solid state.pptxCoprecipitation as a precursor to solid state reaction and kinetic of solid state reaction

Coprecipitation as a precursor to solid state reaction and kinetic of solid state reaction

1. Session -2021-22 Solid state chemistry Presented by:- Kritika yadav M.Sc 4 th sem Chemistry Guided by :- Dr. Abha verma (HOD) Chemistry COPRECIPITATION AS A PRECURSOR TO SOLID STATE REACTIONS And kinetics of solid state reaction J.H GOVT. P.G COLLEGE , BETUL

2. COPRECIPITATION AS A PRECURSOR TO SOLID STATE REACTIONS In normal solid state reactions, the reactants are mixed togethermanually or mechanically and the subsequent reaction rate depends to a large degree on the particle size of the reactants, the degree of homogenisation achieved on mixing and the intimacy of contact between the grains, as well as the obvious effect of temperature. By using coprecipitation procedures, it is sometimes possible to obtain a high degree of homogenisation together with a small particle size and thereby increase the reaction rate.

3. This may be shown by the synthesis of ZnFe20, spinel In one method, oxalates of zinc and iron are used as the reactants, these are dissolved in water in the ratio of 1 1, the solutions then being mixed and heated to evaporate the water Oxalates of zinc and iron are slowly precipitated together and the resulting fine powder is a solid solution that contains the cations mixed together essentially on an atomic scale.
The precipitated solids are filtered off and calcinated (ie, heated) in the normal way, but because of the high degree of homogenisation, much lower reaction temperatures are sufficient for reaction to occur (e.g., -1000°C for the formation of ZnFe2O). The overall reaction may be written as follows Fe2[(COO)2]3 + Zn(COO)2 ZnFe20, +4CO+4CO2

4. The method has also been successfully used for the preparation of other spinels CoFe2O4, MnFe2O4, and NiFe2O4 . The method does not work well in cases where (a) The two reactants have very different solubilities in water, (b) The reactants do not precipitate at the same rate or (c) Supersaturated solutions commonly occur. It is therefore, often notsuitable for a preparation of high purity, accurately stoichiometric phases.

5. KINETICS OF SOLID STATE REACTIONS The above discussions have shown some of the factors that affect the reactions of solids. Several factors are usually involved in a particular reaction and consequently the analysis of kinetic data may be difficult. In heterogeneous reactions between, say, two solids, i.e., actual reaction to form the product occurs at an interface. In kinetic studies ofreaction rates, it is required to know which is the slow, rate controlling step in a reaction.

6. The approaches used to analyse rate data are rather different from those used for gas phase reactions. In the latter, the concept of reaction order is very useful In this, the rate of change of concentration, c, of one species depends on the nth power of concentration, ie, There are at least three clear possibilities for this. Transport of matter to the reaction interface, (b) Reaction at the interface .(c) Transfer of matter away from the reaction interface.

7. where n is the reaction order and k is a constant. From the value of n, important insight into reaction mechanims may be obtained, such as the number of molecules involved in a particular reaction step.I'or most solid state reactions, however, it is usually incorrect and misleading to think in terms of reaction order as the reactions do not involve molecules. In this way, the data may still be represented empirically and it is sometimes found that n is not a simple integer but may be fractional. It is not proposed to cover further the kinetics of solid state reactions here.