Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) Refresh A particular voltaic cell is made from magnesium and iron half- cells. The overall equation for the reaction occurring in the cell is Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) Which statement is correct when the cell produces electricity? Magnesium atoms lose electrons. The mass of the iron electrode decreases. Electrons flow from the iron half-cell to the magnesium half-cell. Negative ions flow through the salt bridge from the magnesium half-cell to the iron half-cell. For each incorrect statement, explain why it is wrong.

Lesson 6-7 Electrolytic cells

Lesson 6-7: Electrolysis 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 a molten salt

Li+(l) + Cl-(l)  Li(s) + ½Cl2(g) Electrolytic Cells Electrolytic cells use electricity to provide the energy for an endothermic redox reaction Electrolysis of lithium chloride: Li+(l) + Cl-(l)  Li(s) + ½Cl2(g) Current is carried by moving ions Cations (+) move to the cathode (negative electrode) Anions (-) move to the anode (positive electrode) As ions need to be able to move, the ionic compound must be either: Molten Dissolved in solution The opposite of a voltaic cell: Voltaic cells turn stored chemical energy into electrical Electrolytic cells turn electrical energy into stored chemical energy

Cations move to cathode An Electrolytic Cell: e- e- Bubbles of gas formed ANODE (+) CATHODE (-) X X M+ MOLTEN SALT or SALT SOLUTION M+ M Layer of metal formed M X- M+ M X- X- Anions move to anode Cations move to cathode

Products of Electrolysis Assuming a molten simple metal salt involving only monatomic ions Cathode – metal Deposited on the surface of the electrode Anode – non-metal Typically as bubbles of gas For example, electrolysis of molten magnesium bromide: Cathode: a layer of magnesium metal (Mg2+ + 2e-  Mg) Anode: bubbles of bromine gas (2Br-  Br2 + 2e-) More complicated systems Aqueous solutions – a range of possibilities depending on the stability of the ions relative to water Polyatomic ions – a mixture of various products

By the end of the next lesson you should: Complete the electrolysis of molten zinc chloride in the fume hood. Instructions here: http://www.nuffieldfoundation.org/practical-chemistry/electrolysis-zinc-chloride Produce an animation showing how electrolysis happens. Could be PowerPoint, flicker book, smart phone animation app, stop-motion (see: http://www.wikihow.com/Create-a-Stop-Motion-Animation )...be creative. Draw a Venn diagram comparing and contrasting electrolytic and voltaic cells Write three potential internal-assessment research questions and on electrolytic or voltaic cells, select one and start producing a plan for it.

Key Points Electrolysis uses electricity to drive endothermic redox reactions Metal salts must be molten or dissolved so ions can move and carry charge The negative cathode reduces positive metal ions The positive anode oxidises negative non-metal ions