Redox Titrations 2010

Transcription

1 Redox: Redox (Reduction-Oxidation Reaction) describes all chemical reactions in which atoms have their oxidation number / oxidation state changed. Redox Chemistery: Redox chemistry is concerned with net electron flow to and from a defined centre during a chemical reaction. A defined centre may be: Reduction: Reduction is a gain of electrons or a decrease in oxidation state by a molecule, atom or ion.

2 Oxidation: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom or an ion. A defined centre is said to be oxidised if the electron density decreases, and reduced if electron density increases, during a reaction. The rule is: Loss of electrons equates with Oxidation and Gain of electrons equates with Reduction The oxidation of a defined centre can be changed in two ways. Firstly by Single Electron Transfer (SET) to the defined centre (reduction) or from the defined centre (oxidation). For example, the iron(iii) ion, Fe 3+, can be reduced to iron(ii), Fe 2+. The reaction can also occur in the oxidation direction. The reduction electron can either be provided by a chemical reducing agent (often a metal) or electrochemically.

3 Electron flow by way of single electron transfer oxidation and reduction can be predicted using standard reduction potential data (below). The second method of changing the oxidation number is by reversal of bond polarisation at the defined centre. Hydrogen is electropositive and it renders the carbon of methane, CH4, electron rich and it is defined as having an oxidation number of -4. However, the carbon of carbon dioxide has an oxidation number of +4 because oxygen is more electronegative than carbon. (Each bond contributes once.) Carbon is able to exist in several oxidation states:

4 The combustion of methane to carbon dioxide is an oxidation of carbon because the oxidation number of carbon increases from -4 to +4 Redox Titration Redox titration is a type of titration based on a redox reaction between the analyte and titrant. Oxidising Agent: An oxidizing agent can be defined as either: An agent that evolvs oxygen atoms, or A substance that gains electrons In both cases, the oxidizing agent becomes reduced, and the reaction bascically is of oxidation reaction. Example: The formation of Iron Oxide ; 4Fe + 3O 2 2Fe 2 O 3 Common Oxidising Agents Ammonium Cerium(IV) Nitrate Chlorate, Perchlorate, Chlorite

5 Chromic & Dichromic Acids Hypochlorite Iodine Nitric Acid Nitrous Oxide Osium Tetraoxide Ozone Permeganate, etc. Reducing Agent: A reducing agent can be described either: A compound that accepts oxygen atoms, or An agent that donate electrons In both cases, element is oxidized byself and other elements are reduced in the reaction. Example: Ferrous is oxidised into ferric: 2 [Fe(CN) 6 ] 4 + Cl 2 2 [Fe(CN) 6 ] Cl Common Reducing Agents Ferrous Iron Nascent Hydrogen Lithium Aluminium Hydride (LiAlH 4 ) Sodium Amalgam Oxalic Acid (C 2 H 2 O 4 ) Formic Acid (HCOOH) Lindlar Catalyst

6 EXPERIMENT The given soln. contains 3.0gm of an impure sample of FeSO 4 dissolved in 100 ml of solution. Determine %age impurity of the sample by Redox Titration. Standard Solution: 0.02 M KMnO 4 Indicator: End Point: KMnO 4 itself. Light Pink Equation: 2KMnO 4 + 8H 2 SO FeSO 4 K 2 SO 4 + 5Fe 2 (SO 4 ) 3 +10H MnSO 4 Formulae: M 1 V 1 =M 2 V 2 Calculations: Suppose, FeSO 4 = M 1 = 0.1M Amount of FeSO 4 dissolved/litre = M 1 * Mol.wt. = 0.1 * 278 = 27.8 gm 1000ml of impure sample contains FeSO 4 = 27.8 gm 100 ml = 27.8/1000 * 100 = 2.78 gm 3 gm of impure sample contains FeSO 4 = gm = 2.78/3*100 = 92.66% Result: %age purity of the given sample is 92.66%.