Chemistry/Physical Setting Electrochemical Cells Br. Jabreal

Electrochemical Processes Chemical processes can either release or absorb energy. Sometimes the energy released is electrical energy. Electrochemistry is a field relating chemical changes and electrical energy. It is applied for many important things: Flashlight and automobile batteries, silverplating, nerve impulse transmission, etc.

Aim: How do voltaic cells produce electrical energy? Do Now: What is a “spontaneous” reaction?

A Spontaneous Redox Reaction Some redox reactions will occur spontaneously. For example, when placing a strip of zinc in an aqueous solution of copper (II) sulfate, Zn will be oxidized and will cause the Cu2+ ions to be reduced.

The spontaneous reaction can be summed up as follows: Zinc atoms lose electrons as they are oxidized to zinc ions. The zinc is now dissolved in the solution. The electrons are transferred to the copper ions, which now become deposited as solid metallic copper.

Half Reactions The redox reaction can be described with half-reactions occurring simultaneously, one for the oxidation, and one for the reduction:

Practice Write the oxidation and reduction half reaction equations for the following reaction: Zn(s) + Fe3+(aq) Zn2+(aq) + Fe(s)

Why do some redox reactions occur spontaneously? The previously mentioned reaction occurs naturally, without applying external work. However, if the same experiment is done in reverse, placing a strip of copper in a solution with Zn2+ ions, the reaction will not occur. For any two metals, the more active metal will spontaneously oxidize and cause the less active metal to be reduced. *Refer to Table J

Practice Given the following redox reaction: Mg(s) + Zn2+(aq)  Zn(s) + Mg2+(aq) State, in terms of the relative activity of metals, why the reaction in this cell occurs. Write a half reaction for the oxidation that occurred.

Practice Zn is above Pb in the activity series. Which of the following statements is correct? (1) Zn will react with Pb2+ (2) Pb2+ will react with Zn2+ (3) Zn2+ will react with Pb (4) Pb will react with Zn2+

Practice

Redox Reactions and Electrochemistry An electrochemical cell is any device that converts chemical energy into electrical energy or electrical energy into chemical energy. All electrochemical cells involve redox reactions Two types: 1. Voltaic Cells 2. Electrolytic Cells

Voltaic Cells Voltaic Cells: Electrochemical cells used to convert chemical energy into electrical energy. Electrical energy is produced by spontaneous redox reactions within the cell. The flow of electrons from the oxidation reaction produces electricity Made up of two separate half-cells: One cell where oxidation takes place One cell where reduction takes place Mnemonic: An Ox, Red Cat Anode oxidation, reduction at cathode

Constructing a Voltaic Cell

Parts of the Voltaic Cell Electrodes: Metal rods placed in solution that carry electrons either to or from a substance Anode: Electrode at which oxidation occurs Cathode: Electrode at which Reduction occurs Electrons flow from anode to cathode. External Wire Connects the electrodes so electrons can flow between them Salt Bridge: Tube containing electrolytes. *Releases ions into half cells to keep solutions electrically neutral.

How a Voltaic Cell Works Electrons produced at anode as it is oxidized Ex. Zn(s)  Zn2+ + 2e- 2. Electrons from anode travel through external circuit to the cathode 3. Electrons enter the cathode and then interact with the ions in solution to reduce them. Ex. Cu2+(aq) + 2e-  Cu(s) 4. To complete the circuit, ions flow from the salt bridge. The ions keep the solutions neutral. If there is an electrical imbalance in the half cells, the voltaic cell will stop functioning.

Note: Salt Bridges Sometimes a porous barrier keeps the two half cells separated and provides the function of the salt bridge. The two solutions cannot mix, but ions can migrate through the porous barrier to keep the solution neutral, allowing the voltaic cell to continue operating.

Electrolytic Cells Electrolytic Cell: Electrochemical cell used to cause a chemical change through the application of electrical energy. Conversion of electrical energy to chemical energy (Electrolysis) Forces a nonspontaneous reaction to occur from this application of electrical energy Requires outside power source, such as a battery

Electrolytic Cell

Electroplating: Electrolytic Cell

Practice

State the form of energy that is converted to electrical energy in the operating cell. State, in terms of the Cu(s) electrode and the Zn(s) electrode, the direction of electron flow in the external circuit when the cell operates Write a balanced equation for the half reaction that occurs in the Cu half-cell when the cell operates. State what happens to the mass of the Cu electrode and the mass of the Zn electrode in the operating cell.

Practice because an electrolytic cell An electrolytic cell differs from a voltaic cell because an electrolytic cell (1) generates its own energy from a spontaneous physical reaction (2) generates its own energy from a nonspontaneous physical reaction (3) requires an outside energy source for a spontaneous chemical reaction to occur (4) requires an outside energy source for a nonspontaneous chemical reaction to occur

Homework Page 687: Question 42 (a and c) Page 688: Question 58 (a,b,c). Use Table J.