Unit 2: Electrochemistry Electrolysis

Electrolytic Cells The term “electrochemical cell” is often used to refer to a: Electrolytic cell – one with a nonspontaneous reaction SOA below SRA – i.e. zinc sulfate and lead solid cell Eocell less than zero= nonspontaneous vs Galvanic Cell – one with a spontaneous reaction SOA over SRA on the activity series Eocell greater than zero = spontaneous Why would anyone be interested in a cell that is not spontaneous? This would certainly not a good battery choice, but by supplying electrical energy to a nonspontaneous cell, we can force this reaction to occur. This is especially useful for producing substances, particularly elements. I.e. the zinc sulfate cell discussed above is similar to the cell used in the industrial production of zinc metal.

An electrolytic cell is a device in which an external source of electrons (electrical circuit) is used to make a non-spontaneous redox reaction take place.

Electrolysis of water 2H2O → 2H2 + O2 Anode: 2H2O→ O2 + 4H+ + 4 e- Cathode: 4H2O + 4e- → 2H2 + 4OH- 6H2O → 2H2 + O2 + [4 H+ + 4OH-] 2H2O → 2H2 + O2

Electrolytic Cells Electrolytic Cell –. The external power source acts as an “electron pump”; the electric energy is used to do work on the electrons to cause an electron transfer Electrons are pulled from the anode and pushed to the cathode by the battery or power supply

Oxidation at anode 2Cl- Cl2 + 2e- Reduction at cathode Na+ + 1e- Na Molten (liquid) state of a pure substance can be the medium of the cell Eg: molten NaCl Oxidation at anode 2Cl- Cl2 + 2e- Reduction at cathode Na+ + 1e- Na

E0net = E0ox + E0red = (-1.36 V) + (-2.71 V) = -4.07 V The net cell voltage of -4.07 Because it is a non-spontaneous reaction the E0net is always less than zero.

Electrolysis of Molten NaCl (Down’s cell) (anode):  2 Cl- → Cl2 + 2 e- (cathode):  2Na+ + 2e- → 2Na 2 NaCl(l) → 2 Na(l) + Cl2(g) 2 NaCl(l)        

Molten NaCl Electrolytic Cell cathode half-cell (-) REDUCTION Na+ + e-  Na anode half-cell (+) OXIDATION 2Cl-  Cl2 + 2e- overall cell reaction 2Na+ + 2Cl-  2Na + Cl2 X 2 Non-spontaneous reaction!

What chemical species would be present in a vessel of aqueous sodium chloride, NaCl (aq)? H2O Will the half-cell reactions be the same or different?

Aqueous NaCl Electrolytic Cell possible cathode half-cells (-) REDUCTION Na+ + e-  Na 2H20 + 2e-  H2 + 2OH- possible anode half-cells (+) OXIDATION 2Cl-  Cl2 + 2e- 2H2O  O2 + 4H+ + 4e- overall cell reaction 2H2O → 2H2 + O2

Procedure for Analyzing Electrolytic Cells Use the redox table to identify the SOA and SRA Don’t forget to consider water for aqueous electrolytes. Write equations for the reduction (cathode) and oxidation (anode) half- reactions. Include the reduction potentials if required. Balance the electrons and write the net cell reaction including the cell potential. E0 cell = E0r cathode - E0r anode If required, state the minimum electric potential (voltage) to force the reaction to occur. (The minimum voltage is the absolute value of E0 cell) If a diagram is requested, use the general outline in Figure 6, and add specific labels for chemical entities.

Analyzing Electrolytic Cells #1 Example: What are the cell reactions and the cell potential of the aqueous potassium iodide electrolytic cell? Identify major entities and identify the SOA and SRA. Write the half-reaction equations and calculate the cell potential. State the minimum electric potential (voltage) to force the reaction to occur. Electrons must by supplied with a minimum of +1.37 V from an external battery or other power supply to force the cell reactions.

Potassium-Iodide Electrolytic Cell In the potassium iodide electrolytic cell, litmus paper does not change colour in the initial solution and turns blue only near the electrode from which gas bubbles. Why? At the other electrode, a yellow-brown colour and a dark precipitate forms. The yellow brown substance produces a purplish-red colour in the halogen test (pg. 805). Why?

Analyzing Electrolytic Cells #2 Example: An electrolytic cell containing cobalt(II) chloride solution and lead electrodes is assembled. The notation for the cell is as follows: Predict the reactions at the cathode and anode, and in the overall cell. Draw and label a cell diagram for this electrolytic cell, including the power supply. What minimum voltage must be applied to make this cell work?

Analyzing Electrolytic Cells #3 Example: An electrolytic cell is set up with a power supply connected to two nickel electrodes immersed in an aqueous solution containing cadmium nitrate and zinc nitrate. Predict the equations for the initial reaction at each electrode and the net cell reaction. Calculate the minimum voltage that must be applied to make the reaction occur.

Electrolytic Cells Summary: An electrolytic cell is based upon a reaction that is nonspontaneous; the Eocell for the reaction is negative. An applied voltage of at least the absolute value of Eocell is required to force the reactions to occur. The SOA undergoes reduction at the cathode (- electrode) The SRA undergoes oxidation at the anode (+ electrode) Electrons are forced by a power supply to travel from the anode to the cathode through the external circuit. Internally, anions move toward the anode and cations move toward the cathode

Electroplating Electroplating is a procedure that uses electrolysis to apply a thin layer of a metal over the surface of another metal.

In electroplating, the anode is made up of the metal you want to coat the surface of another metal with. There is also a salt solution present of the anode metal. While electrolysis is taking place, the anode metal is oxidized and goes into solution as positive ions. These positive ions are then reduced on the surface of the cathode (the metal you wish to coat).

Examples of electroplating Coating jewlery with thin layer of expensive metal. Coating chromium over steel to make rust resistant.

SAMPLE PROBLEM 1 A spoon is to be plated with silver, Ag. Identify the anode and cathode. Write an equation for the reaction taking place at the anode and at the cathode and indicate whether it is oxidation or reduction. What electrolyte is used?

ANSWER 1. Cathode = spoon (metal object to be coated), Anode = silver electrode 2. As electrolysis takes place, the silver anode is oxidized, Ag(s) → Ag+(aq) + 1e- The Ag+ (aq) ions in solution travel to the spoon cathode and are reduced to form neutral Ag(s) on the surface of the spoon (cathode): Ag+ (aq) + 1e- → Ag(s)