Electrochemistry Lesson 3 Galvanic Cells

Review Zn (s) + Cu2+ (aq) → Cu (s) + Zn2+ (aq) In this reaction, copper ions remove two electrons from zinc atoms. Zn becomes Zn2+, a loss of 2 electrons (oxidation) Cu2+ becomes Cu, a gain of 2 electrons (reduction).

Galvanic Cells An example of a galvanic cell is a battery. By separating the zinc metal and the solution containing copper ions, and by placing a metal conductor (wire) between them, electrons that are lost by zinc are forced to travel through the metal conductor to reach the copper ions. These moving electrons contain energy, which can be used to power an electrical device. These reactions are spontaneous reactions, meaning they require no outside assistance or energy input to make them occur. A galvanic cell converts chemical energy from a redox reaction into electrical energy.

Galvanic Cell Diagram

Galvanic Cells (continued) In the above diagram, the oxidation of zinc and the reduction of copper ions occur in separate compartments, called half-cells. Each half-cell contains a solid conductor called an electrode. In each half-cell, electron transfer occurs between atoms on the surface of the electrode and ions in solution. The electrode where oxidation occurs is called the anode (in this case – zinc) The electrode where reduction occurs is called the cathode (in this case – copper) The half-cells are connected to each other using a wire (for electron flow) and a salt bridge (for ion flow) – both are necessary to produce electrical energy. Electrons always flow from the anode to the cathode (from A – C).

Cell Reactions You can predict that zinc is oxidized in the zinc/copper cell without seeing the chemical reaction because zinc is higher than copper in the activity series, therefore copper (II) ions are reduced. The higher up the element, the more likely it is the one that is oxidized. The chemical equation for this reaction can be divided into two parts, called half-reactions.

Cell Reactions Original Reaction: Zn (s) + Cu2+ (aq) → Cu (s) + Zn2+ (aq) Anode Half Reaction: (oxidation) Cathode Half Reaction: (reduction)

Reactions (continued) Atoms from the zinc electrode lose electrons and dissolve as Zn2+ ion in the zinc half cell. Cu2+ ions in the copper half-cell gain electrons and become neutral solid copper atoms. As the cell operates, the mass of the zinc electrode decreases and the mass of the copper electrode increases, and the colour of the copper solution fades as the ions in solution change to solid copper.

Example 1 A copper strip in a solution of copper (II) chloride, and a tin strip in a solution of tin (II) chloride. Anode: Cathode: Overall:

Example 2 An aluminum strip in a solution of aluminum nitrate, and a silver strip in a solution of silver nitrate. Anode: Cathode: Overall:

Salt Bridge A salt bride is a tube that contains a concentrated solution of an electrolyte, such as sodium nitrate. The electrolyte that is chosen for the salt bridge should not react with other chemicals in the cell. It must be higher in activity than all the other metals in the cell. The salt bridge provides ions to prevent charge building from occurring If it is removed, the production of zinc ions results in a build up of positive charges around the zinc electrode, and prevents further production of zinc ions. Similarly, the loss of copper ions at the cathode leaves the solution negatively charged, which prevents electrons in the copper electrode from being transferred to other copper ions. The end result is that the electrode reactions stop occurring, like disconnecting the electrodes.

Cell Potential The cell potential is also called the voltage (V), and measures the change in energy from one side of the cell to the other. Once the potential difference becomes zero, the electron flow stops and the cell is “dead.” This is why batteries eventually stop working.

Videos Basic Galvanic Cell