1741

1742

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: 1743

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. 1744

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. 1745

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn (s) Zn 2+ (aq) + 2 e

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) 1747

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) Overall: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) 1748

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) Overall: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) When a piece of copper metal is placed in an aqueous ZnSO 4 solution: nothing happens. 1749

When a piece of zinc metal is placed in an aqueous CuSO 4 solution: Zinc atoms enter the solution as Zn 2+ ions. Cu 2+ ions convert to Cu atoms which deposit on the Zn metal. oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) Overall: Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) When a piece of copper metal is placed in an aqueous ZnSO 4 solution: nothing happens. One conclusion: zinc has a greater tendency to be oxidized than copper. 1750

1751

It is possible, by a suitable arrangement, to make the electrons flow along an external circuit. This is carried out in a galvanic cell. 1752

It is possible, by a suitable arrangement, to make the electrons flow along an external circuit. This is carried out in a galvanic cell. A galvanic cell operates on the principle that the oxidation of Zn to Zn 2+ and the reduction of Cu 2+ to Cu can be made to take place separately (and simultaneously) with electron transfer taking place through a connecting wire between the two electrodes. 1753

For the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) 1754

For the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) the two half-cell reactions are just the oxidation and reduction steps: 1755

For the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) the two half-cell reactions are just the oxidation and reduction steps: oxidation: Zn (s) Zn 2+ (aq) + 2 e

For the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) the two half-cell reactions are just the oxidation and reduction steps: oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) 1757

For the reaction Cu 2+ (aq) + Zn (s) Zn 2+ (aq) + Cu (s) the two half-cell reactions are just the oxidation and reduction steps: oxidation: Zn (s) Zn 2+ (aq) + 2 e - reduction: Cu 2+ (aq) + 2 e - Cu (s) Note: It is essential to separate the two solutions, otherwise the Cu 2+ ions will react directly with the zinc electrode, and there would be no electron flow through the external wire. 1758

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium. 1759

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium. This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl. 1760

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium. This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl. The electrolyte is usually present dissolved in a gel (such as agar-agar). 1761

In order to complete the electrical circuit, the two solutions must be connected by a connecting medium. This is accomplished by using a salt bridge – which contains an inert electrolyte such as KCl. The electrolyte is usually present dissolved in a gel (such as agar-agar). During the reaction, electrons flow externally from the anode (the Zn electrode) to the cathode (the Cu electrode). 1762

1763

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. 1764

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes. 1765

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes. If electrons flow from the anode to the cathode, then the anode must be labeled negative with respect to the cathode, which is labeled positive. 1766

The fact that electrons flow from one electrode to the other means that there must be a voltage difference (also called a potential difference) between the electrodes. This difference, called the electromotive force, or emf, can be measured by placing a voltmeter in the external circuit between the two electrodes. If electrons flow from the anode to the cathode, then the anode must be labeled negative with respect to the cathode, which is labeled positive. In solution, negative ions move towards the anode, which means the region close to the anode must be positively charged (the Zn 2+ are formed there). 1767

Notation for Electrochemical Cells 1768

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) 1769

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) vertical line means a phase boundary 1770

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn 2+ (1 M) 1771

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn 2+ (1 M) Often ordered in the form: reactant|product 1772

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn 2+ (1 M) Often ordered in the form: reactant|product For example, Zn(s)| Zn 2+ (aq) would indicate 1773

Notation for Electrochemical Cells Zn(s)| Zn 2+ (aq) vertical line means a phase boundary Sometimes concentrations will be indicated, e.g. Zn(s)| Zn 2+ (1 M) Often ordered in the form: reactant|product For example, Zn(s)| Zn 2+ (aq) would indicate Zn Zn 2+ (aq) 1774

For H + (aq)|H 2 (g)|Pt(s) 1775

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. 1776

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) 1777

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) 1778

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) Carbon is also often used as an inert electrode. 1779

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) Carbon is also often used as an inert electrode. For Pt(s)|H 2 (g)|H + (aq), the reaction is H 2(g) 2 H + (aq) 1780

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) Carbon is also often used as an inert electrode. For Pt(s)|H 2 (g)|H + (aq), the reaction is H 2(g) 2 H + (aq) An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by 1781

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) Carbon is also often used as an inert electrode. For Pt(s)|H 2 (g)|H + (aq), the reaction is H 2(g) 2 H + (aq) An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by Fe 2+ (aq), Fe 3+ (aq)|Pt(s) 1782

