Standard Electrode Potentials When the concentrations of Cu 2+ (aq) and Zn 2+ (aq) are both kept at unit activity, the emf of the galvanic cell is 1.10 V at 25 o C. (V is the unit for voltage). 1801

Standard Electrode Potentials When the concentrations of Cu 2+ (aq) and Zn 2+ (aq) are both kept at unit activity, the emf of the galvanic cell is 1.10 V at 25 o C. (V is the unit for voltage). Recall: The activity of a species X can be written as where is called the activity coefficient. 1802

Standard Electrode Potentials When the concentrations of Cu 2+ (aq) and Zn 2+ (aq) are both kept at unit activity, the emf of the galvanic cell is 1.10 V at 25 o C. (V is the unit for voltage). Recall: The activity of a species X can be written as where is called the activity coefficient. For fairly dilute solutions,, so that. 1803

Standard Electrode Potentials When the concentrations of Cu 2+ (aq) and Zn 2+ (aq) are both kept at unit activity, the emf of the galvanic cell is 1.10 V at 25 o C. (V is the unit for voltage). Recall: The activity of a species X can be written as where is called the activity coefficient. For fairly dilute solutions,, so that. We will make the gross assumption (as does the text) that at a concentration of 1 M, and replace unit activity for Cu 2+ (aq) and Zn 2+ (aq) by concentrations of 1 M. 1804

The value of the emf is independent of the amount of solution or the size of the electrodes. 1805

The value of the emf is independent of the amount of solution or the size of the electrodes. The measured emf can be treated as the sum of the two electric potentials arising from the Zn and Cu electrodes. 1806

The value of the emf is independent of the amount of solution or the size of the electrodes. The measured emf can be treated as the sum of the two electric potentials arising from the Zn and Cu electrodes. It is impossible to measure the potential of a single electrode: any complete circuit must by necessity, contain two electrodes. 1807

The value of the emf is independent of the amount of solution or the size of the electrodes. The measured emf can be treated as the sum of the two electric potentials arising from the Zn and Cu electrodes. It is impossible to measure the potential of a single electrode: any complete circuit must by necessity, contain two electrodes. A simple way out of this dilemma is to chose a certain electrode and arbitrarily set its potential value to zero volts. 1808

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. 1809

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. The standard hydrogen electrode is chosen as the reference (abbreviated as SHE). 1810

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. The standard hydrogen electrode is chosen as the reference (abbreviated as SHE). The reaction is 2 H + (aq) + 2e - H 2(g) E 0 = 0 V 1811

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. The standard hydrogen electrode is chosen as the reference (abbreviated as SHE). The reaction is 2 H + (aq) + 2e - H 2(g) E 0 = 0 V (1 M) (1 bar) 1812

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. The standard hydrogen electrode is chosen as the reference (abbreviated as SHE). The reaction is 2 H + (aq) + 2e - H 2(g) E 0 = 0 V (1 M) (1 bar) The symbol for the emf is E cell (some use just E). 1813

This electrode can then be used to determine the potentials of other electrodes by measuring the emf of various cells. The standard hydrogen electrode is chosen as the reference (abbreviated as SHE). The reaction is 2 H + (aq) + 2e - H 2(g) E 0 = 0 V (1 M) (1 bar) The symbol for the emf is E cell (some use just E). The superscript 0 denotes standard state conditions, which for the present case refers to H + (aq) at 1 M, H 2(g) at 1 bar, and a reference temperature of exactly 25 o C is assumed. 1814

For a half-cell reaction at standard conditions, the notation E 0 is employed. Other notation that is employed is or sometimes, this latter one signifying that it is a standard reduction potential. Standard emf: The potential difference between two electrodes which can be measured for a given cell when all solutes are at a concentration of 1 M and all gases are at 1 bar. 1815

Suppose we want to determine the for the reaction Cu 2+ (aq) + 2 e - Cu (s) then set up the cell with a SHE, so that: anode: H 2(g) 2 H + (aq) + 2e - = 0 V cathode: Cu 2+ (aq) + 2 e - Cu (s) = ? overall reaction: H 2(g) + Cu 2+ (aq) 2 H + (aq) + Cu (s) = 0.34 V Since the two values must add to 0.34 V, therefore = 0.34 V for the Cu 2+ half-reaction. 1816

1817

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) 1818

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) can be measured with a SHE, so that: 1819

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) can be measured with a SHE, so that: anode: Zn (s) Zn 2+ (aq) + 2 e - = ? 1820

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) can be measured with a SHE, so that: anode: Zn (s) Zn 2+ (aq) + 2 e - = ? cathode: 2 H + (aq) + 2e - H 2(g) = 0 V 1821

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) can be measured with a SHE, so that: anode: Zn (s) Zn 2+ (aq) + 2 e - = ? cathode: 2 H + (aq) + 2e - H 2(g) = 0 V overall reaction: 2 H + (aq) + Zn (s) H 2(g) + Zn 2+ (aq) = 0.76 V 1822

The standard electrode potential for the reaction Zn 2+ (aq) + 2 e - Zn (s) can be measured with a SHE, so that: anode: Zn (s) Zn 2+ (aq) + 2 e - = ? cathode: 2 H + (aq) + 2e - H 2(g) = 0 V overall reaction: 2 H + (aq) + Zn (s) H 2(g) + Zn 2+ (aq) = 0.76 V Since the two values must add to 0.76 V, therefore = 0.76 V for the Zn half-reaction. 1823

