La cellule de Daniell

Mettre en place des cellules galvaniques de cuivre et de zinc avec deux concentrations différentes de solutions de sel métallique, et mesurer leur différence de potentiel.

Ce que les élèves apprennent

Les fondements scientifiques de cette expérience

Matériel utilisé

Produits chimiques et réactifs

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Questions destinées aux élèves

À quoi s'attendre

<p>The voltage shown is between 1.09 and 1.1 V in each experiment even if the concentrations are different.</p><p>Cu2++2e – → Cu + 0.34 V Zn2+ + 2e –→ Zn – 0.76 V.</p><p>The tendency of the zinc to lose electrons and produce ions is stronger than that of the copper, so that on the zinc electrode there will be a larger concentration of electrons.</p><p>On completing the circuit, electrons will flow from the metal with the higher concentration (potential) to the metal with the lower concentration.</p><p>Zn → Zn2+ + 2e – (oxidation process; anode –) Cu2++2e – → Cu (reduction process, cathode +).</p><p>It is important to remember that cathode is where reductions occur while anode is where oxidations occur.</p><p>In these cells, we use a chemical process to obtain an electric current. There is another kind of cell, the electrolytic cell, in which the application of a direct voltage produces a chemical process. The electrodes are still called anode and cathode but, in this case, the anode (where the reaction is still the oxidation process) is the positive pole, while the cathode (where the reaction is still reduction) is the negative pole.</p><p>When a lead connects the two electrodes, electrons can travel through it (ie there is a current) from the zinc and reach the copper electrode to neutralize its positive charges (reduction of copper ions and production of metallic copper).</p><p>Since electrons are subtracted from the zinc electrode, to restore equilibrium new zinc ions will go into solution whereas.</p><p>The reaction would soon stop because the zinc solution would become increasingly positive and the copper solution increasingly negative but the salt bridge allows electrons to flow between the two solutions.</p><p>Because of these reactions, the zinc electrode (the negative one where there is the oxidation process also called anode) gradually becomes thinner while the copper electrode (the positive one where there is the reduction process also called cathode) becomes thicker.</p><p>Like any battery, this type of battery has a limited life. The electrodes undergo chemical reactions that block the flow of electricity. The electromotive force diminishes and the battery stops working. Usually, what happens is that the production of hydrogen at the copper electrode and the zinc electrode acquiring deposits of oxides both act as barriers between the metals and the electrolytes. This is referred to as the electrodes being polarized. To achieve a longer life and higher voltages and current flows, it is necessary to use electrolytes better suited for the purpose. Commercial batteries, apart from their normal electrolyte, contain chemicals with an affinity for hydrogen which combine with the hydrogen before it can polarize the electrodes.</p>

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