Today s Objectives: and an electrolytic cell. standard cell potentials. Section 14.3 (pp )

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1 Today s Objectives: 1. Identify the similarities and differences between a voltaic cell and an electrolytic cell 2. Predict the spontaneity of REDOX reactions based on standard cell potentials. 3. Recognize that predicted reactions do not always occur. Section 14.3 (pp )

Electrochemical Cells Voltaic Cell electrochemical cell with a spontaneous reaction (E cell > 0) SOA above SRA on the activity series Standard cell used for scientific research Electrolytic cell

2 Electrochemical Cells Voltaic Cell electrochemical cell with a spontaneous reaction (E cell > 0) SOA above SRA on the activity series Standard cell used for scientific research Electrolytic cell electrochemical cell with a non-spontaneous reaction (E cell < 0) supplying electrical energy to a non-spontaneous cell forces a reaction to occur Example: zinc sulfate and lead solid cell (Figure 1 p. 639) SOA below SRA useful for producing substances, particularly elements, such as the industrial production of zinc metal using a cell similar to the zinc sulfate cell (Figure 1 p. 639)

spontaneous cell) electric energy from the external power source acts as an electron pump that is used to do work on the electrons

3 Electrolytic Cells Consists of two electrodes, an electrolyte, and an external power source Uses the decomposition process of electrolysis where supplying electricity forces a non-spontaneous REDOX reaction to occur (reverse chemical reaction of a spontaneous cell) electric energy from the external power source acts as an electron pump that is used to do work on the electrons and cause a transfer of electron Figure 1 p. 639 Electrons are pulled from the anode and pushed to the cathode by the battery or power supply.

4 Comparing Electrochemical Cells p. 640 Think of positive and negative for electrodes as labels, not charges.

5 Secondary Cell Rechargeable cell that illustrate the difference between electric and electrolytic cells functions as an electric cell as it discharges, spontaneously producing electrical energy functions as an electrolytic cell when recharging as electrical energy forces the products to react and reform the original reactants (reverse electrochemical process)

6 Potassium-Iodide Electrolytic Cell litmus paper only turns blue near the electrode where gas evolves. a dark precipitate forms at the other electrode as well as a yellow-brown colored substance that produces a purplish-red color in the halogen test (see p. 805) non-spontaneous reaction water is the SOA (greatest attraction for electrons) gains electrons from the power source (negative terminal) iodide ions are the SRA (loses electrons at anode) Note: same theoretical definition for cathode and anode as voltaic cells p. 805

electrolytic cell? Identify major entities and identify the SOA and SRA.

7 Sample Problem 14.1 p. 642 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. Note: water is often a reactant in aqueous electrolytic cells Write the half-reaction equations and calculate the cell potential. State the minimum electric potential (voltage) to force the reaction to occur. ( ) cell potential indicates that the chemical process is non-spontaneous i.e. requires supply of elections with a minimum of +1.37V from an external power sources to force the cell reactions

Procedure for Analyzing Electrolytic Cells (p. 630) Very similar to analyzing voltaic cells Use the REDOX table to identify the SOA and SRA Remember to consider water for aqueous electrolytes.

8 Procedure for Analyzing Electrolytic Cells (p. 630) Very similar to analyzing voltaic cells Use the REDOX table to identify the SOA and SRA Remember 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. E cell = E r, cathode E r, anode If required, state the minimum electric potential (voltage) to force the reaction to occur. (The minimum voltage is the absolute value of E cell ) To draw a diagram, if requested, add specific labels for chemical entities using the general outline in Figure 6 p. 642

at the cathode and anode, and in the overall cell.

9 Communication Example 1 p. 643 An electrolytic cell containing cobalt(ii) chloride solution and lead electrodes is assembled. The notation for the cell is as follows: a) Predict the reactions at the cathode and anode, and in the overall cell. b) Draw and label a cell diagram for this electrolytic cell, including the power supply. c) What minimum voltage must be applied to make this cell work? a) b) c)

10 Communication Example 2 p. 644 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.

experimental analysis Consider the electrolysis of water SOA H 2 O (l)

11 Hoffman Apparatus (Figure 4 p. 662 ) Used to collect any gas that may be produced in an electrolysis experimental analysis Consider the electrolysis of water SOA H 2 O (l) inert salt to conduct electrical current SRA + Water Electrolysis Demo with a Diagram Simple Electrolysis Apparatus

12 Chloride Anomaly Predict the products of an electrolytic cell containing an aqueous solution of sodium chloride. Expect the electrolysis of water to occur However, experimental evidence indicates dissolved chlorine is present at the anode in the final solution, not oxygen gas as predicted. halogen diagnostic test

Chloride Anomaly using the REDOX table to predict reactions using the SOA and SRA does not always in electrolytic cells In the example of sodium chloride electrolysis, water should react at the anode

13 Chloride Anomaly using the REDOX table to predict reactions using the SOA and SRA does not always in electrolytic cells In the example of sodium chloride electrolysis, water should react at the anode as the SRA, however chloride ions react instead of water molecules The actual reduction potential required for a particular half-reaction and the reported half-reaction reduction potential may be quite different (depending on the conditions or half-reactions) This difference is known as the half-cell overvoltage, which creates an exception to the generalization previously made. Explaining overvoltage is well beyond the scope of this course As an empirical rule, recognize that chlorine gas is produced instead of oxygen gas in situations where chloride and water are the only reducing agents present i.e. this anomaly occurs during the electrolysis of solutions containing chloride ions.

14 Electrolytic Cells Summary p. 645 based on reactions that are non-spontaneous (negative E cell) a minimum applied voltage of at least the absolute value of E cell is required to force the reactions to occur SOA undergoes reduction at the cathode (negative electrode) SRA undergoes oxidation at the anode (positive electrode) Electrons are forced to travel from the anode to the cathode through the external circuit by a power supply. Internally, anions move toward the anode and cations move toward the cathode. +

15 Applications of Electrolysis In the electrolysis of molten salt, metal cations are reduced to metal atoms at the cathode and nonmetal anions are oxidized at the anode reduction potentials on the REDOX table are for standard aqueous solutions do not include molten (liquid) salts Electrorefining is a process used to obtain high grade metals at the cathode from an impure metal at the anode. Electroplating is a process in which a metal is deposited on the surface of an object placed at the cathode of an electrolytic cell.

16 Homework Practice Questions p. 640 #1-4 Practice Questions p. 644 #5-6 Investigation 14.5 (p. 644) - predictions Practice Questions p. 645 #7-15 Read pp Practice Questions p. 649 #16-21 Section 14.3 Review Questions p. 651 #1-16 Section 14.3 Extra Exercises worksheet

11.3. Electrolytic Cells. Electrolysis of Molten Salts. 524 MHR Unit 5 Electrochemistry

11.3. Electrolytic Cells. Electrolysis of Molten Salts. 524 MHR Unit 5 Electrochemistry 11.3 Electrolytic Cells Section Preview/ Specific Expectations In this section, you will identify the components of an electrolytic cell, and describe how they work describe electrolytic cells using oxidation