[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 state of manganese decreases from +7 in the manganite (VII) ion to +2 in the manganese (II) ion. The colour of the solution will turn from purple to colourless. 2) Acidified potassium dichromate (VI), K2Cr2O7 is an oxidizing agent. Hence, it will undergo reduction in the reaction. Cr2O7 2- (aq) + 14 H+ (aq) + 6e-  2 Cr3+ (aq) + 7 H2O (l) The oxidation state of chromium decreases from +6 in the dichromate (VI) ion to +3 in the chromium (III) ion. The colour of potassium dichromate (VI) solution will turn from orange to green. DARRELL ER (COPYRIGHTED) © *The job of an oxidising agent is to oxidise another substance. In the process, the agent itself would undergo reduction. The best way to check if a substance is an oxidising agent is to check whether the substance has undergone reduction.

8 REDUCING AGENTS REDUCING AGENT A reducing agent refers to a reagent that causes another substance to be reduced. A reducing agent itself would undergo oxidation instead. 1) Potassium iodide, KI is an reducing agent. It will undergo oxidation in the reaction. 2I– (aq)  I2 (aq) + 2e– The oxidation state of iodine has increased from -1 in iodide to 0 in iodine. The colour of the solution will turn from colourless to brown. DARRELL ER (COPYRIGHTED) © *The job of an reducing agent is to reduce another substance. In the process, the agent itself undergoes oxidation. The best way to check if a substance is an reducing agent is to check whether the substance has undergone oxidation.

Try it yourself! (TYS Question) Answer:

Try it yourself! (TYS Question) Answer: