8303116-037-Redox-Equilibria-I - Chem Factsheet September...

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Redox Equilibria I: Standard Electrode Potentials and Cells C hem F actsheet September 2002 Number 37 1 To succeed with this topic you need to: be familiar with the concept of equilibrium (Factsheet 09); be able to assign oxidation numbers to elements (Factsheet 11). After working through this Factsheet you will: understand the link between cells and oxidising and reducing powers (redox reactions); have met the Standard Hydrogen Electrode and why it is needed; know the definition for Standard Electrode Potential (SEP) and its symbol E ; be able to use SEP values to find values for different cells; know how SEP values affect reducing and oxiding powers. Examination questions and techniques on SEPs and cells are covered separately in Factsheet 41. When a metal is placed into a solution of its own ions an equilibrium is set up. Redox – revision When a piece of zinc is placed in a copper salt solution the copper ions are displaced – zinc is more reactive than copper ions: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) You will recognise this from the reactivity series of metals at GCSE level. If we now apply oxidation numbers to the same equation: Zn + Cu 2+ Zn 2+ + Cu O.N. 0 +2 +2 0 ie. Zn is being oxidised by the Cu 2+ which is the oxidant (oxidising agent). Cu 2+ is being reduced by the Zn which is the reductant (reducing agent). We can now separate the half equations from the full equation to show the transfer of electrons: Zn Zn 2+ + 2e oxidation – loss of electrons (increase in ON) Cu 2+ + 2e Cu reduction – gain of electrons (decrease in ON) oxidation reduction oxidising agent reducing agent Cells Let us stay with the zinc and copper solution displacement reaction. The full equation shows zinc is more reactive than copper (displacement method) but doesn’t tell us how much more reactive – this is where cells come into the work. Zn 2+ (aq) Zn Cu 2+ (aq) Cu Zn 2+ (aq) + 2e Zn(s) Cu 2+ (aq) + 2e Cu (s) You will recognise the half equations – the diagrams show the half cells. NB: In this work, half equations are always shown with electrons on the left . If these two half cells are connected then the same reaction takes place as
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303116-037-Redox-Equilibria-I - Chem Factsheet September...

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