9.5 Electrolytic Cells

Match up the words with their descriptions Test for Oxygen Test for Hydrogen Electrolyte Relights a glowing splint Makes a ‘popping’ noise when lit The substance being broken down Splitting a substance using electricity Electrolysis Positive electrode Negative electrode Anode Cathode Anion Negative ion Positive ion Cation

Activity Each individual student has 5 sec to draw a part of a voltaic cell.

Salt bridge to allow flow of ions so there is no charge build up in either solution. Electrolyte : allows for charges to move from one electrode to the other. e- Power Source Anode: Oxidation occurs here Positive Electrode Anions are attracted to cathode Cathode: Reduction occurs here Negative Electrode Cations attracted to anode

Assessment Statement Describe, using a diagram, the essential components of an electrolytic cell State that oxidation occurs at the positive electrode (anode) and reduction at the negative electrode (cathode) Describe how current is conducted in an electrolytic cell Deduce the products of the elctrolysis of a molten salt.

Assessment Statement Describe, using a diagram, the essential components of an electrolytic cell

What is Electrolysis? Electrolysis is splitting up substances using electricity (Lysis is latin for splitting) Electric current ions Electric current is used to breakdown a substance made up of ions electrolyte substance being broken down An electrolyte is the name for the substance being broken down

An electrolytic cell is set up with a battery in order to force the movement of electrons through the solution Notice how the cathode and anode are different than in the Voltaic Cell Cathode = (-) Anode = (+)

Remember CATHODE REDUCTION occurs at this electrode ANODE OXIDATION occurs at this electrode

Electrolysis set up Cathode (-ve electrode) Anode (+ve electrode) Electrolyte: this is what will be electrolysed d.c. power supply

battery +- inert electrodes power source vessel e-e- e-e- conductive medium Cell Construction Sign or polarity of electrodes (-)(+)

Assessment Statement State that oxidation occurs at the positive electrode (anode) and reduction at the negative electrode (cathode) Describe how current is conducted in an electrolytic cell.

9.5.2 State that oxidation occurs at the positive electrode (anode) and reduction at the negative electrode (cathode) Electrolytic cells are the opposite of galvanic cells: They convert electrical energy into chemical potential energy. Electricity must be supplied in order to force a non-spontaneous reaction to occur. The spontaneity can be somewhat predicted based on the activity series. Current flows from the species being oxidized to the species being reduced.

Positive Anode Negative Is Cathode Don’t get stressed in the exam: Remember PANIC

9.5.2 State that oxidation occurs at the positive electrode (anode) and reduction at the negative electrode (cathode) A substance conducts electricity and is decomposed by the passage of electrical current is known as an electrolyte The process of decomposing such a material is known as electrolysis We still follow the RedCat/AnOx labelling method, but just as in the conductivity tester Anode = (+) = Oxidation (loss of e) Cathode = (-) = Reduction (gain of e)

9.5.3 Describe how current is conducted in an electrolytic cell. In order to test to conductivity of liquids, a conductivity tester can be used When the current is passed through a metal, it’s not effected

9.5.3 Describe how current is conducted in an electrolytic cell. A conductor is a substance that allows electricity to pass through it: Metals Graphite Aqueous solutions of acids, alkalis Ionic compounds when aqueous or molten An insulator is a substance that does NOT allow electricity to pass through it: Non-metallic elements (except graphite) Dry samples of covalent compounds Solid samples of ionic compounds

Assessment Statement Deduce the products of the elctrolysis of a molten salt.

When an electrical current passes through an ionic substance (molten or in solution), the compound undergoes chemical decomposition For a solution of PbBr 2 Notice the Anode is on the left At the ANODE: Br -1 (l)  ½Br 2 (g) + 1e -1 At the CATHODE: Pb 2+ (l) + 2e -1  Pb (l)

For a solution of NaCl Notice the Anode is on the right At the ANODE: Cl -1 (l)  ½Cl 2 (g) + 1e -1 At the CATHODE: Na + (l) + e -1  Na (l) Metals for cations and migrate toward the cathode Non-metals for anions and migrate toward the anode

+- battery Na (l) electrode half-cell Molten NaCl Na + Cl - Na + Na + + e -  Na2Cl -  Cl 2 + 2e - Cl 2 (g) escapes Observe the reactions at the electrodes NaCl (l) (-) Cl - (+)

+- battery e-e- e-e- NaCl (l) (-)(+) cathode anode Molten NaCl Na + Cl - Na + Na + + e -  Na 2Cl -  Cl 2 + 2e - cations migrate toward (-) electrode anions migrate toward (+) electrode At the microscopic level

Molten NaCl Electrolytic Cell Cathode half-cell (-) REDUCTION Na + + e -  Na Anode half-cell (+) OXIDATION2Cl -  Cl 2 + 2e - Overall cell reaction 2Na + + 2Cl -  2Na + Cl 2 X 2 Non-spontaneous reaction!

Electrolysis Apparatus Cathode (-ve electrode) power supply Anode (+ve electrode) Electrolyte

Electrolysis Apparatus Cathode (-ve electrode) Anode (+ve electrode) Electrolyte: this is what will be electrolysed power supply

Learning check in 2 things Write down 2 things you have learnt so far this lesson

Uses of Electrolysis Examples of electrolysis in action are: making hydrogen and oxygen gas from water 2H 2 O  2H 2 + O 2 2H+ + 2e-  H2 (Reduction at negative electrode/cathode) O 2-  O e- (Oxidation at positive electrode/anode) Making sodium and chloride from molten salt NaCl(l)  Na + Cl 2 Cathode: Na e-  Na Anode: 2Cl e -  Cl 2 Electroplating precious metals onto less expensive metals

Challenge Question How would a cell containing aqueous sodium chloride be different from molten sodium chloride? Write the chemical equations for each. This has come up in an IB question before (even though the electrolysis of aqueous solutions is not in the IB syllabus)

+- Zn + Cl - Zn + positive electrode negative electrode molten zinc chloride (ions free to move) (anode) (cathode)

+- Zn + Cl - Zn + The positively charged zinc ions are attracted to the negative electrode

+- Zn + Cl - Zn + The negatively charged chloride ions are attracted to the positive electrode

True or False Electrolysis is the splitting of a substance using electricity. The anode is negative. Anions are negative Aluminium oxide is found in bauxite. Cathode is negative. Cations are negative.

CompoundAnodeCathode Sodium Bromide Potassium Iodide Calcium Fluoride Magnesium Oxide Lithium Chloride

VoltaicElectrolytic Comparison of Electrochemical Cells

galvanicelectrolytic need power source two electrodes produces electrical current anode (-) cathode (+) anode (+) cathode (-) salt bridge vessel conductive medium Comparison of Electrochemical Cells spontaneous non-spont.

Voltaic/Galvanic Cells Electrolytic Cells anode oxidation occurs here negativeoxidation occurs here positive cathode reduction occurs here positivereduction occurs here negative

1. One person names a topic. 2. All pupils write a key word to do with the topic on a whiteboard 30 secondsname as many key words 3. The person who chose the topic has 30 seconds to name as many key words to do with the topic as they can. sit down 4. If the word the pupil has written down is said they have to sit down. 5. The aim for the topic chooser is to get everyone to sit down. 6. The aim for the pupils writing down a word is to out fox the topic chooser with a relevant but obscure word.