How does electrolysis separate hydrogen and oxygen? How is electrolysis used to extract aluminium? How do electrochemical cells relate to electrolysis? What happens on the cathode during the electrolysis of a copper II sulfate solution? How can I extract sodium and chloride from salt? Why is Delta G negative for electrolysis reactions? If a current flows through the substance when it is molten or dissolved, it is an electrolyte.
Electrolysis Ionic substances contain charged particles called ions. This is what happens during electrolysis: Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised. The number of moles of electrons can be related to the reactants and products using stoichiometry.
The total charge Q , in coulombs is given by. Where t is the time in seconds, n the number of moles of electrons, and F is the Faraday constant. Moles of electrons can be used in stoichiometry problems. The time required to deposit a specified amount of metal might also be requested, as in the second of the following examples.
Converting Current to Moles of Electrons In one process used for electroplating silver, a current of How many moles of electrons passed through the cell? What mass of silver was deposited at the cathode from the silver nitrate solution? The charge is the current I multiplied by the time. From the problem, the solution contains AgNO 3 , so the reaction at the cathode involves 1 mole of electrons for each mole of silver.
Check your answer: From the stoichiometry, 1 mole of electrons would produce 1 mole of silver. Less than one-half a mole of electrons was involved and less than one-half a mole of silver was produced. Check Your Learning Aluminum metal can be made from aluminum ions by electrolysis.
What is the half-reaction at the cathode? What mass of aluminum metal would be recovered if a current of 2. Time Required for Deposition In one application, a 0. How long would it take to deposit the layer of chromium if the current was The density of chromium metal is 7. Solution This problem brings in a number of topics covered earlier. An outline of what needs to be done is:. Solving in steps, and taking care with the units, the volume of Cr required is.
Cubic centimeters were used because they match the volume unit used for the density. The amount of Cr is then. Since the solution contains chromium III ions, 3 moles of electrons are required per mole of Cr. The total charge is then. Check your answer: In a long problem like this, a single check is probably not enough. Each of the steps gives a reasonable number, so things are probably correct. Pay careful attention to unit conversions and the stoichiometry.
Check Your Learning What mass of zinc is required to galvanize the top of a 3. If the zinc comes from a solution of Zn NO 3 2 and the current is The density of zinc is 7. Using electricity to force a nonspontaneous process to occur is electrolysis. Electrolytic cells are electrochemical cells with negative cell potentials meaning a positive Gibbs free energy , and so are nonspontaneous. Electrolysis can occur in electrolytic cells by introducing a power supply, which supplies the energy to force the electrons to flow in the nonspontaneous direction.
Electrolysis is done in solutions, which contain enough ions so current can flow. If the solution contains only one material, like the electrolysis of molten sodium chloride, it is a simple matter to determine what is oxidized and what is reduced. In more complicated systems, like the electrolysis of aqueous sodium chloride, more than one species can be oxidized or reduced and the standard reduction potentials are used to determine the most likely oxidation the half-reaction with the largest [most positive] standard reduction potential and reduction the half-reaction with the smallest [least positive] standard reduction potential.
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