Factors affecting reaction rate catalysts

In Chapter 20, we will investigate in more detail the factors affecting reaction rates, including the way in which a catalyst is able to speed up a reaction and reduce the time it takes for the reaction to reach equilibrium. 

The experiments were carried out in small stainless steel autoclaves having an internal volume of 700 mL. The autoclaves, having been charged with a particular catalyst solution and gas mixture of interest, were mounted vertically in electrically heated ovens. The factors affecting the rate of the reaction are partial pressure of carbon monoxide, partial pressure of ethylene, catalyst concentration, temperature, base concentration/pH, and the nature of the base. Carbon monoxide has an inhibitory effect upon the reaction. The rate of reaction increases linearly with ethylene pressure in the low-pressure regime but exhibits saturation at ethylene pressures exceeding 17 atm. The reaction is second order with respect to catalyst concentration. The nature of the base used deter-... 

We explore four variables that affect reaction rates concentration, physical states of reactants, temperature, and presence of catalysts. These factors can be understood in terms of the collisions among reactant molecules that lead to reaction. 

In Reaction 2.14, heat, pressure, and catalysts are needed to convert ethylene gas to polyethylene. Explain the effects of each of the three conditions (heat, pressure, catalysts) in terms of factors that affect reaction rates. 

Other factors that affect reaction rates are temperature and catalysts. Heating a reaction generally increases the rate at which the reaction occurs by providing the reactant... 

Four factors affect the rates of all reactions (1) the nature of the reactants, (2) reactant concentrations, (3) reactant temperature, and (4) the presence of catalysts. 

Equation (5.113) indicates the factors affecting the rate of the reaction in beds. The further the gas composition from the equilibrium, the faster the process goes. Therefore, when the gas is used for reducing the catalyst bed, the gas should include a minimum concentration of H2O. 

Enzymatic Catalysis. Enzymes are biological catalysts. They increase the rate of a chemical reaction without undergoing permanent change and without affecting the reaction equiUbrium. The thermodynamic approach to the study of a chemical reaction calculates the equiUbrium concentrations using the thermodynamic properties of the substrates and products. This approach gives no information about the rate at which the equiUbrium is reached. The kinetic approach is concerned with the reaction rates and the factors that determine these, eg, pH, temperature, and presence of a catalyst. Therefore, the kinetic approach is essentially an experimental investigation. 

The five factors that can affect the rates of chemical reaction are the nature of the reactants, the temperature, the concentration of the reactants, the physical state of the reactants, and the presence of a catalyst. 

The published values for the activation energies and pre-exponential factors of transesterification and glycolysis vary significantly. Catalysts and stabilizers influence the overall reaction rate markedly, and investigations using different additives cannot be compared directly. Most investigations are affected by mass transport and without knowledge of the respective mass transport parameters, kinetic results cannot be transferred to other systems. 

The properties and composition of the CL in PEM fuel cells play a key role in determining the electrochemical reaction rate and power output of the system. Other factors, such as the preparation and treatment methods, can also affect catalyst layer performance. Therefore, optimization of the catalyst layer with respect to all these factors is a major goal in fuel cell development. For example, an optimal catalyst layer design is required to improve catalyst... 

With many reactions, the rates are affected by materials which are neither reactants nor products. Such materials called catalysts can speed a reaction by a factor of a million or much more, or they may slow a reaction (negative catalyst). 

B. Many external factors other than catalysts can affect the rates of physiologic reactions. 

In Equation 3.1, the suffix i usually designates a reaction product. Ihe rate r,-is negative, in case i is a reactant. Several factors, such as temperature, pressure, the concentrations of the reactants, and also the existence of a catalyst affect the rate of a chemical reaction. In some cases, what appears to be one reaction may in fact involve several reaction steps in series or in parallel, one of which may be rate limiting. 

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

Since reaction rates vary markedly with crystal face, the reaction conditions or the manner of preparation can greatly affect the properties of a catalyst. The formation of surface films, such as oxide, which stabilize facets parallel to certain crystal faces may be an important factor in determining catalytic properties. 

Where intraparticle diffusion appreciably affects the rate of the reaction, reduction in catalyst particle size would be necessary to increase the effectiveness factor and hence conversion. But this may not be possible due to the pressure drop limitations in conventional packed beds. In such situations, the use of Monoliths would provide the advantage of higher effectiveness factor. 

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