Exothermic equilibrium reaction

Although the left to right reaction is exothermic, hence giving a better equilibrium yield of sulphur trioxide at low temperatures, the reaction is carried out industrially at about 670-720 K. Furthermore, a better yield would be obtained at high pressure, but extra cost of plant does not apparently justify this. Thus the conditions are based on economic rather than theoretical grounds (cf Haber process). 

Oxidation Feedstocks generally hydrocarbons Hazard of fire/explosion arises from contact of flammable material with oxygen Reactions highly exothermic equilibrium favours complete reaction... 

The reaction occurs in two steps ammonium carbamate is formed first, followed by a decomposition step of the carbamate to urea and water. The first reaction is exothermic, and the equilibrium is favored at lower temperatures and higher pressures. Higher operating pressures are also desirable for the separation absorption step that results in a higher carbamate solution concentration. A higher ammonia ratio than stoichiometric is used to compensate for the ammonia that dissolves in the melt. The reactor temperature ranges between 170-220°C at a pressure of about 200 atmospheres. 

Another difficulty is that spontaneous chemical reactions do not go to completion. Even if a spontaneous reaction is exothermic, it proceeds only till it reaches equilibrium. But in our golf ball analogy, equilibrium is reached when all of the golf balls are on the lower level. Oui analogy would lead us to expect that an exothermic reaction would proceed until all of the reactants are converted to products, not to a dynamic equilibrium. 

Reactions highly exothermic equilibrium favours complete reaction... 

According to Le Chatelier s principle the equilibrium will be shifted to the right-hand side by high pressures and, since the reaction is exothermic, by low temperatures. Indeed early work by Haber showed that at 200 °C and 300 atmospheres pressure the equilibrium mix would contain 90% ammonia, whilst at the same pressure but at 700 °C the percentage of ammonia at equilibrium would be less than 5%. Unfortunately the activation energy is such that temperatures well in excess of 1000 °C are needed to overcome this energy barrier (Figure 4.1). The conclusion from this is that direct reaction is not a commercially viable option. 

The reactor brings the reaction mixture to equilibrium at the outlet temperature. The reaction is exothermic and the equilibrium constant K is given by ... 

The reaction mixture is heated to speed up the rate at which equilibrium is reached, not to shift the equilibrium toward more product. Heating actually decreases the amount of product present at equilibrium since the reaction is exothermic (AH is negative). 

One of the issues of the industrial process design is related to the heat released by this reaction. A temperature rise will decrease the acetic acid yield, not only because the equilibrium constant becomes lower (the reaction is exothermic see section 2.9) but also because it will reduce the enzyme activity. It is therefore important to keep the reaction temperature within a certain range, for instance, by using a heat exchanger. However, to design this device we need to know the reaction enthalpy under the experimental conditions, and this quantity cannot be easily found in the chemical literature. 

The heat of reaction for vinyl polymers affects the thermal stability of the polymer during extrusion, and the thermal stability is related to the ceiling temperature. The ceiling temperature is the temperature where the polymerization reaction equilibrium is shifted so that the monomer will not polymerize, or if kept at this temperature all the polymer will be converted back to monomer. From thermodynamics the equilibrium constant for any reaction is a function of the heat of reaction and the entropy of the reaction. For PS resin, the exothermic heat of reaction for polymerization is 70 kj/gmol, and the ceiling temperature is 310 °C. Ceiling temperatures for select polymers are shown in Table 2.5. 

Please realize that the effect of temperature on the equilibrium constant depends on which of the two opposing reactions is exothermic and on which is endothermic. You must have information on the heat of a reaction before you can apply Le Chateliers principle to judge how temperature alters the equilibrium. 

The equilibrium position for the monomer-polymer equilibrium in Eq. 3-174 will be dependent on the temperature with increased temperature, resulting in a shift to the left, since the forward reaction is exothermic. The reaction isotherm... 

The reaction is exothermic, hence the highest equilibrium yield is obtained at low temperatures and high pressures. The catalyst functions by inducing the formation of a nitrogen complex with the catalyst surface this complex is far more readily hydrogenated to NH3 than is nitrogen with its triple bond (Somorjai and Salmeron, 1986). 

The final stress to be considered is a change in temperature. To apply Le Chate-lier s Principle with temperature changes, the sign of AH for the reaction needs to be known. The AH in our example is = +131 kilojoules. This indicates that the forward reaction is endothermic and the reverse reaction is exothermic. When the temperature of a system at equilibrium is increased, the equilibrium will favor the endothermic reaction. One way to think of the effect of temperature is to think of energy as a reactant or product. This is seen when the forward and reverse reactions are written as two separate reactions ... 

According to Le Chatelier s Principle, the production of ammonia is favored by a high pressure and a low temperature. The Haber process is typically carried out at pressures between 200 and 400 atmospheres and temperatures of 500°C. While Le Chatelier s Principle makes it clear why a high pressure would be favorable in the Haber process, it is unclear why a high temperature would be desirable because the reaction is exothermic. An increase in temperature shifts an exothermic reaction to the left. Even though the equilibrium shifts to... 

The forward reaction is endothermic, and the reverse reaction is exothermic, according to the Le Chatelier s principle. If we change the temperature of the system, it will shift in a way that will decrease the effect of the change. If the temperature of the system is raised, the equilibrium will proceed to the right (products) to decrease the temperature, according to the Le Chatelier s principle. If the reaction mixture is cooled down, the equilibrium will shift to the left (reactants) to increase the temperature. 

In the above reaction, the forward reaction is exothermic, and the reverse reaction is endothermic. If the system is heated (the temperature raised), the equilibrium will shift to the left to counteract the effect of the change. 

The reaction of H2 with N2 is an equilibrium reaction. The forward reaction is exothermic, meaning increasing the system s temperature tends to shift the reaction toward the reactants. 

The reaction is exothermic, hence as the gases cool the reaction equilibrium is shifted to the right... 

For each reaction in a surface chemistry mechanism, one must provide a temperature dependent reaction probability or a rate constant for the reaction in both the forward and reverse directions. (The user may specify that a reaction is irreversible or has no temperature dependence, which are special cases of the general statement above.) To simulate the heat consumption or release at a surface due to heterogeneous reactions, the (temperature-dependent) endothermicity or exothermicity of each reaction must also be provided. In developing a surface reaction mechanism, one may choose to specify independently the forward and reverse rate constants for each reaction. An alternative would be to specify the change in free energy (as a function of temperature) for each reaction, and compute the reverse rate constant via the reaction equilibrium constant. 

From (8.34c) or (8.35), it is easy to see that if the chosen reaction is endothermic (AH° > 0), then a T increase tends to promote product formation (the reaction shifts right ). Conversely, if the reaction is exothermic (AH° < 0), a temperature increase promotes formation of reactants (the equilibrium shifts left ). Such conclusions appear intuitive from the perspective of Le Chatelier s principle, and indeed we shall show in Section 8.6 that such Le Chatelier-like conclusions arise from deep theoretical roots that permeate the Van t Hoff equation and many other thermodynamic relationships. 

Another effect is that increasing temperature might shift the reaction equilibrium towards the reactants. This applies especially to hydrogenation processes, since most of them are exothermic. 

This reaction is reversible and so the ideas of Le Chatelier (Chapter 11, p. 178) can be used to increase the proportion of sulfur trioxide in the equilibrium mixture. The forward reaction is exothermic and so would be favoured by low temperatures. The temperature of 450 °C used is an optimum temperature which produces sufficient sulfur trioxide at an economical rate. Since the reaction from left to right is also accompanied by a decrease in the number... 

---