[CHEM] Chapter 6.2 - Redox Reaction

1 DARRELL ER (COPYRIGHTED) © TOPIC 6.2: REDOX REACTION DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Relatively straight forward chapter • Closely linked to chapters like Metals (Displacement reactions) & Periodic Table & Acid Bases • Usually tested in MCQs Tested as add-on to other chapters 🡪 Chemical Equations • Light overall weightage • Constitute to 2.5% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 REDOX REACTION OXIDATION REDUCTION IONIC HALF EQUATIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 REDOX REACTION REDOX REACTION A redox reaction refers to the chemical reaction where one substance undergoes reduction, while another substance undergoes oxidation. Both processes always occur together. Oxidation happens when: - Gaining of oxygen - Loss of hydrogen - Loss of electrons - Increase in oxidation state Reduction happens when: - Loss of oxygen - Gaining of hydrogen - Gaining of electrons - Decrease in oxidation state DARRELL ER (COPYRIGHTED) © In fact, the best way to identify if a substance had undergone oxidation or reduction is to check its oxidation state. Gaining of oxygen, loss of hydrogen, loss of electrons will lead to the increase in oxidation state. Hence, they can be considered a subset of increase in oxidation state. Similarly, losing oxygen, gain in hydrogen, gain in electrons will lead to the decrease in oxidation state. Once again, they can be considered a subset of decrease in oxidation state. Hence, by identifying the change in oxidation state is the most accurate way of deducing whether the substance had undergone oxidation or reduction. INCREASE OXIDATION STATE DECREASE OXIDATION STATE Gain of oxygen Loss of electrons Loss of hydrogen Gain of hydrogen Gain of electrons Loss of oxygen

5 IONIC HALF EQUATIONS IONIC HALF-EQUATIONS Steps: 1) Write out the balanced chemical equation for the reaction. Mg (s) + CuSO4 (aq)  MgSO4 (aq) + Cu (s) 2) Convert the chemical equation to its ionic equation. Mg (s) + Cu2+ (aq)  Mg2+ (aq) + Cu (s) 3) Identify the substance that undergoes oxidation and reduction respectively. Oxidation: Mg (s)  Mg2+ (aq) Reduction: Cu2+ (aq)  Cu (s) 4) Balance the number of charges on both sides of each half- equation using electrons. Oxidation: Mg (s)  Mg2+ (aq) + 2e- Reduction: Cu2+ (aq) + 2e-  Cu (s) DARRELL ER (COPYRIGHTED) ©

6 OXIDISING & REDUCING AGENTS KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

7 OXIDISING AGENTS OXIDISING AGENTS An oxidising agent refers to a reagent that causes another substance to be oxidised. An oxidising agent itself would undergo reduction instead. 1) Acidified potassium manganate (VII), KMnO4 is an oxidizing agent. Hence, it will undergo reduction in the reaction. MnO4– (aq) + 8 H+ (aq) + 5e–  Mn2+ (aq) + 4 H2O (l) The oxidation