8303126-045-Standard-Electrode

8303126-045-Standard-Electrode - Chem Factsheet January...

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The Feasibility of Reactions C hem F actsheet January 2003 Number 45 1 To succeed with this topic you need to: ensure you are fully able to write equations for cells and work of E ê After working through this Factsheet you will: understand the concepts of ‘feasibility’ of a chemical reaction and ‘spontaneous change’; be able to use E ê data to decide if a chemical reaction is ‘feasible’ or not; have met the concept of the ‘anti-clockwise rule’ as one way of assessing if a reaction is ‘feasible’; be able to write balanced chemical equations from half equations using the ‘electron balance method; have met the concept of ‘disproportionation’; understand how ‘non-standard’ conditions affect the ‘feasibility’ of a reaction. Introduction We need to pick out some key points from earlier work before we start: 1. REDOX means reduction and oxidation 2. OXIDATION NUMBERS define whether reduction or oxidation is taking place 3. HALF EQUATIONS are equilibria and relate oxidised and reduced forms 4. SEP (E ê ) VALUES (+/-) show the tendency for oxidation or reduction for a particular half equation 5. THE TABLE OF E ê VALUES below shows the trend in oxidising/ reducing power for particular half equations 6. COMBINING HALF EQUATIONS based on reduction and oxidation using E ê values (+/-) gives the BALANCED CHEMICAL EQUATION ‘Feasibility’ - will these two substances react or not? E ê Values and Feasibility The best example you can use to help you look at these key questions is the cell made by combining zinc and copper. The Data (a) Zn 2+ + 2e ¾ Zn E ê = 0.76 V Cu 2+ + 2e ¾ Cu E ê = +0.34 V Using the data (b) Zn/Zn 2+ is above Cu/Cu 2+ in the table i.e. zinc will be the cathode i.e. the electron provider so needs to be written the other way round Zn Zn 2+ + 2e i.e. oxidation O.N. 0 +2 (Zn is the reducing agent because it is oxidised ) Copper is the anode i.e. the electron acceptor so stays in the same order as in the table Cu 2+ + 2e Cu i.e. reduction +2 0 (Cu is the oxidising agent because it is reduced ) Writing the overall equation (the cell reaction) (c) Zn Zn 2+ + 2e Cu 2+ + 2e
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8303126-045-Standard-Electrode - Chem Factsheet January...

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