Example 4.3 A Conceptual Example Explain the observations illustrated in Figure 4.6. Change in electrical conductivity as a result of a chemical reaction (a) When the beaker contains a 1 M solution of acetic acid, CH3COOH, the bulb in the electric circuit glows only very dimly. (b) When the beaker contains a 1 M solution of ammonia, NH3, the bulb again glows only dimly. (c) When the two solutions are in the same beaker, the bulb glows brightly. What happens when the two solutions are mixed is described in Example 4.3. Chapter Four Reactions that Form Precipitates ? There are limits to the amount of a solute that will dissolve in a given amount of water. (even NaCl is < 6 M) ? If the maximum concentration of solute is less than about 0.01 M, we refer to the solute as insoluble in water. ? When a chemical reaction forms such a solute, the insoluble solute comes out of solution and is called a precipitate. Chapter Four Silver Iodide Precipitation A solution containing silver ions and nitrate ions, when added to ? ? a solution containing potassium ions and iodide ions, forms ? ? a precipitate of silver iodide. What is the net ionic equation for the reaction that has occurred here? (Hint: what species actually reacted?) Chapter Four ? With these guidelines we can predict precipitation reactions. ? When solutions of sodium carbonate and iron(III) nitrate are mixed, a precipitate will form. ? When solutions of lead acetate and calcium chloride are mixed, a precipitate will form. Chapter Four Example 4.4 Predict whether a precipitation reaction will occur in each of the following cases. If so, write a net ionic equation for the reaction. (a) Na2SO4(aq) + MgCl2(aq) ? ? (b) (NH4)2S(aq) + Cu(NO3)2(aq) ? ? (c) K2CO3(aq) + ZnCl2(aq) ? ? Chapter Four Chapter Four Example 4.6 One cup (about 240 g) of a certain clear chicken broth yields 4.302 g AgCl when excess AgNO3(aq) is added to it. Assuming that all the Cl? is derived from NaCl, what is the mass of NaCl in the sample of broth? Chapter Four Reactions Involving Oxidation and Reduction ? Oxidation: Loss of electrons ? Reduction: Gain of electrons ? Both oxidation and reduction must occur simultaneously. ? A species that loses electrons must lose them to something else (something that gains them) . ? A species that gains electrons must gain them from something else (something that loses them). ? Historical: ?oxidation? used to mean ?combines with oxygen?; the modern definition is much more general. Chapter Four Oxidation Numbers ?An oxidation number is the charge on an ion, or a hypothetical charge assigned to an atom in a molecule or polyatomic ion. ? Examples: in NaCl, the oxidation number of Na is +1, that of Cl is ?1 (the actual charge). ?In CO2 (a molecular compound, no ions) the oxidation number of oxygen is ?2, because oxygen as an ion would be expected to have a 2? charge. ? The carbon in CO2 has an oxidation number of +4 (Why?) It is neutral Chapter Four Rules for Assigning Oxidation Numbers 1. For the atoms in a neutral species?an isolated atom, a molecule, or a formula unit?the sum of all the oxidation numbers is 0. 2. For the atoms in an ion, the sum of the oxidation numbers is equal to the charge on the ion. 3. In compounds, the group 1A metals all have an oxidation number of +1 and the group 2A metals all have an oxidation number of +2. 4. In compounds, the oxidation number of fluorine is ?1. 5. In most compounds, hydrogen has an oxidation number of +1. 6. In most compounds, oxygen has an oxidation number of ?2. 7. In binary compounds with metals, group 7A elements have an oxidation number of ?1, group 6A elements have an oxidation number of ?2, and group 5A elements have an oxidation number of ?3. Chapter Four Example 4.7 What are the oxidation numbers assigned to the atoms of each element in (a) KClO4 (b) Cr2O72? (c) CaH2 (d) Na2O2 (e) Fe3O4 Chapter Four Identifying Oxidation?Reduction Reactions ? In a redox reaction, the oxidation number of a species changes during the reaction. ? Oxidation occurs when the oxidation number increases (species loses electrons). ? Reduction occurs when the oxidation number decreases (species gains electrons). ? If any species is oxidized or reduced in a reaction, that reaction is a redox reaction. ? Examples of redox reactions: displacement of an element by another element; combustion; incorporation of an element into a compound, etc. Chapter Four A Redox Reaction: Mg + Cu2+ ..Mg2+ + Cu r Four ? the products are Cu metal and Mg2+ ions. Electrons are transferred from Mg metal to Cu2+ ions and ? Oxidation?Reduction Equations ? Redox equations must be balanced according to both mass and electric charge. ? A complete method for balancing such equations will be presented in Chapter 18. ? For now, our main goals will be to: ? Identify oxidation?reduction reactions. ? Balance certain simple redox equations by inspection. ? Recognize, in all cases, whether a redox equation is properly balanced. Chapter Four Oxidizing and Reducing Agents ?An oxidizing agent causes another substance to be oxidized. ? The oxidizing agent is reduced. ?A reducing agent causes another substance to be reduced. ? The reducing agent is oxidized. Mg + Cu2+ ? Mg2+ + Cu What is the oxidizing agent? What is the reducing agent? Chapter Four Oxidation Numbers of Nonmetals ?The maximum oxidation number of a nonmetal is equal to the group number. ? For nitrogen, +5. ? For sulfur, +6. ? For chlorine, +7. ?The minimum oxidation number is equal to the (group number ?8). Chapter Four Activity Series of Some Metals In the activity series, any metal above another can displace that other metal. Will lead metal react with Fe3+ ions? Will iron metal dissolve in an acid to produce H2 gas? Mg metal can react with ? ?Cu2+ ions to form Cu metal. Chapter Four Example 4.8 A Conceptual Example Explain the difference in what happens when a copper-clad penny is immersed in (a) hydrochloric acid and (b) nitric acid, as shown in Figure 4.14. - NO3 can do the job Cu + 4 H+ + 2 NO3Cu cannot reduce H+ ? Cu 2+ + 2 NO2 + 2H2O & be oxidized apt Applications of Oxidation and Reduction ? Everyday life: to clean (bleach) our clothes, sanitize our swimming pools (?chlorine?), and to whiten teeth (peroxide). Cl2 + H2O .. ? H+ + Cl-+ HOCl (aq) The hypochlorous acid sanitizes the pool. The HCl must be neutralized. ? In foods and nutrition: redox reactions ?burn? the foods we eat; antioxidants react with undesirable free radicals. Nitrites in foods mess with hemoglobin. Ascorbic acts as an anti oxidant (gets oxidized) - C6H8O6 + 2H+ + 2 NO2 .. ? C6H6O6 + 2 H2O + 2NO
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