Factors influencing reaction speeds

Many variables can influence the speed of a reaction. Their number cannot be known a priori, so they need to be determined by experience. The two main variables are  

The influence of temperature on the speed of reactions was found very early on in chemistiy. In general the speed of a reaction increases with temperature, but exceptions to this rule are known (for instance reaction [1.R5] between nitric oxide and oxygen in order to obtain nitrogen dioxide has a speed that decreases as the temperature increases). 

The consequence is that experimenters often confine themselves to this raw result. The computed parameter H is often called the activation energy of the reaction. This definition is also used for a particular class of reactions, which will be called elementary reactions (see Chapter 2), with a precise physical meaning that is not found for the other types of equations. In the general case and in the absence of any additional information, we prefer to call this parameter the temperature coefficient , which is thus defined from the experiment by  

Note 1.6 - In a reactor, this is mainly true in industrial reactors, the temperature is not always the same at all points and thus the speeds differ both in time and space. 

The relationships between reaction speed and concentrations of reactants, products or catalysts are extremely complex and varied. We will see that the diversity of these relations is a real treasure and makes speed-concentration relationships at constant temperature the main tool of justification for a mechanism. 

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Getting Things Moving The metabolic processes of cold-blooded animals like this baby Nile crocodile (Crocodylus niloticus) speed up as the temperatures rise toward midday. In this chapter, you ll see how temperature, as well as several other factors, influences the speed of a reaction. 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

Works on increase of an overall performance of HHP were simultaneously carried out. For example, in [2] a number of the factors influencing specific output power of HHP has been considered. Properties of metal hydrides (absorbing ability, speeds of reactions, porosity of a covering, the characteristic of a heat transmission of a hydride bed) were analyzed for optimum selection. It has been shown that in pressings from powder metal hydrides gas permeability and effective specific heat conductivity of a bed Xes should be in common optimized in the certain range of a weight share of an additional heat-conducting material. 

The heating rate has of course a considerable effect on the factor a (time corresponding to 50 % of volatile matter production). The time is delayed by a factor of 3 for the heating rate 2 C/min, compared to the heating rate 20 C/min. The factor b (indicator of the reaction speed at the point of 50 % production) is also influenced, but significantly less. The reaction is more rapid in the case of heating rate 20 °C/min (b = 7.4) than of a 2 C/min rate (b = 8.9). 

The behaviour of chemical warfare agents in the sea environment depends both on the chemical and the physico-chemical properties of the substances as well as external factors such as temperature, salinity, pH value and turbulence in the water. In the Baltic Sea, for instance, the pH value does not vary much—it is slightly alkaline (pH 8). Salinity and temperature are therefore the main factors which will influence the chemical reaction. Solubility and reaction speed increase if temperature increases. With a temperature increase of lOoC for example, the speed of a chemical reactions doubles. 

From the interaction diagram as given in Fig. 13.3, the gel effect is identified as a possible source of chemical instabilities. Nevertheless, there are two other chemical effects that influence the stability of reactive extrusion The ceiling temperature and the phase separation. The stability of a reactive extrusion process is influenced by these three factors because they have a direct influence on the viscosity and reaction speed and therefore on the hydrodynamic instabilities. 

The initiation speed of the reaction (I) influences the speed of destructive processes that can be seen in the Table 23.2-23.4. The rise in the initial concentrations of H2O2, FeSO and the temperature really increases the concentration of PVA radical intermediators subjected to destruction in the reaction system. It leads in its turn to lowering the dependence of kinematic viscosity from the mentioned factors (Table 23.2-23.4). 

By a knowledge of how various factors influence the rale of a reaction, it becomes possible to bring the reaction under control, i.e., the speed of a reaetkm can be regulated to gain the desired effect. The economic viability of the process can Ihcrefore be ascertained. 

In this chapter, we begin to focus on chemical changes and their interaction with diffusion. We are particularly interested in cases in which diffusion and chemical reaction occur at roughly the same speed. When diffusion is much faster than chemical reaction, then only chemical factors influence the reaction rate these cases are detailed in books on chemical kinetics. When diffusion is not much faster than reaction, then diffusion and kinetics interact to produce very different effects. 

The dump temperature of the compound was varied by changing the mixer s rotor speed and fill factor while keeping the other mixing conditions and the mixing time constant. Under the assumption that the final dump temperature is the main parameter influencing the degree of the sUanization reaction, the effect of the presence of ZnO on the dynamic and mechanical properties of the compound was investigated. ZnO was either added on the two-roll mill or in the mixer. 

To summarize, the kinetics of the silanization reaction are strongly influenced by the efficiency of the devolatilization process. The degree of devolatilization mainly depends on processing conditions (e.g., rotor speed and fill factor), mixer design (e.g., number of rotor flights, size of the mixer), and material characteristics. The diffusion coefficient of the volatile component in the polymeric matrix is of minor influence. 

Mechanical, physical, or chemical external irritants act not only at the place of occurrence, but the excitation can be also transferred along the whole plant [3,6-21]. The speed of transfer depends on many factors, such as the intensity of the irritation, temperature, chemical treatment, or mechanical wounding it is also influenced by previous excitations. The excitation reaction travels in both directions, from the top of a stem to roots and conversely, but not always at identical rates. The transfer of excitation has a complicated character accompanied by an internal change in cells and tissues. 

The water temperature will also influence the electrocoagulation process. A1 anode dissolution was investigated in the water temperature range from 2 to 90°C. The A1 current efficiency increase rapidly when the water temperature increase from 2 to 30°C. The temperature increase will speed up the destructive reaction of oxide membrane and increase the current efficiency. However, when the temperature was over 60°C, the current efficiency began to decrease. In this case, the volume of colloid Al(OH)3 will decrease and pores produced on the A1 anode will be closed. The above factors will be responsible for the decreased current efficiency. 

When gum formation proceeds, the minimum temperature in the catalyst bed decreases with time. This could be explained by a shift in the reaction mechanism so more endothermic reaction steps are prevailing. The decrease in the bed temperature speeds up the deactivation by gum formation. This aspect of gum formation is also seen on the temperature profiles in Figure 9. Calculations with a heterogenous reactor model have shown that the decreasing minimum catalyst bed temperature could also be explained by a change of the effectiveness factors for the reactions. The radial poisoning profiles in the catalyst pellets influence the complex interaction between pore diffusion and reaction rates and this results in a shift in the overall balance between endothermic and exothermic reactions. 

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