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REDOX (REDUCTION & OXIDATION) II 08 OCTOBER 2013 Lesson Description In this lesson, we: Revise Oxidation Numbers. Write half-reactions to show Reduction and Oxidation. Combine half-reactions to make net-ionic reactions. Key Concepts Revision of Oxidation Numbers: Oxidation numbers tell us the number of electrons that have been removed. Oxidation is the loss of Electrons. In the formula MgO we could see this: Mg loses 2 electrons. This means that the oxidation number is +2. What about oxygen? O gains two electrons. This means that the oxidation number is -2. What are half-reactions? Half-reactions tell the same story as a normal equation but in two parts. Let's look at the example from earlier: 2Mg + O2 -> 2MgO 2+ The magnesium atom turned from an element into a Mg ion in the product. It also lost two electrons: Mg 2+ -> Mg - + 2e So we can see that electrons came out of the reaction and this is oxidation (electrons are lost!). Now let's do the same for the oxygen: - O2 + 4e -> 2O 2- Notice how the electrons are on the left? This is because they are going into the reaction. This is reduction. Can half-reactions combine to make a whole? You may have guessed that half-reactions are exactly that, half the story. We can combine them again. Step 1: Write out the reactions. 2+ Mg -> - O2 + 4e - Mg + 2e (oxidation) -> 2O (reduction) 2- Step 2: Balance the electrons to cancel them out. 2+ 2Mg -> 2Mg - O2 + 4e - + 4e (oxidation) 2- -> 2O (reduction) Step 3: Write them together 2Mg + O2 -> 2+ 2- 2Mg + 2O (net ionic) Terminology Reduction The gain of electrons. Oxidation The loss of electrons. Half-reaction The reaction (including electrons) showing the path of only one atom or ion. Net-ionic reaction The combined half reactions, excluding the electrons. Demonstration If we burn magnesium in air, the white powder that is formed doesn’t resemble the silver magnesium nor the colourless oxygen gas. Try to trace out the paths of the magnesium and oxygen into ions. Questions Question 1 Write out half-reactions for the following reactions: 2Na + Cl2 -> 2NaCl 2Sr + Br2 -> 2 SrBr2 2Fe + 3O2 -> 2Fe2O3 Question 2 Combine the half-reactions below into net-ionic reactions. + - a.) Na -> Na + e Cl2 + 2e -> 2Cl 2+ - b.) Mg -> Mg + 2e 2O2 + 4e -> 2O 3+ c.) Al -> Al + 3e 2S + 2e -> S - Question 3 Write complete half –reactions and net-ionic reactions between the following elements: a.) Oxygen + Iron => Iron(III)oxide b.) Magnesium + Bromine => Magnesium bromide c.) Iodine + Calcium => Calcium iodide