Electrolysis of Molten salts

Transcription

1 Electrolytic Cells In electrolysis we use electrical energy to bring about chemical change. An otherwise non-spontaneous reaction is driven in the forward direction by application of a voltage. Electrolytic Cell: Electrical energy from an external source drives a non-spontaneous reaction. Examples of electrolysis: 1. Recharging a secondary battery. 2. Electrolysis of molten salts. 3. The breakdown of water into O2 and H2 gases. 4. Electrolysis of aqueous salts. 5. Electroplating of metals. 6. Purification of metals. Voltaic Cell-Discharge The processes occurring during the discharge and recharge of a lead-acid battery. Electrolytic Cell-Recharge As with a voltaic cell, oxidation occurs at the anode and reduction takes place at the cathode. An external voltage source supplies the cathode with electrons, which is negative, and removes them from the anode, which is positive. Electrochemistry 1 Electrolysis of Molten salts Molten salts can be transformed into their elements by electrolysis, a process that, splits the ionic compound decomposing it to its elements. Inert electrodes are used in these cases. Conditions are non-standard! Standard Reduction Potentials do not apply! Not an aqueous system! Example: Electrolysis of molten NaCl. cathode Voltage Source anode What half reaction occurs: at the anode? + at the cathode? + What is the overall reaction with phase labels? (The melting point of NaCl is 801 C, the melting point of Na is 97.8 C.) Note the sign conventions Question: of anode and cathode. A mixture of KI and LiBr is melted and electrolyzed. What products are formed at the lowest applied voltage that will produce a reaction? What is the overall cell reaction? During electrolysis of a mixture of molten salts - the more easily oxidized species (stronger reducing agent) reacts at the anode. How do you decide? - the more easily reduced species (stronger oxidizing agent) reacts at the cathode. How do you decide? Electrochemistry 2

2 Electrolysis of Water Electrolysis of water decomposes (splits the water) into its elements. 2 H 2 O(l) > 2 H 2 (g) + O 2 (g) A 9 V battery is sufficient. An inert salt (one that will not react) must be present in the water to transport charge. Oxidation takes place at the anode: 2 H 2 O(l) > O 2 (g) + 4 H + (aq) + 4 e Reduction takes place at the cathode: 2 H 2 O(l) + 2 e > H 2 (g) + 2 OH (aq) a Anode Cathode In practice, the half-reaction voltages required are as follows and are not the same as in a table of SRP s: (-) Reduction: 2 H 2 O(l) + 2 e > H 2 (g) + 2 OH (aq) (E red V) E red (Actual Over voltage ) 1 V (+) Oxidation: 2 H 2 O(l) > O 2 (g) + 4H + (aq) + 4 e (E ox V) E ox (Actual Over voltage ) 1.4 V These are the voltages we will use when analyzing a aqueous system. What is the actual-voltage that must be applied (E cell ) for the electrolysis of water? Electrochemistry 3 Electrolysis of AQUEOUS Salts Under aqueous conditions we must include the possible oxidation or reduction of water as well as the salt ions. When an aqueous salt solution is electrolyzed: - The strongest oxidizing agent (most easily reduced) is reduced, and - The strongest reducing agent (most easily oxidized) is oxidized. Anode For example consider the electrolysis of 1 M NaI(aq): Possible cathode reactions: a) Na + + e - > Na(s) E red = V Wire mesh b) 2 H 2 O(l) + 2 e - > H 2 (g) + 2 OH E red -1 V (nonstandard) Possible anode reactions: a) 2 I > I 2 (s) + 2 e - E ox = V + (Zoom View) 1M NaI(aq) b) 2 H 2 O(l) > O 2 (g) + 2 H e - E ox -1.4 V (nonstandard) Determine the net redox reaction at the minimum voltage needed to produce a reaction. Electrodes are inert. Therefore, there is no oxidation of the anode electrode. Electrochemistry 4 What voltage is needed?

