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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

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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

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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

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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

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

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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

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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

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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

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

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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

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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

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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

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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

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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

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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

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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

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

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

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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

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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

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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

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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

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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

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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

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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

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The standard oxidation potential for the Zn electrode reaction: Zn (s) Zn 2+ (aq) + 2 e - 1831

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

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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

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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

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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 = -0.76 V 1835

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Calculation of 1836

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

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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

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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) = -0.76 V 1839

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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) = -0.76 V Cu 2+ (aq) + 2 e - Cu (s) = 0.34 V 1840

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

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

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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

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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

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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

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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

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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

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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

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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 - 1849

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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

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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

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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 -0.42 V. 1852

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Spontaneity of Redox Reactions 1853

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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

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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

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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

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Li + (aq) + e - Li = - 3.05 V 1857

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Li + (aq) + e - Li = - 3.05 V This reaction has the one of the most negative values, making Li + one of the weakest oxidizing agents. 1858

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Li + (aq) + e - Li = - 3.05 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

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Li + (aq) + e - Li = - 3.05 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

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