Conditions That Affect Reaction Rates

Recall from Section 13.8 that the average kinetic energy of a collection of molecules is directly proportional to the temperature (K). 

The reason that a reaction occurs faster as the temperature is increased is that the speeds of the molecules increase with temperature. So at higher temperatures, the average collision is more energetic. This makes it more likely that a given collision will possess enough energy to break bonds and to produce the molecular rearrangements needed for a reaction to occur. 

Higher temperatures y Higher speeds y More high-energy collisions y More collisions that break bonds y Faster reaction 

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When molecules collide, a certain minimum energy called the activation energy (Eg) Is needed for a reaction to occur. If the energy contained In a collision of two BrNO molecules Is greater than Eg, the reaction can go over the hump to form products. If the collision energy Is less than Eg, the colliding molecules bounce apart unchanged. 

I AIMS To understand activation energy. To understand how a I cataiyst speeds up a reaction. 

Comparison of the activation energies for an uncat-aiyzed reaction (f ) and for the same reaction with a cataiyst present ( ,). Note that a cataiyst works by iowering the activation energy for a reaction. 

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We can draw a very useful general conclusion from this simple binary system that is applicable to more complex processes changes in production rate can be achieved only by changing conditions in the reactor. This means something that affects reaction rate in the reactor must vary holdup in liquid-phase reactors, pressure in gas-phase reactors, temperature, concentrations of reactants (and products in reversible reactions), and catalyst activity or initiator addition rate. Some of these variables affect the conditions in the reactor more than others. Variables with a large effect are called dominant. By controlling the dominant variables in a process, we achieve what is called partial control. The term partial control arises because we typically have fewer available manipulators than variables we would like to control. The setpoints of the partial control loops are then manipulated to hold the important economic objectives in the desired ranges. 

When the conditions that affect the rate of a chemical reaction are changed in a system at equilibrium, the rates of the forward and the reverse reaction may be affected differently. If these rates become different, more reactants or products are produced. The direction of the shift of an equilibrium can be predicted qualitatively using LeChatelier s principle. LeChatelier s principle states that if a stress is applied to a system at equilibrium, the equilibrium will tend to shift in a direction to relieve that stress. 

Methods in which some property related to substrate concentration (such as absorbance, fluorescence, chemiluminescence, etc.) is measured at two fixed times during the course of the reaction are known as two-point kinetic methods. They are theoreticahy the most accurate for the enzymatic determination of substrates. However, these methods are technically more demanding than equifibrium methods and all the factors that affect reaction rate, such as pH, temperature, and amount of enzyme, must be kept constant from one assay to the next, as must the timing of the two measurements. These conditions can readily be achieved in automatic analyzers. A reference solution of the analyte (substrate) must be used for calibration. To ensure first-order reaction conditions, the substrate concentration must be low compared to the K, (i.e., in the order of less than 0.2 X K, . Enzymes with high K , values are therefore preferred for kinetic analysis to give a wider usable range of substrate concentration. 

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. 

The Collision Theory of Chemical Reactions Energy Changes During a Molecular Collision Conditions That Affect the Rate of a Chemical Reaction The Development of a Chemical Equilibrium Le Chatelier s Principle The Equilibrium Constant The Significance of the Value of K... 

Conditions That Affect the Rate of a Chemical Reaction... 

What conditions might alter the equilibrium state Concentration and temperature These are factors that affect the rate of reaction. Equilibrium is attained when the rates of opposing reactions become equal. Any condition that changes the rate of one of the reactions involved in the equilibrium may affect the conditions at equilibrium. 

However, some sets of reactants can undergo both a forward and a reverse reaction under the same set of conditions. This circumstance leads to a state called chemical equilibrium. Before we take up equilibrium, however, we have to learn about the factors that affect the rate of a chemical reaction. 

It must be also considered that the reaction rates of different thermal processes which can occur simultaneously are influenced by the treatment conditions (temperature, heating rate, pressure, static or dynamic atmosphere). This will affect the relative quantities of the products formed and in some cases also their nature, when recombination reactions give rise to secondary degradation products. On account of its sensitivity and resolution power Py-GC/MS will also provide useful information on minor components present in a material, including low molecular weight additives and pigments. 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

For a particular gas-carbon reaction. Equation (39), with one reservation, leads to the conclusion that under identical reaction conditions (i.e., Cg, Dfree, and S are constant), the rate of reaction in Zone III is independent of the type of carbon reacted. The reservation is that in the carbon-oxygen reaction, the nature of the carbon may affect the CO-CO2 ratio leaving the surface and hence the reaction rate per unit of oxygen diffusing to the surface. Unfortunately, little data are available on reactivities of different carbons where the reaction has been conducted completely in Zorn III. Day (2Ii) reports that the reaction rates of petroleum coke, graphitized lampblack, and graphitized anthracite rods agree within 12 % at a temperature of 1827° and at a constant gas velocity. 

Heat is a form of energy that affects the rate at which chemical reactions take place. The shelf life of a product can therefore be considerably reduced by exposure to elevated temperatures. Odor deterioration and discoloration will develop much faster under these conditions, and this is exploited in the accelerated testing of perfumes for stability in final products. Some products are manufactured at quite high temperatures, and perfumers have to keep this in mind in the formulation of their fragrances. 

Before entering into discussions of the conditions that affect chemical reaction rates and reactor design, it is necessary to account for the various chemical species entering and leaving a reaction system. This accounting process is achieved through overall mole balances on individual species in the... 

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