331_lecture_notes_chp_1_f08

331_lecture_notes_chp_1_f08 - 1.1 INTRODUCTION WHAT IS AN...

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1.1 INTRODUCTION WHAT IS AN ELECTROCHEMICAL REACTION? One way to answer this question is to try to distinguish electrochemical reactions from other chemical reactions. You should recall from earlier courses that reactions can occur via the collisions between thermally energized molecules and the formation and breaking of chemical bonds in the instant the molecules are in contact. Furthermore, in chemical reactions, bonds that are formed result from the sharing of valence electrons between the reacting atoms. What characterizes electrochemical reactions is that the process of forming or breaking bonds results in either the complete release of an electron or the consumption of an electron that has come from somewhere else. Electrons actually move within the system. In ordinary chemical reactions, there is no separate movement of electrons. In many cases, electrochemistry provides another means of carrying out a given overall reaction. Consider the case of 2HI H 2 + I 2 This reaction can be accomplished two ways as follows: 1. chemical reaction - heat up pure HI gas until it decomposes H I H I thermal collisional activation I I H H H ……. I H ……. I bond + breaking
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1.2 2. electrochemical reaction The following processes have occurred: bulk solution: 2HI 2H + (aq) + 2I - (aq) (1) Pt cathode: 2H + + 2e - H 2 (2) Rh anode: 2I - I 2 + 2e - (3) Overall (1) + (2) + (3) 2HI H 2 + I 2 In the electrochemical way, the reactant HI molecules do not collide with each other but with separated sources and sinks of electrons. cathode 2H + Pt power supply 2e - Rh H 2 I 2 + 2I - anode Immerse strips of Pt and Rh into an aqueous solution of HI. Hook the Rh to the positive terminal of a voltage source, Pt to the negative end. Increase voltage until you begin to see gas bubbles coming off each electrode. HI I - + H + water (gas) 2e - electrons to electrode donates electrons from electrode accepts I 2 H 2
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1.3 Some Basic Electrochemical Cells (Devices) We can think of electrons as being as much of a reactant or product as, let’s say, an ion diffusing to a surface and reacting. One important aspect of electrochemical reactions is that electrons can only move within an electronic conductor, eg. metal or semiconductor, whereas ions can only move within an ionic conductor, eg. electrolyte. You are probably familiar with the following type of electrochemical cell for the electrolysis of H 2 O in H 2 SO 4 . e - 1 SO 4 2- , HSO 4 - i i e - H 2 O ½ O 2 + 2H + + 2e - 2H + + 2e - H 2 H + + 2 Voltage is applied between two metal strips immersed in an aqueous solution so that a current begins to flow. Current is simply a measure of the flow rate of electrons through the circuit. Electrons begin to flow because the two electrode reactions begin to occur. In the case shown, the power supply pumps electrons or negative charge carriers
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331_lecture_notes_chp_1_f08 - 1.1 INTRODUCTION WHAT IS AN...

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