Spontaneous Reactions Problem Set Standard Reduction Potential Table

Spontaneous Reactions Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/17/2015 Spontaneous Reactions  Reading assignment: Chang: Chapter 4.4, 19.1  Features of reactions which are spontaneous will be identified.  Electrochemical spontaneity will be described as a means to identify spontaneous reactions.

6/17/2015 Which of the Following Reactions Are Spontaneous? The neutralization of hydrochloric acid by potassium hydroxide Ba(OH) 2 · 8 H 2 O(s) + 2 NH 4 SCN(s) Ba(SCN) 2 (aq) + 2 NH 3 (g) + 10 H 2 O( l )

6/17/2015 Spontaneous Reactions are Favorable Reactions KCl(aq)+ H 2 O( l ) HCl(aq) + KOH(aq) Ba(OH) 2 · 8 H 2 O(s) + 2 NH 4 SCN(s) Ba(SCN) 2 (aq) + 2 NH 3 (g) + 10 H 2 O( l ) exothermic endothermic

6/17/2015 Which of the Following Reactions Are Spontaneous? Melting of an ice cube at -10 °C Melting of an ice cube at 20 °C

6/17/2015 Spontaneous Reactions are Favorable Reactions H 2 O(s) H2O(l)H2O(l) 20 °C) H 2 O(s) H2O(l)H2O(l) -10 °C) endothermic exothermic

6/17/2015 Spontaneous Reaction  A reaction that occurs by itself. Not "driven" or forced to proceed by some outside energy source.  Thermodynamic concept Nature moves towards equilibrium "spontaneously".  Not a kinetic concept. No indication of how long it takes. Does not mean immediate or instantaneous.

6/17/2015 Spontaneous Reaction  No direct relationship to exothermicity Many common examples of spontaneous reactions are exothermic, but spontaneous reactions can also be endothermic.  Spontaneity does depend on temperature.

6/17/2015 Is the Reaction of Aluminum Metal and Copper(II) Ion Spontaneous? Al(s) + Cu 2+ (aq) A spontaneous reaction has a positive electrochemical cell potential ( E cell > 0). To predict the spontaneity of a reaction, the cell potential ( E cell ) must be determined.

6/17/2015 Procedure Determine the Standard Cell Potential  Identify the reactants and products undergoing oxidation and reduction.  Balance each half-cell reaction.  Add together the oxidation and reduction half-cell equations to give the target equation.  Determine the cell potential.

6/17/2015 Aluminum Metal Reacts With Copper(II) Ion Al(s) + Cu 2+ (aq)Al 3+ (aq) + Cu(s) Cu 2+ (aq) Cu(s)+ 2 e – Al(s)Al 3+ (aq)+ 3 e –

6/17/2015 Aluminum Metal Reacts With Copper(II) Ion Identify the reactants and products of the target equation Al(s) + Cu 2+ (aq)Al 3+ (aq) + Cu(s) Break the equation into reduction and oxidation half-cells Cu 2+ (aq) Cu(s)+ 2 e – Al(s)Al 3+ (aq)+ 3 e – Multiply each half-cell equation so that the electrons cancel 3  ( ) 2  ( ) 2323

6/17/2015 Does Copper(II) Ion React with Aluminum Spontaneously? 2 Al(s) + 3 Cu 2+ (aq)2 Al 3+ (aq) + 3 Cu(s) Al(s)Al 3+ (aq) + 3 e – 2  ( ) Cu 2+ (aq) + 2 e – Cu(s) 3  ( ) 2323 Determine the half-cell potential for each half-cell equation E°½=E°½= V E°½=E°½= V The sum of the two half-cell potentials is the cell potential E ° cell = V

6/17/2015 Notes About This Reaction Copper(II) Ion Reacts With Aluminum  If the cell potential is positive ( E cell > 0), then the reaction is spontaneous.  If the standard cell potential is positive ( E ° cell > 0), then the reaction is spontaneous under standard conditions.  E ° cell = V

Two Kinds of Cell Potentials E cell E ° cell standard conditions all gases at 1 atm pressure all solutions at 1 M concentrations any concentration any pressure Observed Cell Potential Standard Cell Potential When would the observed cell potential equal the standard cell potential?

The reaction of aluminum and copper(II) ion is spontaneous under standard conditions. Aluminum metal is put into a solution containing 1 M copper(II) ion and 1 M aluminum ion. copper(II) ion aluminum ion 2 Al(s) + 3 Cu 2+ (aq)2 Al 3+ (aq) + 3 Cu(s)

6/17/2015 Notes about this reaction 6 e – + 2 Al(s) + 3 Cu 2+ (aq)2 Al 3+ (aq) + 3 Cu(s) + 6 e – This is a six electron process (n=6), corresponding to the number of electrons transferred (cancelled). What is the mass-action expression for this reaction?

6/17/2015 Notes about this reaction 2 Al(s) + 3 Cu 2+ (aq)2 Al 3+ (aq) + 3 Cu(s) Q = [Al 3+ ] 2 [Cu 2+ ] 3 The mass-action expression for this process is

6/17/2015 Cell Potentials  The magnitude of the cell potential can aid in identifying the most probable product. The larger the cell potential, the more favorable the reaction. When aluminum metal reacts with copper(II) ion, what oxidation state of copper is produced? zero (copper metal) aqueous copper(I) ion

6/17/2015 Determine the Standard Cell Potential E ° ½ = E ° cell = Cu 2+ (aq) + e – Cu + (aq) Al(s)Al 3+ (aq) + 3 e – Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq)

6/17/2015 Determine the Standard Cell Potential E°½=E°½= E°½=E°½= E ° cell = 3  ()Cu 2+ (aq) + e – Cu + (aq) Al(s)Al 3+ (aq) + 3 e – 1  () Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) V V V 33

6/17/2015 Questions about this system 3 e – + Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) + 3 e – E ° cell = V Is the reaction spontaneous or non-spontaneous under standard conditions? spontaneous

6/17/2015 Questions about this system 6 e – + 2 Al(s) + 3 Cu 2+ (aq)2 Al 3+ (aq) + 3 Cu(s) + 6 e – E ° cell = V 3 e – + Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) + 3 e – E ° cell = V Does copper metal or copper(I) ion preferentially form? copper metal

Does copper metal or copper(I) ion preferentially form? Al(s) + Cu 2+ (aq) Cu + (aq) + Al 3+ (aq) Cu(s) + Al 3+ (aq) V V

6/17/2015 Questions about this system 3 e – + Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) + 3 e – E ° cell = V How many electrons are transferred? 3

6/17/2015 Questions about this system Q = [Al 3+ ] [Cu + ] 3 [ Cu 2+ ] 3 3 e – + Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) + 3 e – E ° cell = V What is the mass-action expression for this process?

6/17/2015 Questions about this system 3 e – + Al(s) + 3 Cu 2+ (aq)Al 3+ (aq) + 3 Cu + (aq) + 3 e – E ° cell = V If this reaction actually occurred, what experimental conditions would be necessary to obtain a cell potential of V? Aluminum metal is placed in a solution of 1 M copper(II) ion and 1 M copper(I) ion and 1 M aluminum ion at 25 °C.

Could copper(I) ion undergo disproportionation spontaneously? Disproportionation is the process in which a compound undergoes auto-oxidation and reduction. 2 Cu + (aq)Cu 2+ (aq) + Cu(s) Break this reaction into an oxidation half-cell and a reduction half-cell. If the cell potential is positive, the reaction is spontaneous. Help!

6/17/2015 E°½=E°½= Cu + (aq) + e – Cu (s) V