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CHEM B1110 Lecture Notes - Lecture 22: Spectroscopy, Chlorophyll, Oxidation State

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3 Nov 2016

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Purdue University

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Gain of oxygen (why its called oxidation) De(cid:272)rease i(cid:374) o(cid:454)idatio(cid:374) (cid:374)u(cid:373)(cid:271)er (agai(cid:374) wh(cid:455) it is called reduction) Loss of oxygen (why it is called reduction) Equal to the charge on a monoatomic ion. +1 for hydrogen (except in metal hydrides, e. g. nah then its -1) 2 for oxygen in everything except for peroxides. The sum of oxidation numbers equals the charge on an ion. Different molecules can absorb light of different colors (energies) A less concentrated red solution will still reflect red but it won"t be as strong and it will also reflect a very weak green light while it still absorbs the blue light. A leaf looks green because it absorbs blue and red wavelengths and reflects yellow and green. A spectrometer measures how much light is absorbed at each wavelength across the uv and visible range (about 200 - 800nm) Beer discovers that the amount of light transmitted through a red solution decreases for more concentrated solutions.

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Introduction An important type of reaction mechanism is the transfer of electrons from one species to another effecting a change in oxidation state. This gain or loss of electrons is referred to as reduction and oxidation, respectively. For example, consider the two balanced, chemical equations (1 & 2) that follow Cu (s) D Cu (aq) 2e (1) Cl2(aq) 2e D 2Cl (aq) (2) The first equation (1) shows elemental copper metal with a oxidation state of zero forming aqueous copper(Il) ions having an oxidation state of +2. This gain in oxidation number, 0 to +2, is called oxidation. The second equation (2) show elemental chlorine having an oxidation state of zero forming chloride ions in aqueous solution with an oxidation state of This decrease in oxidation number, 0 to -1, is called reduction. In any chemical reaction, the electrons gained or lost are conserved, i.e.. there is no net gain or loss of electrons. In fact, the two reactions illustrated above could occur simultaneously with the electrons lost by copper, gained by the chlorine in aqueous solution. Since the copper provides the electrons that subsequently reduce the chlorine copper is referred to as the reducing agent. Likewise, since the chlorine is accepting the electrons from copper, which subsequently oxidizes the copper, chlorine is referred to as he oxidizing agent. The two reactions above (1 & 2) are called half-reactions, as much as they each constitute one-half of the total reaction Equation 1 is the oxidation half reaction showing Cu(s) losing two electrons. In Equation 2, Cla(aq) gains two electrons and is the reduction half-reaction. If these two half-reaction are added together the result is the total oxidation-reduction reaction or "redox" reaction. Adding the two equations yields (3). Cu(s) Cl2(aq) D Cu (aq) 2CI (aq) (3) Whether or not the above reaction is spontaneous proceeds from reactants to products without input of energy into the system, may be determined by observing the reaction mixture if the reactants and products differ greatly in their appearance. Copper metal is a bright copper color and chlorine in aqueous solution will appear yellow. The Cu ion may turn the liquid part of the mixture blue, however, the Cris colorless and cannot be observed directly. Copper, like many of the transition metals, exhibits a variety of oxidation states depending upon the chemical environment in which it is found. In a strong reducing environment copper may exist as the free metal, Cu(s). An example of such a situation is illustrated in the following chemical equation (4). 4Cuo(s) CH4(g) D 4Cu(s) CO2(g) 2H2O(l) (4)