Redox Reactions PDF

KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM OXIDATION AND REDUCTION (REDOX) REACTIONS 1. A redox reaction is a chemical reaction that involves reduction and oxidation that occurs simultaneously....

KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM OXIDATION AND REDUCTION (REDOX) REACTIONS 1. A redox reaction is a chemical reaction that involves reduction and oxidation that occurs simultaneously. 2. In a redox reaction, both reduction and oxidation are going on side-by-side. 3. Oxidation and reduction can be defined in terms of a. loss or gain of oxygen b. loss or gain of hydrogen c. transfer of electrons d. change in oxidation number 4. The redox reactions that you need to know includes: a. Redox reaction in aqueous solution: changing of iron (II) ions to iron (III) ions and vice versa displacement of hydrogen displacement of halogens transfer of electrons at a distance b. Electrochemistry c. Corrosion of metals d. Combustion of metals e. Rusting of iron f. Extraction of metal 1 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM REDOX IN TERMS OF OXYGEN 1. Oxidation is the process of gaining oxygen. 2. Reduction is the process of losing oxygen. 3. For example, in the extraction of iron from its ore: O - g = Iron (III) oxide loses oxygen. Iron (III) oxide is reduced to iron. This is a reduction process. Carbon monoxide gains oxygen. Carbon monoxide is oxidised to become carbon dioxide. This is an oxidising process. More examples: Combustion of Magnesium in Air: Magnesium is oxidised to become magnesium oxide. 2 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM Displacement of copper (II) oxide by Carbon: Copper (II) oxide is reduced to become copper metal Carbon is oxidised to become carbon dioxide. Displacement of lead (II) oxide by Zinc: Zinc is oxidised to become zinc oxide. Lead (II) oxide is reduced to become lead metal 3 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM REDOX IN TERMS OF HYDROGEN 1. Oxidation is the process of losing hydrogen. 2. Reduction is the process of gaining hydrogen. 3. For example, the reaction between ammonia and bromine: Ammonia loses hydrogen. Ammonia is oxidised to become nitrogen. This is an oxidation process. Bromine gains hydrogen. Bromine is reduced to become hydrogen bromide. This is a reduction process. Hydrogen Sulphide Reacts with Chlorine: Hydrogen sulphide is oxidised to become sulphur. Chlorine is reduced to become hydrogen peroxide. 4 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM Copper (II) oxide Reacts with Ammonia: Copper (II) oxide is reduced to become copper(II) metal. Ammonia is oxidised to become nitrogen gas. mso- myst ga 8 ⑧ (2mg) + I (02) + = UNDERSTANDING OXIDATION STATES Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state. Oxidation State of Some Elements 1. The oxidation state of an element is zero. Example: Element Oxidation State Mg 0 H2 0 Br2 0 5 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM 2. For a simple ion with single atom, the oxidation state is equal to the charge. Example: Ion Oxidation State Cu2+ +2 Br– -1 O2- -2 Al3+ +3 3. Some elements almost always have the same oxidation states in their compounds: Example 1: The oxidation state of oxygen is always -2 except peroxide, which is -1. Compound Oxidation state of oxygen H2O -2 H2SO4 -2 ZnO -2 KClO3 -2 H2O2 -1 6 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM Example 2: The oxidation state of hydrogen is always +1 except hydride, which is -1. Compound Oxidation state of hydrogen NH3 +1 HCl +1 NaOH +1 MgH2 -1 NaH -1 4. The sum of the oxidation states of all the atoms or molecule in a neutral compound is zero. Example: Ion Sum of Oxidation State H2O 0 CO2 0 NH3 0 7 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM 5. The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion. Example: Ion Sum of Oxidation State NO3– -1 CO32- -2 PO43- -3 NH4+ +1 Oxidation number in compounds 1. You will have come across names like iron (II) sulphate and iron (III) chloride. The (II) and (III) are the oxidation states of the iron in the two compounds: +2 and +3 respectively. That tells you that they contain Fe2+ and Fe3+ ions. Take a look below: Formula Name of the compound FeCl2 Iron (II) chloride FeCl3 Iron (III) chloride 8 KSSM FORM 5 CHEMISTRY CHAPTER 1: REDOX EQUILIBRIUM MnO2 Manganese (IV) oxide Mn(NO3)2 Manganese (II) nitrate PbCl2 Lead (II) chloride 2. Transition metals always show difference oxidation state as shown in the table below. Metal Oxidation state Fe +2, +3 Cu +1, +2 Mn +2, +4, +6, +7 REDOX IN TERMS OF CHANGE OXIDATION STATES REMEMBER! Oxidation involves an increase in oxidation state Reduction involves a decrease in oxidation state 1. Another way to determine oxidation and reduction is to see the change of the oxidation state after a reaction. 2. An atom is said to be oxidised when its oxidation state increases. 3. An atom is said to be reduced when its oxidation state decreases. 9