1824

1825

1826

Standard reduction potential: The voltage associated with a reduction at an electrode when all solutes are 1 M and all gases are at 1 bar. 1827

Standard reduction potential: The voltage associated with a reduction at an electrode when all solutes are 1 M and all gases are at 1 bar. It is most common to table information as reductions potentials. 1828

1829

Standard reduction potential: The voltage associated with a reduction at an electrode when all solutes are 1 M and all gases are at 1 bar. It is most common to table information as reductions potentials. Standard oxidation potential: The voltage associated with an oxidation at an electrode when all solutes are 1 M and all gases are at 1 bar. 1830

The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e

The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e - is = 0.76 V 1832

The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e - is = 0.76 V When we reverse the half-cell reaction, we must change the sign of. 1833

The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e - is = 0.76 V When we reverse the half-cell reaction, we must change the sign of. Thus the standard reduction potential for the reaction: Zn 2+ (aq) + 2 e - Zn (s) 1834

The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e - is = 0.76 V When we reverse the half-cell reaction, we must change the sign of. Thus the standard reduction potential for the reaction: Zn 2+ (aq) + 2 e - Zn (s) is = V 1835

Calculation of 1836

Calculation of Example: Calculate for the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) assuming a table of is available. 1837

Calculation of Example: Calculate for the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) assuming a table of is available. From the table of values the following is available 1838

Calculation of Example: Calculate for the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) assuming a table of is available. From the table of values the following is available Zn 2+ (aq) + 2 e - Zn (s) = V 1839

Calculation of Example: Calculate for the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) assuming a table of is available. From the table of values the following is available Zn 2+ (aq) + 2 e - Zn (s) = V Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V 1840

The overall reaction is stripped down to the two half- equations: 1841

The overall reaction is stripped down to the two half- equations: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) 1842

The overall reaction is stripped down to the two half- equations: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V 1843

The overall reaction is stripped down to the two half- equations: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V Zn (s) Zn 2+ (aq) + 2 e - = 0.76 V 1844

The overall reaction is stripped down to the two half- equations: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V Zn (s) Zn 2+ (aq) + 2 e - = 0.76 V Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) = 1.10 V 1845

The overall reaction is stripped down to the two half- equations: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V Zn (s) Zn 2+ (aq) + 2 e - = 0.76 V Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) = 1.10 V This is the simplest approach to calculate values, and the approach I recommend using. 1846

There is an alternative approach, that is based on the formula: where and are the values for the cathode and anode reactions, pulled directly from a standard table of reduction potentials. 1847

There is an alternative approach, that is based on the formula: where and are the values for the cathode and anode reactions, pulled directly from a standard table of reduction potentials. The reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) has the two half-cell reactions: 1848

There is an alternative approach, that is based on the formula: where and are the values for the cathode and anode reactions, pulled directly from a standard table of reduction potentials. The reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) has the two half-cell reactions: anode: Zn (s) Zn 2+ (aq) + 2 e

There is an alternative approach, that is based on the formula: where and are the values for the cathode and anode reactions, pulled directly from a standard table of reduction potentials. The reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) has the two half-cell reactions: anode: Zn (s) Zn 2+ (aq) + 2 e - cathode: Cu 2+ (aq) + 2 e - Cu (s) 1850

There is an alternative approach, that is based on the formula: where and are the values for the cathode and anode reactions, pulled directly from a standard table of reduction potentials. The reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) has the two half-cell reactions: anode: Zn (s) Zn 2+ (aq) + 2 e - cathode: Cu 2+ (aq) + 2 e - Cu (s) Therefore 0.34 V – (–0.76 V) = 1.10 V 1851

A large number of mistakes are made when using this approach. The most common one is that the reaction involving Zn is an oxidation, so students reverse the sign of the value in the table for the Zn half-reaction, but retain the minus sign in the formula, thereby getting the wrong answer of V. 1852

Spontaneity of Redox Reactions 1853

Spontaneity of Redox Reactions Under standard state conditions, a redox reaction is spontaneous in the forward direction if the standard emf of the cell is positive. 1854

Spontaneity of Redox Reactions Under standard state conditions, a redox reaction is spontaneous in the forward direction if the standard emf of the cell is positive. The more positive the value, the greater the tendency for the substance to be reduced. For example, F 2(g) + 2 e - 2 F - (aq) = 2.87 V, is one of the largest values, 1855

Spontaneity of Redox Reactions Under standard state conditions, a redox reaction is spontaneous in the forward direction if the standard emf of the cell is positive. The more positive the value, the greater the tendency for the substance to be reduced. For example, F 2(g) + 2 e - 2 F - (aq) = 2.87 V, is one of the largest values, which makes F 2 one of the strongest oxidizing agents available. 1856

Li + (aq) + e - Li = V 1857

Li + (aq) + e - Li = V This reaction has the one of the most negative values, making Li + one of the weakest oxidizing agents. 1858

Li + (aq) + e - Li = V This reaction has the one of the most negative values, making Li + one of the weakest oxidizing agents. If we reverse the reaction: Li Li + (aq) + e - = 3.05 V 1859

Li + (aq) + e - Li = V This reaction has the one of the most negative values, making Li + one of the weakest oxidizing agents. If we reverse the reaction: Li Li + (aq) + e - = 3.05 V Li is one of the strongest reducing agents available. 1860