Spontaneous surface reactions

Spontaneous surface reactions vs. stimulated surface reactions (by polarization). 

The spontaneous redox reaction shown in Figure 19-7 takes place at the surfaces of metal plates, where electrons are gained and lost by metal atoms and Ions. These metal plates are examples of electrodes. At an electrode, redox reactions transfer electrons between the aqueous phase and the external circuit. An oxidation half-reaction releases electrons to the external circuit at one electrode. A reduction half-reaction withdraws electrons from the external circuit at the other electrode. The electrode where oxidation occurs is the anode, and the electrode where reduction occurs is the cathode. 

An additional surprise is, that the reaction rates of these spontaneous back-reactions 153 - 152 (X = Br, Cl) are temperature-independent between 8.5 and 25 K. In our opinion this unusual kinetic behavior might be the outcome of a double two-state reaction involving a surface crossing with the triplet state of... 

First, consider the gradient of cA. Since A is consumed by reaction inside the particle, there is a spontaneous tendency for A to move from the bulk gas (cAg) to the interior of the particle, first by mass transfer to the exterior surface (cAj) across a supposed film, and then by some mode of diffusion (Section 8.5.3) through the pore structure of the particle. If the surface reaction is irreversible, all A that enters the particle is reacted within the particle and none leaves the particle as A instead, there is a counterdiffusion of product (for simplicity, we normally assume equimolar counterdiffusion). The concentration, cA,at any point is the gas-phase concentration at that point, and not the surface concentration. 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

This chapter discusses the fluid-solid and solid-solid reactions used to produce ceramic powders. The first aspect of this discussion is the spontaneity of a particular reaction for a given temperature and atmosphere. Thermodynamics is used to determine whether a reaction is spontaneous. The thermod3mamics of the thermal decomposition of minerals and metal salts, oxidation reactions, reduction reactions, and nitridation reactions is discussed because these are often used for ceramic powder synthesis. After a discussion of thermodynamics, the kinetics of reaction is given to determine the time necessary to complete the reaction. Reaction kinetics are discussed in terms of the various rate determining steps of mass and heat transfer, as well as surface reaction. After this discussion of reaction kinetics, a brief discussion of the types of equipment used for the synthesis of ceramic powders is presented. Finally, the kinetics of solid—solid interdiffusion is discussed. 

Once the thermod3mamics of chemical reaction is determined as spontaneous, the reaction kinetics will establish the importance of this reaction to the degradation of the ceramic powder in the solvent. Reaction kinetics of this t3rpe between a solid and a (liquid) fluid were discussed in Chapter 5. Under some conditions the reaction kinetics are very slow, limited by either a slow surface reaction or a slow product layer diflusion. As a result, this reaction can be n ected in its importance to the ceramic powder s chemical stability. Unfortunately little information is found in the literature on the reaction kinetics for ceramic powders reacting with organic solvents. Therefore, trial and error seems to be the only dependable way to determine the chemical stability of ceramic powders in nonaqueous solvents. This is the way that the chemical decomposition of YBa2Cu3Q,. in alcohols was determined. 

A very dangerous fire hazard when exposed to heat, flame, or by spontaneous chemical reaction. A severe explosion hazard when shocked or exposed to O3, heat, or flame. Nitroglycerin is a powerful explosive, very sensitive to mechanical shock, heat, or UV radiation. Small quantities of it can readily be detonated by a hammer blow on a hard surface, particularly when it has been absorbed in filter paper. It explodes when heated to 215°C. Frozen nitroglycerin is somewhat less sensitive than the liquid. However, a half-thawed or partially thawed mixture is more sensitive than either one. When heated to decomposition it emits toxic fumes of NOx. 

Because of their monofiinctionality, the title compounds may be used as a silicone offset for many organic molecules and additives. While the monosilanol oil is prone to spontaneous condensation reactions and has limited pot life, the amino- and chlorofiinctional fluids will confer the silicone haptics, surface tension, weatherability, and other sought-after siloxane properties onto the organic formulation. 

Although corrosion is a serious problem for many metals, we will focus on the spontaneous electrochemical reactions of iron. Corrosion can be pictured as a short-circuited galvanic cell, in which some regions of the metal surface act as cathodes and others as anodes, and the electric circuit is completed by electron flow through the iron itself. These electrochemical cells form in parts of the metal where there are impurities or in regions that are subject to stress. The anode reaction is... 

