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Chapter 26 – Electricity from Chemical Reactions. Galvanic Cells. An electric current flows through the wire and light globe. This part of the cell is called the external circuit. The current flows because a chemical reaction is taking place in the cell.
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Galvanic Cells • An electric current flows through the wire and light globe. • This part of the cell is called the external circuit. • The current flows because a chemical reaction is taking place in the cell. • Initially, there is little indication that a reaction is taking place in the beakers. • However, it it left for several hours: • the zinc metal corrodes • The copper metal is covered with a furry deposit • The blue copper sulfate solution loses some colour.
Galvanic Cells cont… • If the light globe is replaced with a galvanometer, the galvanometer will indicate that electrons flow from the zinc electrode, through the wire and to the copper electrode. • Current flows only if a salt bridge is present. • These observation provide information on what is happening in the cell: • The reaction is a redox reaction • The zinc electrode is eating away, forming zinc ions in solution • The oxidation of the zinc metal releases electrons, which flow to the copper electrode • Electrons are accepted by the copper ions in solution when they collide with the copper electrode. • Copper acts as the oxidant and zinc as the reductant.
Why is electrical energy released? • A galvanic cell is designed so that the two half cells are totally separated the each other. • Because the oxidant and reductant do not come into contact, electrons can only be transferred through an external circuit. • This flow creates an electrical current. • Chemical energy of the reactants is transferred into electrical energy.
Half Cells • The species in each half cell form a conjugate redox pair. • In general, if one member of the conjugate pair in a half cell is a metal, it is usually used as the electrode. • If not metal is present, than an inert electrode such as platinum is used. • If one of the conjugate pairs is a gas, a special gas electrode is used. • Half cells also usually contain spectator ions and that particles that make up the solvent. • The electrode where oxidation occurs is called an anode. In galvanic cells the anode, where electrons are produced, is the negative terminal. • The electrode at which reduction occurs is called the cathode. In galvanic cells the cathode, where electrons are consumed, is the positive terminal.
Salt Bridge • If there was no salt bridge, the solution in one half cell would accumulate negative charge and the other a positive charge. • These accumulations would prevent further reaction and prevent the production of electricity. • The salt bridge contains ions that are free to move so they can balance the charges formed. • Cations move to the cathode and anions move to the anode. • This is called the internal circuit.
The Electrochemical Series • The electrochemical series is valid only for the conditions under which it was determined. • The series that you will be supplied with applies at a temperature of 25°C, a pressure of 1 atm and 1M concentration of solutions. • When reading the electrochemical series, the higher one is written forwards and undergoes reductions, while the lower one is written in reverse and undergoes oxidation.
Potential Difference • A current flows in these cells because one half cell has a greater tendency to push electrons into the external circuit than the other half. Chemists say that there is a potential difference between the two half cells. • The potential difference is sometimes called the emf. • The potential difference is measured in volts and measures the amount of energy supplied by a fixed amount of charge flowing from a galvanic cell. • An indication of the cell voltage can be found using the electrochemical series.
Potential Difference cont… • Potential differences of cells at standard conditions can be found using the E° values in the electrochemical series. • These standard half cell potentials give a numerical measure of the tendency of a half cell reaction to occur as a reduction reaction. • E° values are measured by connecting two half cells to a reference cell. • A hydrogen half cell is used for this purpose and its E° is assigned as a 0. • The potential difference of a cell at standard conditions is the difference between the E° values of its two half cells. Cell potential difference = higher half cell E° - lower half cell E°
Predicting Direct Redox Reactions • in a galvanic cell, the higher half reaction in the electrochemical series undergoes reduction in the forward direction and the lower reaction undergoes oxidation in the reverse. • This principle applies equally to redox reactions that occur when reactants are mixed directly.
Limitations of Predictions • The standard half cell potentials are measured under standard conditions and as such it cannot be assumed that these figures are correct if the temperature were to be changed. • The electrochemical series also does not given an indication of the rate of reactions.
