Week Ten Lesson 3: Redox Reactions PDF

WEEK TEN LESSON 3  Oxidation-Reduction (Redox) Reactions Oxidation is the: (i) addition of oxygen (ii) removal of hydrogen (ii) addition of electronegative element (iv) removal of electron(s) Reduction is the: (i) addition of hydrogen (ii) removal of oxygen (iii) addition of ele...

WEEK TEN LESSON 3  Oxidation-Reduction (Redox) Reactions Oxidation is the: (i) addition of oxygen (ii) removal of hydrogen (ii) addition of electronegative element (iv) removal of electron(s) Reduction is the: (i) addition of hydrogen (ii) removal of oxygen (iii) addition of electropositive elements (iv) addition of electron(s)  Oxidation-Reduction (Redox) Reactions Oxidizing Agents are substances/species that accept electrons (electron acceptors). They are also the substances/species that are reduced during redox reaction. Reducing Agents are substances/species that donate electrons (electron donors). They are also the substances/species that are oxidized during redox reaction. Test for Oxidizing Agents: (I) Using Hydrogen Sulphide: A yellow precipitate of sulphur is deposited when hydrogen sulphide gas is passed through an oxidizing agent because the suphide ion, S2- is oxidized to sulphur, S.  Oxidation-Reduction (Redox) Reactions (ii) Using Iron(ii)chloride,FeCl2: Oxidizing agents turn the colour of FeCl2 from green to brown because Fe2+ which is green is oxidized to Fe3+. Test for Reducing Agent: (i)Using Acidified PotassiumTetraoxomanganate (vii), KMnO4 : Reducing agents decolorize the purple colour of acidified KMnO4 because the purple colour of Mn7+ is reduced to Mn2+. (ii) Using Acidified Potassium Heptaoxodichromate (vi), K2Cr2O7 : Reducing agents change the colour of acidified K2Cr2O7 from orange to green because Cr2O7- ion which is orange is reduced to Cr3+ which is green.  Oxidation Number The oxidation number (or oxidation state) of an atom in a substance as the actual charge of the atom if it exists as a monatomic ion, or a hypothetical charge assigned to the atom in the substance by simple rules. An oxidation–reduction reaction is one in which one or more atoms change oxidation number, implying that there has been a transfer of electrons.  Rules for Assigning Oxidation Numbers 1 Elements: the oxidation number of an atom in an element is zero. 2 Monatomic ions: The oxidation number of an atom in a monatomic ion equals the charge on the ion. 3 Oxygen: the oxidation number of oxygen is -2 in most of its compounds. (An exception is O in H2O2 and other peroxides, where the oxidation number is -1.) 4 Hydrogen: the oxidation number of hydrogen is +1 in most of its compounds. (The oxidation number of hydrogen is -1 in binary compounds with a metal, such as CaH2.) 5 Halogens: the oxidation number of fluorine is -1 in all of its compounds. Each of the other halogens (Cl, Br, I) has an oxidation number of -1 in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen. 6 Compounds and ions: the sum of the oxidation numbers of the atoms in a compound is zero. The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge on the ion.  Redox Titrations (Oxidimetry and Reductimetry) Redox titration is the technique employed to determine the amount of a reducing agent required to reduce completely a given amount of an oxidizing agent and to determine the amount of oxidizing agent required to completely oxidize a reducing agent. The solutions of the following substances are commonly used as oxidizing agents: KMnO4 , KIO3 and Iodine. The reducing agents are solutions of iron (ii) salts, ethanedioic acid (oxalic acid), sodium ethanoate (oxalate), potassium iodide, and sodium trioxodisulphate (iV), (sodium thiosulphate).  Redox Titrations (Oxidimetry and Reductimetry) All the basic principles involved in acid-base titrations are also applicable to redox reactions. When mole concept is applied to redox agents, the amounts of the substances involved can be determined.  End-point of a Redox Titration Indicators such as methyl orange and phenolphthalein, are not used in redox titrations involving purple KMnO4 solution and a colourless reducing agent, an external indicator is not required because KMnO4 acts as its own indicator. The end-point is the first permanent pink colour imparted on the colourless reducing agent. In a redox titration involving iodine solution and a colourless reducing agent, starch solution is used as the indicator. The end-point is signaled by a change in colour of the mixture from blue-black to colourless.  