In the lemon battery shown in Figure 17.9, a different chemical reaction occurs at each of the metal-strip electrodes. The electrode made of the metal that is more easily oxidized becomes the anode—the electrode at which the oxidation reaction occurs. The second electrode becomes the cathode, and a reduction reaction proceeds at its surface. The substance in a lemon that is most easily reduced is the abundant hydrogen ion of the electrolyte. When these two reactions occur together, in the same cell, they combine to produce a spontaneous redox reaction. This type of reaction is represented by the equation below, where M is the metal that is oxidized. 

In this model of a lemon battery, the level of the electron sea is raised or lowered by the chemical reactions at the electrode surfaces, creating a potential difference across the battery. A spontaneous oxidation reaction raises the electron pressure (potential) at the anode, and a spontaneous reduction reaction reduces the pressure at the cathode. The "sea level" in the lemon juice is uniform throughout and is intermediate between the levels at the two electrodes. 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

It is quite impossible to determine the absolute potential difference across a sin e met /solution interface, and the potential must be evaluated indirectly from the e.m.f. of a cell comprising the interface under consideration and another electrified interface. The e.m.f. of the cell can be determined readily by a suitable measuring device such as a potentiometer, vacuum-tube voltmeter or an electrometer, which are capable of measuring the e.m.f. with the minimum passage of electrical charge. This is essential since if a significant current is allowed to pass, the electrodes (electrified interfaces) become polarised and the e.m.f. will be less than the equilibrium value. Consider the determination of the interfacial potential at the surface of a zinc electrode in equilibrium with Zn ions in solution. In order to determine the potential it is necessary to couple it with another electrode, and for the purpose of this discussion the equilibrium between ions in solution and gas will be chosen, i.e. the reversible hydrogen electrode in which the equilibrium between and H takes place at a platinised-platinum surface. The spontaneous cell reaction will be... 

VOLTAIC CELLS We consider voltaic cells, which produce electricity from spontaneous redox reactions. Solid electrodes serve as the surfaces at which oxidation and reduction take place. The electrode where oxidation occurs is the anode, and the electrode where reduction occurs is the cathode. 

Fig. 14.2. The drainpipe A+BC -> AB+C (for a fictitious collinear system). The surface of the potential energy for the motion of the nuclei is a function of distances 1 aB and 1 bc On the left side, there is the view of the surface, while on the right side, the corresponding maps are shown. The barrier positions are given by the crosses on the right figures. Panels (a) and (b) show the symmetric entrance and exit channels with the separating barrier. Panels (c) and (d) correspond to an exothermic reaction with the barrier in the entrance channel ( an early barrier ). Panels (e) and (f) correspond to an endothermic reaction with the barrier in the exit channel ( a late barrier ). This endothermic reaction will not proceed spontaneously, because due to the equal width of the two channels, the reactant s free energy is lower than the product s free energy. Panels (g) and (h) correspond to a spontaneous endothermic reaction, because due to the much wider exit channel (as compared to the entrance channel), the free energy is lower for the products. Note that there is a van der Waals complex well in the entrance channel just before the barrier. There is no such well in the exit channel.

Many of the ions contained in seawater have very high reduction potentials—higher than Fe(r). This means that spontaneous electrochemical reactions will occur with the Fe(i), causing the iron to form ions and go into solution while at the same time, the ions in the sea are reduced and plate out on the surface of the iron. 

In most cases, the surface of a metal or an alloy, and often also of a polymer, differs from the bulk. This is due to spontaneous reactions with the environment like oxidation, which can lead either to passivation or to ongoing destruction. In alloys, which account for the majority of technically important materials, the composition of the surface often differs from the bulk composition to dealloying or to segregation processes that accompany surface reactions. 

Consider an electrode at state

reduction reaction, we must go from < i to < 3. Although the potential energy of the electrode is increased to promote this reaction and support charge transfer across the double layer of the electrode, the actual surface overpotential relative to the SHE will decrease (see Figure 4.5), since we are moving toward a cathodic reduction reaction product in this example. 

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