C6.2 Changes at the electrodes Pages 104-105 Peer-assessed levelled assessment – text books are needed for this.

Specification link AQA Specification links 4.3.1 When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes. Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis. 4.3.4 The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved. At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen. At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced. This happens because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged.

Big Q: What changes happen at each electrode? Saturday, 27 April 2019Saturday, 27 April 2019 Big Q: What changes happen at each electrode? LO what happens to the ions during electrolysis how water affects the products of electrolysis how we can represent the reactions at each electrode using half equations. HIGHER

Electrolysis of Lead Bromide Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis: Pb2+ ions gain electrons at the cathode and become Pb atoms Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules So lead forms at the negative electrode and bromine forms at the positive electrode. Electrolysis of Lead Bromide

Effect of water and halide groups Halides H+ and OH- ions Hydrogen needs to be less reactive to form Hydrogen gas forms at cathode Oxygen gas forms at anode Halogen (Cl-) Forms as gas at anode Halide > hydroxide > other negatively charged ions

Changes at the electrodes – a levelled assessment Changes at the electrodes – a levelled assessment. Work through the levels, starting at the lowest one. Swap books when your teacher tells you and write a WWW/ EBI using the grading grid for help Level Success criteria Tick 4 State why ions move to the electrodes during electrolysis (figure 1 will help- you could draw this and describe it) State what happens to ions when they reach the electrodes 5 Describe clearly what reduction and oxidation are in relation to electrolysis and use the example of lead bromide given to illustrate (be careful, it does NOT involve oxygen!). Find out what OILRIG stands for, write it down, highlight it (and remember it!). What do we mean by a half equation in electrolysis? [H] 6 Describe what the effect of water has on electrolysis of aqueous solutions. Make sure you include how reactivity of elements is involved. You should draw a diagram to help you with this. 7 Describe what halides are (you may need a periodic table to help you here). And describe their effect on electrolysis. Explain what a half equation is. [H] Write step by step instructions on how to write a half equation. [H] 8 Draw the table in your book and balance the half equations at the bottom of the help sheet.[H]

Progress check

Homework – Electrolysis of aqueous solutions 1. https://www.youtube.com/watch?v=6WjC_Vi4roA 2. https://www.youtube.com/watch?v=mL7mkqyLpSo Copper Sulfate Solution Elements involved: Sodium Chloride Solution Diagram: What is produced at the Anode? What is produced at the Cathode?