Applications of Redox Reactions Oxidation-reduction (redox) reactions include the formation of a compound from its elements (and the reverse process), all combustion reactions, the generation of electricity in batteries, the production of cellular energy, and many others. In fact, redox reactions are so widespread that many do not occur in solution at all. The key chemical event in an oxidation-reduction (or redox) reaction is the net movement of electrons from one reactant to another. The movement occurs from the reactant (or atom in the reactant) with less attraction for electrons to the reactant (or atom) with more attraction for electrons. This process occurs in the formation of both ionic and covalent compounds:  Combination Redox Reactions In a combination redox reaction, two or more reactants, at least one of which is an element, form a compound: Combining Two Elements Two elements may react to form binary ionic or covalent compounds. Here are some important examples: Metal and nonmetal form an ionic compound. A metal, such as aluminum, reacts with a nonmetal, such as oxygen. The change in O.N.s shows that the metal is oxidized, so it is the reducing agent; the non-mental is reduced, so it is the oxidizing agent:  Combination Redox Reactions Two nonmetals form a covalent compound. In one of thousands of examples, ammonia forms from nitrogen and hydrogen in a reaction that occurs in industry on an enormous scale:  Combination Redox Reactions 2. Two nonmetals form a covalent compound. In one of thousands of examples, nitrogen (IV) oxide forms from nitrogen and oxygen in a reaction: Similarly, many nonmetal halides combine with additional halogen to form “higher” halides:  Decomposition Redox Reactions In a decomposition redox reaction, a compound forms two or more products, at least one of which is an element: In any decomposition reaction, the reactant absorbs enough energy for one or more bonds to break. The energy can take several forms, but the most important are decomposition by heat (thermal) and by electricity (electrolytic). The products are either elements or elements and smaller compounds.  Thermal Decomposition When the energy absorbed is heat, the reaction is called a thermal decomposition. (A Greek delta, Δ, above the yield arrow indicates strong heating is required.) Many metal oxides, chlorates, and perchlorates release oxygen when strongly heated. Heating potassium chlorate is a method for forming small amounts of oxygen in the laboratory; the same reaction occurs in some explosives and fireworks:  Thermal Decomposition Notice that the lone reactant is the oxidizing and the reducing agent. Electrolytic Decomposition In the process of electrolysis, a compound absorbs electrical energy and decomposes into its elements. In the early 19th century, the observation of the electrolysis of water was crucial for establishing atomic masses: Many active metals, such as sodium, magnesium, and calcium, are produced industrially by electrolysis of their molten halides:  Thermal Decomposition Disproportionation is a type of redox reaction where a substance acts as both the oxidizing and reducing agents in a reaction. In the process, an atom in the reactant occurs in both lower and higher states in the products: 2NO(g) + O2(g) →2NO2(g)  Displacement Redox Reactions In any displacement reaction, the number of substances on the two sides of the equation remains the same, but atoms (or ions) exchange places. There are two types: In double-displacement (metathesis) reactions, such as precipitation and acid-base reactions atoms (or ions) of two compounds exchange places; these reactions are not redox processes: In single-displacement reactions, one of the substances is an element; therefore, all single-displacement reactions are redox processes:  Displacement Redox Reactions In solution, single-displacement reactions occur when an atom of one element displaces the ion of another: if the displacement involves metals, the atom reduces the ion; if it involves nonmetals (specifically halogens), the atom oxidizes the ion. In two activity series—one for metals and one for halogens—the elements are ranked in order of their ability to displace hydrogen (for metals) and one another. Activity Series