Electrochemistry Electrochemical Process The conversion of chemical energy into electrical energy and the conversion of electrical energy into chemical energy are electrochemical process. Recall that an electric current is a flow of electrons in a circuit. All electrochemical processes involve redox reactions. When a strip of zinc metal is placed into an aqueous solution of copper (II) sulfate, the zinc becomes copper plated. Zn(s) + Cu 2+ (aq) --> Zn 2+ (aq) + Cu(s) Electrons are transferred from the zinc atoms to copper ions. This is a redox reaction which occurs spontaneously. A spontaneous reaction is a reaction that occurs be itself, with an ongoing input of energy. The half reactions are as follows: oxidation Zn(s) --> Zn 2+ (aq) + 2e - reduction Cu 2+ (aq) + 2e - --> Cu(s) As the reaction proceeds, zinc atoms lose electrons as they are oxidized to zinc ions. At the same time copper ions in solution gain electrons and are reduced to copper atoms and are deposited as metallic copper. The Galvanic Cell or Voltaic Cell Galvanic or voltaic cells are electrochemical cells that are used to convert chemical energy into electrical energy. The energy is produced by spontaneous redox reactions. A half cell is one part of a voltaic cell in which either oxidation or reduction takes place. The two half cells are separated by a salt bridge or porous barrier, which is a tube containing a conducting solution. The zinc and copper rods in a voltaic cell are the electrodes. An electrode is a conductor in a circuit that carries electrons to or from a substance. The electrode at which oxidation occurs is the anode. Electrons are produced here, thus the anode is labeled the negative electrode. The electrode at which reduction occurs is the cathode. Electrons are consume at the cathode and therefore labeled the positive electrode.
1. Electrons are produced at the zinc rod according to the oxidation half reaction, Zn(s) --> Zn 2+ (aq) + 2e - because it is oxidized, the zinc rod is the anode or negative electrode. 2. The electrons leave the zinc anode and pass through the external circuit to the copper rod. (If a bulb is in the circuit, it will light) 3. Electrons enter the copper rod and pass to copper ions in solution. There the following reduction half reaction occurs, Cu 2+ (aq) + 2e - --> Cu(s). 4. At the copper rod, copper ions are reduced. The copper rod is the cathode or positive electrode. 5. To complete the circuit, ions move through the aqueous solutions via the salt bridge or porous barrier to maintain the neutrality of each half cell. 6. The two half reaction can be summed to get the overall cell reaction, Zn(s) + Cu 2+ (aq) --> Cu(s) + Zn 2+ (aq).
A Dry Cell
Lead Storage Batteries
Cell Voltage/Potential Water spontaneously flows from a high position to a lower position. Water flows from a state of higher gravitational potential energy to a state of lower gravitational potential energy. As water flows downhill, it can do work, such as turning a water wheel or a turbine. The chemical changes that take place in galvanic cells are also accompanied by changes in potential energy. Electrons spontaneously flow from a position of higher potential energy at the anode to a position of lower potential energy at the cathode. The moving of electrons can do work, such as lighting a bulb or turning a motor. The voltage of a cell measures its tendency to do electrical work. The electrical potential of a voltaic cell is the ability of the cell to produce an electric current. The standard cell potential is a measure of: E O cell = E O cathode - E O anode For the cell example used above, the half-reactions and standard reduction potentials are found in your data booklet. This table lists standard reduction potentials. They are arranged in decreasing order. The half-reactions at the top of the table have a greater tendency to occur as reduction. The half-reactions at the bottom of the table have the least tendency to occur as reduction. The magnitude of the voltage is a numerical measure of the tendency for the oxidized species in the half-reaction to accept electrons in comparison to H + ions. If the sign if positive, the oxidized species have a greater tendency to accept electrons than do H + ions. If the sign is negative, the oxidized species have less tendency to accept electrons than do H + ions. Cu 2+ (aq) + 2e - --> Cu(s) E O = 0.342 V Zn 2+ (aq) + 2e - --> Zn(s) E O = -0.762 V You know that electrons flow to the copper, therefore it is the cathode and that electrons flow from the zinc, therefore it is the anode. Therefore the cell potential is: E O cell = 0.342 V - (-0.762 V) = 1.104 V The reverse half-reaction of a reduction is an oxidation. The half-reaction potential for an oxidation half-reaction is called an oxidation potential. If the reduction half-reaction is as follows: Zn 2+ (aq) + 2e - --> Zn(s) E O = -0.762 V
Then the oxidation half-reaction is: Zn(s) --> Zn 2+ (aq) + 2e - E O = +0.762 Therefore the cell potential can be calculated using another formula: E O cell = E O red + E O ox Cu 2+ (aq) + 2e - --> Cu(s) E O = 0.342 V Zn(s) --> Zn 2+ (aq) + 2e - E O = +0.762 E O cell = 0.342 V + 0.762 V = 1.104 V
Electrolytic Cells For a voltaic cell, you have learned that the overall reaction is spontaneous, and that they cell potential has a positive value. A voltaic/galvanic cell converts chemical energy to electrical energy. Electrons flow from a higher potential energy to a lower potential energy. It can be described as water flowing downhill Although water flows downhill spontaneously, you can also pump water uphill. This process requires energy because it moves water from a position of lower potential energy to a position of high potential energy. Electrolytic cells are electrochemical cells that use electrical energy (DC) to cause a chemical change to occur. Specifically they use energy from an external source to force a non spontaneous redox reaction to occur. The process, whereby electric energy is converted to chemical energy is called electrolysis. Examples of Electrolysis Decomposition of Water Two inert electrodes, such as platinum or graphite are immersed in water to which H2SO4(aq) has been added. The electrodes are connected to a direct current power source and the voltage gradually increased until a chemical reaction occurs. Electroplating This term applies to the process of coating an object with a thing layer of a metal such as silver, copper, chromium, etc. An electrical current is pass through an aqueous solution of salt which contains the ion of the metal to be deposited. The object to be plated is the cathode. The anode sometimes is composted of the metal being deposited.
The cathode is the copper spoon to be plated with silver. The anode is an inert bar of graphite. Both electrodes are immersed in an aqueous solution of 1.0 M silver nitrate.