3 Electrochemical cells

Electrochemical cells •A cell has two half–cells. •The two half cells have to be connected with a salt bridge. •Simple half cells will consist of a metal (acts an electrode) and a solution of a compound containing that metal (eg Cu and CuSO4 ). •These two half cells will produce a small voltage if connected into a circuit. (i.e. become a Battery or cell). Salt Bridge The salt bridge is used to connect up the circuit. The free moving ions conduct the charge. A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually Potassium Nitrate. The salt should be unreactive with the electodes and electrode solutions.. E.g. potassium chloride would not be suitable for copper systems as Chloride ions can form complexes with copper ions. A wire is not used because the metal wire would set up its own electrode system with the solutions. Why does a voltage form? In the cell pictured above When connected together the zinc half-cell has more of a tendency to oxidise to the Zn2+ ion and release electrons than the copper half-cell. (Zn
Zn2+ + 2e- ) More electrons will therefore build up on the zinc electrode than the copper electrode. A potential difference is created between the two electrodes. The zinc strip is the negative terminal and the copper strip is the positive terminal. This potential difference is measured with a high resistance voltmeter, and is given the symbol E. The E for the above cell is E= +1.1V. Why use a High resistance voltmeter? The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit. In this state it is possible to measure the maximum possible potential difference (E). The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing. What happens if current is allowed to flow? If the voltmeter is removed and replaced with a bulb or if the circuit is short circuited, a current flows. The reactions will then occur separately at each electrode. The voltage will fall to zero as the reactants are used up. The most positive electrode will always undergo reduction. Cu2+ (aq) + 2e-
Cu(s) (positive as electrons are used up) The most negative electrode will always undergo oxidation. Zn(s)
Zn2+ (aq) + 2e- (negative as electrons are given off) N Goalby chemrevise.org Zinc electrode copper electrode 1M zinc sulphate solution 1M copper sulphate solution Salt bridge Electron flow