For H + (aq)|H 2 (g)|Pt(s) The Pt is employed as an inert metal electrode. So the reaction is 2 H + (aq) H 2(g) (The Pt is not involved in the chemistry.) Carbon is also often used as an inert electrode. For Pt(s)|H 2 (g)|H + (aq), the reaction is H 2(g) 2 H + (aq) An electrode consisting of a platinum wire dipping into a solution of iron (II) and iron (III) is denoted by Fe 2+ (aq), Fe 3+ (aq)|Pt(s) There is no phase boundary between the Fe 2+ and Fe 3+ solutions. 1783

A comma is used to separate half-cell components that are in the same phase. 1784

A cell is represented as: Zn(s)| Zn 2+ (aq)||Cu 2+ (aq)|Cu(s) 1785

A cell is represented as: Zn(s)| Zn 2+ (aq)||Cu 2+ (aq)|Cu(s) The || represents a salt bridge. 1786

A cell is represented as: Zn(s)| Zn 2+ (aq)||Cu 2+ (aq)|Cu(s) The || represents a salt bridge. It also indicates the divide between the two half cells. 1787

A cell is represented as: Zn(s)| Zn 2+ (aq)||Cu 2+ (aq)|Cu(s) The || represents a salt bridge. It also indicates the divide between the two half cells. Often ordered so that it is in the form: anode compartment|| cathode compartment 1788

A cell is represented as: Zn(s)| Zn 2+ (aq)||Cu 2+ (aq)|Cu(s) The || represents a salt bridge. It also indicates the divide between the two half cells. Often ordered so that it is in the form: anode compartment|| cathode compartment The contents of the salt bridge are usually omitted – they are not part of the chemistry of the cell. 1789

Example: Write the balanced reactions occurring at the anode and cathode for the cell: C|I - (aq)|I 2 (s)||MnO 4 - (aq), H + (aq), Mn 2+ (aq)|C 1790

Example: Write the balanced reactions occurring at the anode and cathode for the cell: C|I - (aq)|I 2 (s)||MnO 4 - (aq), H + (aq), Mn 2+ (aq)|C anode 2 I - (aq) I 2(s) + 2e

Example: Write the balanced reactions occurring at the anode and cathode for the cell: C|I - (aq)|I 2 (s)||MnO 4 - (aq), H + (aq), Mn 2+ (aq)|C anode 2 I - (aq) I 2(s) + 2e - cathode MnO 4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq) + 4H 2 O 1792

Example: Write the balanced reactions occurring at the anode and cathode for the cell: C|I - (aq)|I 2 (s)||MnO 4 - (aq), H + (aq), Mn 2+ (aq)|C anode 2 I - (aq) I 2(s) + 2e - cathode MnO 4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq) + 4H 2 O Note that the C serves as an inert electrode, and is not involved in the chemistry. 1793

1794

Example: The following chemistry is made to occur in an electrochemical cell: 1795

Example: The following chemistry is made to occur in an electrochemical cell: 14H + (aq) + Cr 2 O 7 2- (aq) + 6e - 2 Cr 3+ (aq) + 7H 2 O 1796

Example: The following chemistry is made to occur in an electrochemical cell: 14H + (aq) + Cr 2 O 7 2- (aq) + 6e - 2 Cr 3+ (aq) + 7H 2 O Fe 2+ (aq) Fe 3+ (aq) + e

Example: The following chemistry is made to occur in an electrochemical cell: 14H + (aq) + Cr 2 O 7 2- (aq) + 6e - 2 Cr 3+ (aq) + 7H 2 O Fe 2+ (aq) Fe 3+ (aq) + e - Write the cell notation that describe this cell. 1798

Example: The following chemistry is made to occur in an electrochemical cell: 14H + (aq) + Cr 2 O 7 2- (aq) + 6e - 2 Cr 3+ (aq) + 7H 2 O Fe 2+ (aq) Fe 3+ (aq) + e - Write the cell notation that describe this cell. It is a solution reaction, so use an inert electrode such as C or Pt. 1799

Example: The following chemistry is made to occur in an electrochemical cell: 14H + (aq) + Cr 2 O 7 2- (aq) + 6e - 2 Cr 3+ (aq) + 7H 2 O Fe 2+ (aq) Fe 3+ (aq) + e - Write the cell notation that describe this cell. It is a solution reaction, so use an inert electrode such as C or Pt. C|Fe 2+ (aq),Fe 3+ (aq)||Cr 2 O 7 2- (aq), H + (aq), Cr 3+ (aq)|C 1800