To obtain good rates of reaction, you typically 1)The area of contact between reacting solids need the diffusion coefficient to be larger than he contact between reactants we want to use starting reagents with large surface area. Consider the . The diffusion coefficient increases wit numbers for a temperature, rapidly as you approach the melting point. This concept is leads to lumber of Crystallites=1 Tamman's Rule: Extensive reaction will not occur til the temperature reaches at least 2/3 of the umber of Crvst Iting point of one or more of the reactants. urface Area=6x10= number of Crystallites=10 Surface Area= 6x106 cm2 Pelletize to encourage intimate contact between crystallites! 2) The rate of diffusion Consider for Example: the tWo ways to increase the rate of diffusion are to Synthesis of Sr, CrTaO Encrease introduce defects by starting with reagents that 1) Possible starting reagents decompose prior to or during reaction, such as PSr Metal- Hard to handle, prone to oxidation carbonates or nitrates sSrO· Picks up CO2& water,mp=2430°C MSI(NO ,-mp=570C, may pick up some water 3)The rate of nucleation of the product phase PSrCO,-decomposes to SrO at 1370.C dWe can maximize the rate of reactants with crvstal structur nucleat Ta metal-mp=2996°C lilar to that of the pa2O3-mp=1800°C roduct (topotactic and epitactic reactions). MCr Metal- Hard to handle, prone to oxidation rO3-mp=2435°C 2) we To make 5.04 g of Sr, CrTaO, (FW=504.2 g/mol: 0.01 moD Ta2O3mp=2070K→引3mp=1380K(107°O complete the Cr2O3mp=2710K→mp=1807K(1534°O CO,+Ta, O +Cr, 0 2Sr CrTao+cOA Although you may get a complete reaction by heating to 1150C, in practice there will still be a fair amount of ou need unreacted CrO TaO322095g(0.005mo) Cr30.7600g(O preliminary fire at 1400C should be used to prevent the a 500-1600C. The initial heating cycle should be slow, or 3)Grind in a mortar and pestle for 5-15 minutes, then CO, from violently decomposing and spilling out of press a pellet crucible 4) Applying Tammans rule to each of the reagents 5)If the sample is pelletized, the reaction with an alumina crucible should be rather small For the highest purity rCO3→SrO1370°(1643K products, a platinum crucible should be used SrO mp=2700K→引3mp=1800K(1527O 6)All of the elements are in stable highly oxidized states the product, so that heating in air should be appropriate4 To obtain good rates of reaction, you typically need the diffusion coefficient to be larger than ~ 10-12 cm2/s. The diffusion coefficient increases with temperature, rapidly as you approach the melting point. This concept is leads to Tamman’s Rule: Extensive reaction will not occur until the temperature reaches at least 2/3 of the melting point of one or more of the reactants. Rates of Reaction are controlled by three factors: 1) The area of contact between reacting solids To maximize the contact between reactants we want to use starting reagents with large surface area. Consider the numbers for a 1 cm3 volume of a reactant Edge Length = 1 cm number of Crystallites = 1 Surface Area = 6 cm2 Edge Length = 10 mm number of Crystallites = 109 Surface Area = 6x103 cm2 Edge Length = 100Å number of Crystallites = 1018 Surface Area = 6x106 cm2 Pelletize to encourage intimate contact between crystallites! 2) The rate of diffusion Two ways to increase the rate of diffusion are to Increase temperature Introduce defects by starting with reagents that decompose prior to or during reaction, such as carbonates or nitrates. 3) The rate of nucleation of the product phase We can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product (topotactic and epitactic reactions). Consider for Example: the Synthesis of Sr2CrTaO6 1) Possible starting reagents Sr Metal – Hard to handle, prone to oxidation SrO - Picks up CO2 & water, mp = 2430°C Sr(NO3 ) 2 – mp = 570°C, may pick up some water SrCO3 –decomposes to SrO at 1370°C Ta Metal –mp = 2996°C Ta2O5 –mp = 1800°C Cr Metal – Hard to handle, prone to oxidation Cr2O3 – mp = 2435°C Cr(NO3 ) 3•nH2O –mp = 60°C, composition inexact 2) Weigh out starting reagents To make 5.04 g of Sr2CrTaO6 (FW = 504.2 g/mol; 0.01 mol) to complete the reaction: 4SrCO3+Ta2O5+Cr2O3®2Sr2CrTaO6+4CO2 you need: SrCO3 2.9526 g (0.02 mol) Ta2O5 2.2095 g (0.005 mol) Cr2O3 0.7600 g (0.005 mol) 3) Grind in a mortar and pestle for 5-15 minutes, then press a pellet 4) Applying Tamman’s rule to each of the reagents: SrCO3 ® SrO 1370°C (1643 K) SrO mp = 2700 K ® 2 / 3 mp = 1800K (1527°C) Ta2O5 mp = 2070 K ® 2 / 3 mp = 1380K (1107°C) Cr2O3 mp = 2710 K ® 2 / 3 mp = 1807K (1534°C) Although you may get a complete reaction by heating to 1150°C, in practice there will still be a fair amount of unreacted Cr2O3 . Therefore, to obtain a complete reaction it is best to heat to 1500-1600°C. The initial heating cycle should be slow, or a preliminary fire at 1400°C should be used to prevent the SrCO3 from violently decomposing and spilling out of the crucible. 5) If the sample is pelletized, the reaction with analumina crucible should be rather small. For the highest purity products, a platinum crucible should be used. 6) All of the elements are in stable highly oxidized states in the product, so that heating in air should be appropriate