Application of Le Chatelier's Principle and Catalysis - Lab Report Example

Chemical Industry: A Study On Application Of Le Chatelier's Principle And Catalysis 1 Introduction Innovations in science and technology since the early 20th Century have lead to the development of certain standard and efficient procedures in the field of industrial chemistry. Although scientists are on the look out for better industrial processes, there exist some processes that give high yield at optimum cost. The explanation for every chemical process used has its roots in physical chemistry - thermodynamics, chemical equilibrium, and structure. The objective of this study is to determine the role of two aspects of physical chemistry in the development of industrial processes: 1. Dynamic equilibrium and the use of Le Chatelier's principle in high yield commercial chemical processes. 2. Catalysis and its role in chemical technology. In this study, we consider four common processes used in the industry: the Haber process for production of ammonia, manufacture of ethanol from ethene, cracking of petroleum fractions, and esterification. An understanding of the Le Chatelier's principle and catalysis is essential for proceeding with this study. 2 Background 2.1 Dynamic equilibrium and Le Chatelier's principle The French chemist Henri Le Chatelier proposed a principle for equilibrium reactions that is commonly known as Le Chatelier's Principle: "A system at equilibrium, when subjected to a disturbance, responds in a way that tends to minimize the effect of the disturbance." (Atkins & de Paula, 2006) Disturbances to a system at equilibrium cause different changes depending on the reaction: Increase in temperature: Exothermic reaction favours reactants, as the heat is absorbed to counterbalance the increase in temperature. Endothermic reaction favours products as the absorbed heat facilitates a larger amount of reactants to convert to products (Atkins & de Paula, 2006). Change in reactant concentration: An increase in concentration of a reactant in a dynamic equilibrium causes the reaction to favour products, as the excess reactant reacts with other reactants. A decrease in concentration of a reactant favours the reverse reaction as the products are converted to reactants in order to balance the decrease in reactant concentration. (Clark, 2002; Morrison & Boyd, 1992) Change in product concentration: A decrease in product concentration favours the forward reaction to balance the decrease. An increase in product concentration favours the reverse reaction. (Atkins & de Paula, 2006) Change in pressure: Increasing the pressure of a system at equilibrium causes changes in the partial pressures of reactants and products. The rate constant is independent of the change in pressure of a system, but depends on ratio of partial pressures of the reactants and products. The reaction moves in the direction where number of moles is less. (Atkins & de Paula, 2006) 2.2 Catalysis A catalyst accelerates a chemical reaction without undergoing a net chemical change. The catalyst reduces the energy of activation by altering the path of reaction to avoid the rate determining step, which is the slowest step in a reaction (Atkins & de Paula, 2006). For example, decomposition of hydrogen peroxide, which is a slow reaction at room temperature, requires activation energy of 76 kJ/mol. In the presence of iodide ions, this activation energy drops to 57 kJ/mol and rate constant increases by 2000. Catalysts are classified as: 1. Homogenous catalysts, which are of the same phase as reactants. Bromide acts as a homogenous catalyst during decomposition of hydrogen peroxide in aqueous solution. These catalysts usually act as proton donors or acceptors. 2. Heterogeneous catalysts, which are of a different phase from reactants. Nickel catalyses the gas-phase hydrogenation of ethylene to ethane. In this catalysis a reactant is chemisorbed into the surface of a catalyst and modified to facilitate the reaction. Therefore, rate of reaction depends on the surface area of the catalyst. According to Atkins & de Paula (2006), almost the entire chemical industry today is based on the development, selection, and application of catalysts. Figure 1 Effect of catalyst in a reaction (Source: Atkins & de Paula, 2006) 2.3 Catalyst and dynamic equilibrium A catalyst only accelerates a reaction by lowering the energy of activation. As the energy of activation is reduced, both forward and reverse reactions are equally favoured. So, there is no net effect on the reactant and product amounts. The system only reaches equilibrium faster. (Clark, 2002) 3 Study 3.1. Haber process for the production of ammonia: Ammonia finds many applications in different industries, as an organic solvent and as a precursor to many chemicals such as urea and nitric acid, among others. Haber process is commercially used to manufacture ammonia in large scale. Nitrogen from air combines with hydrogen from methane to form ammonia in an exothermic reaction. Both the gasses are passed through the reactor and cooler through several cycles to improve yield. On