°V = more favorable the reduction ● A cell’s standard potential (E°cell) can be calculated from the reduction half-reaction standard potentials for the processes occurring at the cathode and anode ○ E°cell = E°cathode - E°anode ● Voltaic cells (thermodynamically favorable reactions) have a positive overall cell potential ● Electrolytic cells (thermodynamically unfavorable reactions) have a negative overall cell potential ● ΔG° is proportional to: ○ The negative of the cell potential for the redox reaction from which it is constructed ○ Moles of electrons (n) transferred in the redox reaction ○ ΔG° = -nFE°cell ■ F = Faraday’s Constant = the amount of charge (q), in coulombs (C), per mole of electrons = 96,485 C/mol e● +E°cell = -ΔG° ● -E°cell = +ΔG° ΔH° ΔS° ΔG° K E° Favorable? + + - at high T + at low T >1 at high T <1 at low T + at high T - at low T YES at high T NO at low T - - + at high T - at low T <1 at high T >1 at low T - at high T + at low T NO at high T YES at low T + - + <1 - NO - + - >1 + YES