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. 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.

Galvanic Cells cont…

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.