9.4 – Voltaic Cells
9.4.1 – Explain how a redox reaction is used to produce electricity in a voltaic cell
In a half-cell, a metal electrode is placed in an aqueous solution of its ions. Two half cells are connected to form a voltaic cell, allowing electrons to flow during the redox reaction and produce electrical energy. The half-cells are joined by a wire to transfer electrons and a salt bridge to transfer ions.
The direction of electron flow is determined by the reactivity of the metals that form the half cells. In the diagram below, the electrons flow from the zinc half-cell to the copper half-cell because zinc is located higher up the reactivity series than copper. In any voltaic cell, the voltage produce during the reaction will depend on the difference in reactivity between the two metals.
In this setup, the anode is sitting in a solution of Zn(NO3)2 and the cathode is in a solution of Cu(NO3)2. As the reaction proceeds, the NO3– ions in the copper cell flow up into the salt bridge while the Cu2+ ions attach to the cathode as the electrons flow down, maintaining charge balance in the cell. The NO3– ions replace those moving into the zinc solution to join the Zn2+ ions being produced. Hence, the salt bridge is essential for maintaining each half-cell as electrically neutral. The salt usually used in KNO3 because it will not react with any other substances in the cells.
9.4.2 – State that oxidation occurs at the negative electrode (anode) and reduction occurs at the positive electrode (cathode)
The metal that is more reactive will form the anode, since electrons flow from it to the other cell. The electrons are lost from the electrode in an oxidation reaction. The ions formed as the electrons leave the electrode then move into the solution.
On the other hand, the metal that is less reactive will receive the electrons, forming the cathode. It accepts the electrons in a reduction reaction. The ions in the solution will combine with the electrons to form solid metal on the electrode.