Factors Affecting Reaction Rates

Chemical kinetics is concerned with the rate of reaction and factors affecting the rate, and chemical thermodynamics is concerned with the position of equilibrium and factors affecting equilibrium. 

In this section, you used collision theory and transition state theory to explain how reaction rates are affected hy various factors. You considered simple reactions, consisting of a single-step collision between reactants. Not all reactions are simple, however. In fact, most chemical reactions take place via several steps, occurring in sequence. In the next section, you will learn about the steps that make up reactions and discover how these steps relate to reaction rates. 

For industrial reactors, the effectiveness factor (i]) is used to provide a measure of the actual reaction rate, as affected by operating conditions, in comparison to the intrinsic reaction kinetics. Assuming that the trickle bed reactor shown in Fig. 4 is operated so that interphase transport of one of the reactants, steps 2 or 8 above, is controlling. 

Enzyme kinetics refers to the quantitative analysis of all factors that determine the catalytic potential of an enzyme. As presented in section 1.3, enzyme activity represents the maximum catalytic potential of an enzyme that is reflected by the initial rate of the catalyzed reaction. Several factors affect the expression of such potential, being the most important the concentrations of active enzyme, substrates and inhibitors, temperature and pH. In the case of insolubilized enzymes or multiphase systems, other variables that reflect mass transfer constraints must be considered. 

To summarize, in the present section, we have demonstrated that the stilbene photoisomerization is a training area, a relatively simple and convenient model reaction for a thorough investigation of detailed mechanisms of photochemical reactions and factors affecting the photochemical conversion rate. Theoretical and experimental data in this area vdll pave the way for practical application of stilbenes as switching materials and biophysical probes (Chapter 10). 

The rate of corrosion is controlled by the rate of dissolved oxygen reaction on the surface, which is controlled by the rate of transport of dissolved oxygen to the surface. Similar mass transfer mechanisms control the rate of transfer of fouling ions to the surface. In a system containing a corrosion inhibitor, the transfer of inhibitor to the surface is also controlled by the same factors. Once the reactants are at the surface, reaction rates are affected by temperature. Temperature also affects the properties of the fluid film in contact with the surface. For example, viscosity typically decreases in the film on the surface of a hot wall, facihtat-ing reactant transfer. It is difficult to generalize about the affect of temperature on corrosion rates in a system treated with corrosion inhibitor. Increased temperature is likely to accelerate the corrosion inhibition reactions as well as the corrosion reaction. The net change in corrosion rate could be either an increase or a decrease, depending upon the effectiveness of the corrosion inhibition treatment. 

Ammonia s3mthesis is a reversible and exothermic reaction without any side reaction. With the rising of temperature the reaction rate constant increases while the equilibrium constant decreases. For a given reactant composition, the reaction rate is affected by two contradictory factors thus there exists an optimum reaction temperature. 

Scale-Up Principles. Key factors affecting scale-up of reactor performance are nature of reaction zones, specific reaction rates, and mass- and heat-transport rates to and from reaction sites. Where considerable uncertainties exist or large quantities of products are needed for market evaluations, intermediate-sized demonstration units between pilot and industrial plants are usehil. Matching overall fluid flow characteristics within the reactor might determine the operative criteria. Ideally, the smaller reactor acts as a volume segment of the larger one. Elow distributions are not markedly influenced by... 

In all cases, water and carbonic acid, the latter of which is the source of protons, are the main reactants. The net result of the reaction is the release of cations (Ca " ), Mg ", K", Na" ) and the production of alkalinity via HCO. When ferrous iron is present in the lattice, as in biotite, oxygen consumption may become an important factor affecting the weathering mechanism and the rate of dissolution. 

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

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 rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Now that we have a good picture of how SN2 reactions occur, we need to see how they can be used and what variables affect them. Some SN-2 reactions are fast, and some are slow some take place in high yield and others, in low yield. Understanding the factors involved can be of tremendous value. Let s begin by recalling a few things about reaction rates In general. 

Reacting species, predominant, 80 Reaction coordinate, 133 Reaction heat, 135 additivity of, 111 measurement of, 111 Reaction rates, 124 factors affecting, 125 Reactions, 38,129 acid-base, 188 balancing, 42, 217, 219 calcium carbonate decomposition, 143... 

Although the mean relative speed of the molecules increases with temperature, and the collision frequency therefore increases as well, Eq. 16 shows that the mean relative speed increases only as the square root of the temperature. This dependence is far too weak to account for observation. If we used Eq. 16 to predict the temperature dependence of reaction rates, we would conclude that an increase in temperature of 10°C at about room temperature (from 273 K to 283 K) increases the collision frequency by a factor of only 1.02, whereas experiments show that many reaction rates double over that range. Another factor must be affecting the rate. 

MULTIPLE FACTORS AFFECT THE RATES OF ENZYME-CATALYZED REACTIONS... 

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