Synthesis of some compounds must be carried outunder a special atmosphere.: an noble gas, argon, may be used too preventoxidation to higher oxidation state.an oxidizing gas, oxygen, might be used to form ahigh oxidation state.: a reducing gas, hydrogen, might be used to form alow oxidation stateEXHAUSTGAS02
Synthesis of some compounds must be carried out under a special atmosphere. • an noble gas, argon, may be used too prevent oxidation to higher oxidation state. • an oxidizing gas, oxygen, might be used to form a high oxidation state. • a reducing gas, hydrogen, might be used to form a low oxidation state
PreparativemethodsDisadvantages of ceramic methodDespite its widespread use, the simple ceramicmethod has several disadvantages.Firstly, high temperatures are generally required(2300K in the example we gave). This requires a largeinput of energy. In addition, the phase or compounddesired may be unstable or decompose at such hightemperatures
Preparative methods Disadvantages of ceramic method Despite its widespread use, the simple ceramic method has several disadvantages. Firstly, high temperatures are generally required (2300K in the example we gave). This requires a large input of energy. In addition, the phase or compound desired may be unstable or decompose at such high temperatures
PreparativemethodsDisadvantages of ceramic methodA second disadvantage is the slowness of solicstate reactions. In general, the solids are not raisedto their melting temperatures, and so the reactionoccurs in the solid stateRaising the temperature enables reaction at theinterface and diffusion through the solid to proceedfaster than they do at room temperatures
A second disadvantage is the slowness of solid state reactions. In general, the solids are not raised to their melting temperatures, and so the reaction occurs in the solid state. Raising the temperature enables reaction at the interface and diffusion through the solid to proceed faster than they do at room temperatures. Preparative methods Disadvantages of ceramic method
Preparative methodsDisadvantages of ceramic methodEven so, diffusion is often the limiting stepBecause of this, it is important that the startingmaterials are ground to give a small particle sizeand very well mixed to maximize the surfacecontact area and minimize the distance that thereactants have to diffuseA third disadvantage is that the product is oftennothomogeneous in composition
Preparative methods Disadvantages of ceramic method Even so, diffusion is often the limiting step. Because of this, it is important that the starting materials are ground to give a small particle size, and very well mixed to maximize the surface contact area and minimize the distance that the reactants have to diffuse. A third disadvantage is that the product is often not homogeneous in composition
PreparativemethodsMicrowave synthesisTo use microwave heating in solid state synthesisat least one component of the reaction mixture mustabsorbmicrowaveradiationThe speed of the reaction process is thenincreased by both increasing the rate of the solidstate reaction and by increasing the rate of diffusion.which as we mentioned earlier is often the rate-limiting step
Preparative methods Microwave synthesis To use microwave heating in solid state synthesis at least one component of the reaction mixture must absorb microwave radiation. The speed of the reaction process is then increased by both increasing the rate of the solid state reaction and by increasing the rate of diffusion, which as we mentioned earlier is often the ratelimiting step