cooling, ammonia gas liquefies and is separated. Iron acts as the catalyst with potassium hydroxide as promoter Figure 2 Schema of Haber process. Source: http://www.chemguide.co.uk/physical/equilibria/haber.html (March 19, 2009) 3.1.1 Analysis of factors affecting the process 3.1.1.1 Catalyst In the process, α iron is used as a catalyst and potassium hydroxide as a promoter. The catalyst is heterogeneous: reactants are in gas phase while catalyst is in solid phase. Considerations: Le Chatelier's principle: As the catalyst does not cause any net change in the dynamic equilibrium, Le Chatelier's principle does not apply. Reaction rate: The catalyst causes a lowering of activation energy for the reaction. Increased surface area of the iron catalyst would mean an increase in reactant adsorption and greater yield. In order to widen the lattice and to enlarge the catalyst surface area, a metal oxide is used as promoter (Cotton & Wilkinson, 1988). 3.1.1.2 Reactant concentrations In the reaction, 1 mol of nitrogen gas reacts with 3 mol of hydrogen gas to form 2 moles of nitrogen gas to form ammonia. Considerations Le Chatelier's principle: According to the principle, an increase in concentration of one of the reactants should favour the formation of ammonia gas. However, an increase in nitrogen gas concentration when hydrogen gas concentration is fixed would imply that more and more unreacted nitrogen molecules pass through the reactor - which is a waste of money. Catalysis: Excess of a reactant would imply greater chemisorption by the catalyst. If adequate number of moles of the other reactant is not available, catalyst space is wasted. 3.1.1.3 Product concentration The actual yield of the reaction is about 15%. However, the reactants are passed through the reactor in repeated cycles while the product ammonia gas is liquefied at low temperature and high pressure. Considerations: Le Chatelier's principle: Removal of the product favours the forward reaction. So, more product (ammonia gas) is formed. Catalysis: As the reaction is reversible, removal of the product prevents reverse catalysis during repeated cycles. 3.1.1.4 Temperature The reactor temperature is at 400–450°C. Considerations: Le Chatelier's principle: According to the principle, higher temperatures in an exothermic reaction should favour the reverse reaction, where heat is absorbed to give the reactants. Lower temperatures would favour the formation of ammonia as heat is released in the balance. However, the temperature of the reactor is not low. Rate of reaction: Rate of a reaction is dependent on the temperature: the reaction is slow at lower temperatures. The system then takes longer to reach equilibrium, which would increase the cost of production. Increasing the temperature accelerates the reaction. 3.1.1.5 Pressure The reactor pressure can vary between 100 atm and 1000 atm, but many ammonia manufacturers use a pressure of 200 atm. Considerations: Le Chatelier's principle: Increasing pressure favours the forward reactions as the number of moles of ammonia is less than that of the reactants. Catalysis: At higher pressures, reactant molecules have a greater chance of colliding with the catalyst surface and getting attached. Cost: Higher pressure reactors are expensive to build, to operate, and to maintain. So, manufacturers use a pressure of 200 atm. 3.2 Production of ethanol from ethene Ethanol is manufactured commercially by passing ethene (ethylene) gas and steam through a reactor. The reaction is mentioned below (Clark, 2002): The process uses phosphoric acid catalyst (solid phase) and takes place at 300°C and pressure of 60–70 atm. The reaction yield is 5% and repeated cycles can give a yield of up to 95%. Ethanol is removed by condensation (Clark, 2002). Water in ethanol is removed by treatment with magnesium and distillation (Morrison & Boyd, 1992). Figure 3 Schema of ethanol production. Source: http://www.chemguide.co.uk/physical/equilibria/ethanolflow.gif (March 19, 2009) Figure 3 gives a schematic representation of the process. The factors affecting the process are analyzed in Table 1. Table 1 Factors affecting Haber process Factor Used Consideration Catalyst Phosphoric acid Le Chatelier's principle: No effect on equilibrium mixture. Reaction rate: Catalyst increases rate of reaction Reactant concentration Ethene: 1 mol Water: 0.6 mol Le Chatelier's principle: Excess of ethene favours ethanol formation. Catalysis: Excess of steam could dilute the catalyst, so an excess of ethene is used. Product Concentration Ethanol is removed by condensation Le Chatelier's principle: Removal of ethanol favours forward reaction and increases yield through repeated cycles. Catalysis: Ethanol present can further react with catalyst to give other products or it can undergo reverse catalysis. Temperature 300°C Le Chatelier's principle: As the reaction is exothermic, higher temperature would favour reverse reaction. Reaction rate: Higher temperature increases the reaction rate and allows the system to reach equilibrium faster Pressure 60–70 atm Le Chatelier's principle: Higher pressures will