8303145-051-Redox-Tit-Rations

8303145-051-Redox-Tit-Rations - Chem Factsheet April 2003...

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Redox Equilibria IV - Redox Titrations C hem F actsheet April 2003 Number 51 1 Exam Hint: - At A2 level you need to learn some of the specific half- equations relevant to redox titrations – THESE HAVE BEEN MARKED FOR YOU IN THIS FACTSHEET. However, it would be a mistake to try to learn all the titration equations – you need to use the METHOD for working out the full equation using: (a) E ê values and the anti-clockwise rule, (b) balancing equations using the ‘electron method’. 1) Potassium manganate(VII) titrations N.B. Used to be called ‘potassium permanganate’. Potassium manganate (VII) is usually found in laboratories as purple crystals which are then dissolved in water to form the solutions used in titrations. The solution is a dark purple colour and is a strong oxidising agent. However, if it is not acidified (with dilute sulphuric acid) it produces a brown precipitate of MnO 2 which makes accurate titrations impossible (you cannot see the end-point). You must learn this half equation MnO 4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) Note the following points about the half-equation: (a) MnO 4 is the ion in KMnO 4 , potassium manganate(VII), which is purple . (b) In MnO 4 the Mn has oxidation number = +7 (O = -2). (c) 5e are involved – this will be the number when we are balancing half- equations to give the full equation. (d) The Mn in MnO 4 is REDUCED to Mn 2+ (O.N. = +2) because it is the OXIDISING AGENT. N.B. Mn 2+ (aq) is faintly pink in colour but at the concentrations used in the titrations the solution appears colourless. In potassium manganate(VII) titrations there is no indicator added because the KMnO 4 acts as a self-indicator . In the titration the purple KMnO 4 solution is in the burette and as it is added to the solution in the conical flask the purple colour is ‘absorbed’. At the end-point one drop of KMnO 4 solutions produces a pale pink colour in the flask (a very slight excess of the KMnO 4 solution). Standardising potassium manganate (VII) solution A ‘standardised solution’ is one whose concentration (mol dm -3 ) is known accurately by titrating it against an accurately made-up solution. KMnO 4 (aq) is standardised by titrating it with sodium ethanedioate, Na 2 C 2 O 4 , solution. Before working through this Factsheet you should: Understand and be confident with titration calculations and using E ê values (Factsheets No.23 and No. 45). After working through this Factsheet you will: have met the specific reactions of potassium manganate (VII) and sodium thiosulphate/iodine (which are the quoted examples for redox titrations); be able to use the anti-clockwise rule and the ‘balancing equations using electrons’ method to write titration reactions from half-equations; have revised the method for calculations involving titrations.
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8303145-051-Redox-Tit-Rations - Chem Factsheet April 2003...

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