3 Electrolysis of AQUEOUS Salts Determine the products that will form, using the minimum voltage needed to produce a reaction, for electrolysis of 1 M SnCl 2 (aq) at 25 C, (-) All Possible Cathode Rxns: + 1 M SnCl2(aq) (+) All Possible Anode Rxns: Electrodes are inert. Therefore, there is no oxidation of the anode electrode. What is the net reaction and expected minimum voltage needed: Electrochemistry 5 Electrolysis of AQUEOUS Salts Write the balanced redox reaction that occurs at the minimum voltage needed to produce a reaction when the electrolysis of 1 M Cu(NO 3 ) 2 (aq) is conducted using: a) A Pt anode and a Pt cathode b) A Cu anode and Cu cathode Electrochemistry 6

4 Energy (J): 1 J = kg m 2 /s 2 Overview of Electrical Units Coulomb (C): fundamental unit of electric charge. One electron has a charge of 1.602x10-19 C. 1 C = 6.241x10 18 e = x10-5 mole e Voltage (V): the emf the electrons feel. 1 V = 1 J/C Faraday (F): Current (A): absolute charge of one mole of electrons. F = x10 4 C/mole the number of electrons that flow per second. ampere (A or Amp), 1 A = 1 C/s Work (w): Voltaic cell (system can do work): The maximum work that can be done: wmax = -nfecell = G ( units are J) Electrolytic cell (work done on system by Eexternal): w = nfeext ( units are J) Electrochemistry 7 Stoichiometry of Electrolysis Faraday s Law of Electrolysis states that the amount of substance produced at each electrode is directly proportional to the quantity of charge flowing through the cell. (This assumes 100% efficiency, not achievable.) AMOUNT (mol) of substance oxidized or reduced (or mol product) M (g/mol) balanced half-reaction AMOUNT (mol) of electrons transferred Faraday constant (9.6485x 10 4 C/mol e ) CHARGE (C) time (s) Recall: 1 Amp = 1 C/sec MASS (g) of substance oxidized or reduced (or g product) Practice Problem: How many minutes does it take to form 10.0 L of oxygen gas at 28 C and a total pressure of 1.00 atm by electrolysis of water if a current of 1.3 A is passed through the electrolytic cell. Assume 100% efficiency. The vapor pressure of water at 28 C is 28.3 torr. CURRENT (A) What mass of hydrogen gas forms? Electrochemistry 8

5 Electrolytic Refining (Purification) of Metals Example: Refining of Copper In the electrolytic refining of copper a current of 1.50 A is passed through the cell for 2.00 hours. If the cell operates at 75.0% efficiency, what mass of pure copper is obtained? Electrochemistry 9 Electroplating - Surface Coating of a Metal Ni(s) is oxidized at the anode. Ni 2+ (aq) is reduced at the cathode. Net Reaction? Practice Problem: A piece of steel is to be electroplated with nickel by the electrolysis of a NiSO 4 (aq) solution. a) If the process is 80.0% efficient, what mass of Ni is plated out if a current of 374 ma is maintained for 3.80 hours? b) If the cathode has a surface area of 100 cm 2, what average thickness of Ni metal was deposited? The density of Ni is 8.91 g/cm 3. c) How much work was done on the system if the external voltage was 5.76 V? During electrolysis, Ni atoms are transferred from the nickel anode to the steel cathode, plating the steel with a thin layer of nickel atoms. E cell = 0 V Only a small emf is needed! d) Is this amount of work done on the system equal in magnitude to the total energy lost by the voltage source? Explain your answer. Electrochemistry 10

6 Electrical Energy and Electrolysis Practice Problem: a) Calculate the total energy in Joules required to produce 1.00 kg of Mg from the electrolysis of molten MgCl 2 if the applied emf is 5.00 V. Assume that the process is 90% efficient. b) If the time to complete the process was 52.0 minutes, what was the average current flow during this time? c) Can Mg be produced by electrolysis of a 1 M MgCl2(aq) solution? Provide clear reasoning for your answer. Electrochemistry 11