favour ethanol formation as the number of moles is lesser than that of reactants. Catalysis: Higher pressures increase collision and attachment of reactants to catalytic surface. By products: Polymerization may occur at higher pressure. Cost: High pressure involves increased building, operation and maintenance costs 3.3 Catalytic cracking of petroleum for fuels Many fuels are produced from petroleum through catalytic cracking of petroleum fractions such as gas oil (Morrison & Boyd, 1992). The larger chain compounds in petroleum, which are in gas phase are brought into contact with zeolites (aluminosilicate complexes) which are in finely divided solid phase at temperatures of 450–550°C and slight pressure (Morrison & Boyd, 1992 Clark, 2002). Through this process, high yields of smaller, branched alkanes and alkenes are obtained. The molecules break in different ways to yield different smaller molecules, all of which comprise fuels. An example of such a breakage is shown in the equation below (Morrison & Boyd, 1992): . The factors affecting the cracking process are analyzed in the Table 2. Table 2. Factors affecting cracking of petroleum fractions Factor Used Consideration Catalyst Zeolite Le Chatelier's principle: No effect. Reaction rate: Catalyst increases rate of reaction Reactant concentration Reactant is recycled Le Chatelier's principle: Higher concentrations of reactants favour the forward reaction. Catalysis: Recycling ensures that unreacted long chains receive more opportunities to bond with the catalyst and increase the yield. Product concentration Products are removed continuously Le Chatelier's principle: Removal of products favours forward reaction and increases yield. Catalysis: Removal of products may prevent undesirable cracking to some extent. Temperature 450–550°C Le Chatelier's principle: As the reaction is endothermic (energy is required to break up the bonds), an increase in temperature favours product formation. Catalyst: Some undesirable cracking may take place at higher temperature and catalyst may start bonding with the carbon coke. This could decrease efficiency of the catalyst. Pressure 60–70 atm Le Chatelier's principle: Higher pressures should not favour formation of multiple products with higher number of moles. Catalysis: Chances of carbon atoms colliding with catalyst increases with pressure. Also, more carbon atoms in the same molecule can attach to the catalyst at higher pressures, leading to formation of isomers and highly branched products. Cost: The cost of maintaining higher pressure is less than the profit from the products. 3.4 Esterification of adipic acid Adipic acid, a dicarboxylic acid reacts with ethyl alcohol in the presence of sulphuric acid catalyst and heat to form diethyl adipate or adipic acid diethyl ester. The reaction takes place in a distillation column to remove water through azeotropic distillation (Morrison & Boyd, 1992). In the reaction, an excess of ethanol is used, while toluene is the organic solvent. At 75°C, an azeotropic mixture of water, ethyl alcohol, and toluene is removed. Removal of water molecules prevents hydrolysis of ester. Ethanol can be separated and used again. Table 3 analyzes the factors affecting the esterification process. Table 3. Factors affecting esterification Factor Used Consideration Catalyst Sulphuric acid Le Chatelier's principle: No effect on equilibrium mixture. Reaction rate: Catalyst increases rate of reaction Reactant concentration Excess ethanol Le Chatelier's principle: Excess of ethanol favours product formation. Product concentration Water is removed Le Chatelier's principle: Removal of water shifts the equilibrium in favour of the products. Catalysis: Reverse catalysis is prevented. Temperature 100°C Le Chatelier's principle: As reaction is endothermic, higher temperature favours forward reaction. Reaction rate: Higher temperature increases the reaction rate and allows the system to reach equilibrium faster. Distillation: As soon as water is formed, it is removed as an azeotropic distillate because of the temperature. Ethyl adipate has a boiling point of 245°C and is not removed easily. 4 Conclusions The factors affecting efficiency were analyzed for four different chemical industrial processes: Haber process for ammonia, ethanol synthesis from ethane, cracking of petroleum products, and esterification of adipic acid. Special emphasis was given to the application of catalysis and Le Chatelier's principles. While all processes were designed to increase the yields, at times some of the principles were compromised for other considerations such as improving the reaction rate. Bibliography Atkins, P., & de Paula, J. (2006). Atkins’ Physical Chemistry, Eighth Edition. Oxford: Oxford University Press. Clark, J. (2002). Chemguide: Helping you understand chemistry. Retrieved March 19, 2009, from < http://www.chemguide.co.uk/>. Cotton, F.A., & Wilkinson, G. (1988). Advanced Inorganic Chemistry, Fifth Edition. New York: John Wiley & Sons. Morrison, R.T., & Boyd, R.N. (1992) Organic Chemistry, Sixth Edition. Englewood Cliffs, NJ: Prentice Hall, Inc. Read More