Title: Lesson 6a Electrolytic Cells (HL) Learning Objectives: – Describe electrolytic cells – Identify at which electrode oxidation and reduction takes place – Understand how current is conducted in electrolytic cells – Deduce the products of electrolysis of an aqueous solution

Main Menu Refresh Consider the following standard electrode potentials. Zn 2+ (aq) + 2e –  Zn(s)E o = –0.76 V Cl 2 (g) + 2e –  2Cl – (aq)E o = V Mg 2+ (aq) + 2e –  Mg(s)E o = –2.37 V  What will happen when zinc powder is added to an aqueous solution of magnesium chloride? A. No reaction will take place. B. Chlorine gas will be produced. C. Magnesium metal will form. D. Zinc chloride will form.  Justify your answer.

Electrolysis of aqueous solutions (HL) Water itself can be oxidized or reduced so predicting products at the electrode is more tricky… So when a solute M + A - is in an aqueous solution, there is more than one redox reaction possible at each electrode: When there is more than one possibility, selective discharge comes into play. The outcome is determined by:

Electrolysis of Water As seen in chapter 8, ionization of pure water is extremely low, so it is not a good conductor of electricity. The addition of NaOH is added when this electrolysis is performed. 3 The overall balanced net ionic equation is: (balance cathode equation by multiplying by 2 to balance electrons transferred, add equations together, cancel out common species) 4H 2 O(l) + 4e - + 4OH - (aq)  2H 2 (g) +4OH - (aq) + 2H 2 O(l) + O 2 (g) + 4e - 2H 2 O(l)  2H 2 (g) + O 2 (g)

Electrolysis of NaCl(aq) NaCl(aq) is sometimes known as brine. Electrolysis leads to the production of H2(g), Cl2(g), and NaOH(aq), all of which are of commercial importance. When concentration of Cl - is low, H 2 O is oxidized. When concentration of NaCl is greater than 25% by the mass of solution, the Cl - is oxidized. Hence, saturated solutions of NaCl are used in industry. 3 The overall balanced net ionic equation (when Cl - is discharged) is: (add equations together, cancel out common species) 2H 2 O(l) + 2e - + 2Cl - (aq)  H 2 (g) +2OH - (aq) + Cl 2 (g) + 2e - 2H 2 O(l) + 2Cl - (aq)  H 2 (g) +2OH - (aq) + Cl 2 (g)

Main Menu Predict the products at the anode and cathode for electrolysis of the following aqueous solutions. For each one, write the overall equation for the reaction.  KCl  NiSO 4  PbI 2  ZnCl 2  LiOH

Electrolysis of CuSO 4 (aq) – Carbon/Inert electrodes Electrolysis of this solution yields different products depending on the nature of the electrodes Sulfates do not tend to oxidize. In sulfate, the oxidation state of sulfur is +6, corresponding to the stable noble gas core of [Ne] which it will not want to give up. 3 The overall balanced net ionic equation is: (multiply cathode equation to balance electrons transferred, add equations together, cancel out common species) 2Cu 2+ (aq) + 4e - + 2H 2 O(l)  4H + (aq) + O 2 (g) + 4e - +2Cu(s) 2Cu 2+ (aq) + 2H 2 O(l)  4H + (aq) + O 2 (g) + 2Cu(s)

Electrolysis of CuSO 4 (aq) – Copper electrodes Electrolysis of this solution yields different products depending on the nature of the electrodes Using copper instead of graphite electrodes means that the copper electrodes now participate in the electrolysis (they are termed active electrodes as opposed to inert electrodes).

Application of using copper electrodes Purification of copper Uses electrolysis of a solution containing Cu 2+ (aq) with an impure copper anode and a small pure copper cathode. As the cell runs, the anode erodes as copper is transferred to the cathode. Cathode increases in mass and impurities collect at the bottom of the cell.

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Factors affecting the amount of product in electrolysis Amount of product at each electrode is dependent on quantity of electric charge passed through the cell. Charge = Current x Time Where: Charge, Q measured in coulomb (C) Current, I measured in amperes (A) Time, t measured in seconds (s) The charge carried by one mole of electrons, known as a Faraday (F) is C.

You can see that the same quantity of electric charge produces twice as much copper from Cu + as from Cu 2+ … So, charge on ion influences amount of product!

Summary Amount of product formed in electrolysis is determined by: 1.Current 2.Time 3.Charge on the ion

Electroplating The process of using electrolysis to deposit a layer of metal on top of another metal or other conductive object. An electrolytic cell used for electroplating has the following features: An electrolyte containing the metal ions to be deposited; A cathode made of the object to be plated; Sometimes the anode is made of the same metal which is to be coated because it be oxidized to replenish the supply of ions in the electrolyte Reduction of the metal ions at the cathode leads to their deposition. Altering the current and time determines thickness. Purpose: Decorative (expensive or attractive metal coating) Corrosion control (sacrificial protection – zinc being oxidised instead of iron)

Summary of voltaic and electrolytic cells Voltaic cell chemical energy  electrical energy Positive value for E means the reaction proceeds towards equilibrium Spontaneous Electrolytic cell Electrical energy  chemical energy Negative value for E means the reaction is driven away from equilibrium by the electric current Non-spontaneous

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Main Menu Investigating Electroplating  In this short experiment, you will electroplate some small metal objects with a variety of different metals  Follow the instructions hereinstructions here