Removing Products or Adding Reactants

EXERCISE IS.n Phosphorus pentachloride, PCI5, decomposes when heated. 

If the initial concentration of PCI5 is l.OOmol/L, what is the equilibrium composition of the gaseous mixture at 160°C The equilibrium constant at 160°C is 0.0211. 

A and B react to produce C according to the following chemical equation  

Amounts of A and B are added to an equilibrium reaction mixture of A, B, and C such that when equilibrium is again attained, the amounts of A and B are doubled in the same volume. How is the amount of C changed  

Obtaining the maximum amount of product from a reaction depends on the proper selection of reaction conditions. By changing these conditions, you can increase or decrease the yield of product. There are Ihree ways to alter the equilibrium composition of a gaseous reaction mixture and possibly increase the yield of product 

---

Changing the concentrations by removing products or adding reactants to the reaction vessel. 

The equilibrium position shifts to the left if a reactant is removed or a product is added [reactant] decreases or [product] increases. 

One way to increase the yield of a desired product is to change concentrations in a reaction mixture by removing a product or adding a reactant. Consider the methana-tion reaction,... 

It is possible to use K to calculate the extent to which reaction occurs when an equilibrium is disturbed by adding or removing a product or reactant To show how this is done, consider the effect of adding hydrogen iodide to the HI-H2-I2 system (Example 12.7). 

A semibatch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another contained as a batch in a vessel, or a product may be removed intermittently or continuously from the vessel as reaction proceeds. The reaction may be single-phase or multiphase. As in a batch reactor, the operation is inherently unsteady-state and usually characterized by a cycle of operation, although in a more complex manner. 

If a reaction is at equilibrium and products are added (or reactants are removed), the reaction goes to the left. 

AH° = +2802 kj. Suppose that the reaction is at equilibrium. State the effect (tend to shift toward the formation of reactants, tend to shift toward the formation of products, or have no effect) that each of the following changes will have on the equilibrium composition (a) the partial pressure of 02 is increased (b) the system is compressed (c) the amount of C02 is increased (d) the temperature is increased (e) some of the C6H1206 is removed (f) water is added (g) the partial pressure of C02 is decreased. 

While still useful for large-scale esterification of fairly robust carboxylic acids, Fischer esterification is generally not useful in small-scale reactions because the esterification depends on an acid-catalyzed equilibrium to produce the ester. The equilibrium is usually shifted to the side of the products by adding an excess of one of the reactants—usually the alcohol—and refluxing until equilibrium is established, typically several hours. The reaction is then quenched with base to freeze the equilibrium and the ester product is separated from the excess alcohol and any unreacted acid. This separation is easily accomplished on a large scale where distillation is often used to separate the product from the by-products. For small-scale reactions where distillation is not a viable option, the separation is often difficult or tedious. Consequently Fischer esterification is not widely used for ester formation in small-scale laboratory situations. In contrast, intramolecular Fischer esterification is very effective on a small scale for the closure of hydroxy acids to lactones. Here the equilibrium is driven by tire removal of water and no other reagents are needed. Moreover the closure is favored entropically and proceeds easily. 

General procedure for deprotection of mono- and polymethyl-aryl ethers with boron tribromide.41 To a 10-ml flask fitted with a septum and magnetic stirrer bar are added reactant (3.6 mmol) and 5 ml of dichloromethane. An inert atmosphere is established and maintained. This mixture is cooled in a dry ice/propan-2-ol bath and boron tribromide [0.13 ml, 1.32 mmol (for monomethyl ethers), or 0.38 ml, 4 mmol (for dimethyl ethers)] is added through the septum by use of a syringe. The cold bath is removed and the mixture stirred for 30 minutes, poured into ice water, stirred for 30 minutes, saturated with salt and extracted with dichloromethane. The extract is dried and concentrated. The purity of the product is established by h.p.l.c. analysis on a Waters Associates 6000A model using both refractive index and u.v. absorbance detectors with a Waters 3.9mm i.d. x 30cm p-Bondapack Ci8 reverse phase column. 

Adding reactant to or removing product from a system at equilibrium will drive the reaction in the forward direction. 

If ammonia had been added instead of nitrogen, the system would have shifted to the left to consume ammonia. So we can paraphrase Le Chatelier s principle for this case as follows If a gaseous reactant or product is added to a system at equilibrium, the system will shift away from the added component. If a gaseous reactant or product is removed, the system will shift toward the removed component. 

In summary, to use Le Chatelier s principle to describe the effect of a temperature change on a system at equilibrium, treat energy as a reactant (in an endothermic process) or as a product (in an exothermic process), and predict the direction of the shift as if an actual reactant or product is added or removed. Although Le Chatelier s principle cannot predict the size of the change in A, it can correctly predict the direction of the change. 

An equilibrium can be forced in the direction of the products by adding a reactant or by removing a product. It can be forced in the direction of the reactants by adding a product or removing a reactant. 

Not all reactions attain equilibrium they may occur too slowly, or else products or reactants may be continually added or removed. Such is the case with most reactions in biological systems. On the other hand, some reactions, such as typical acid-base neutralizations, achieve equilibrium very rapidly. 

Electrochemical reactions at an electrode snrface differ from normal heterogeneous chemical reactions in that they involve the participation of one or more electrons that are either added to (reduction) or removed from (oxidation) the reactant species. The explicit inclusion of electrons as reactants or products means that the reaction rate depends on the electric potential. Electron transfer processes occur within a small portion of the double layer immediately adjacent to the electrode surface (10 to 50 mn in thickness) where solution-phase electroneutrality does not hold and where very strong electric fields (on the order of 10 V/cm) exist during a charge transfer reaction. We begin the analysis of electrochemical kinetics by defining a generic electrode reaction ... 

Liquid-phase Reactions. liquid-phase reactions in which oxidation is secured by the use of oxidizing compounds need no special apparatus in the sense that elaborate means must be provided for temperature control and heat removal. There is usually provided a kettle form of apparatus, closed to prevent the loss of volatile materials and fitted with a reflux condenser to return vaporized materials to the reaction zone, provided with suitable means for adding reactants rapidly or slowly as may be required and for removing the product and provided with adequate jackets or coils through which heating or cooling means may be circulated as required. Examples of such apparatus are scattered throughout this book, and no specific examples are required here. 

Predict the direction of the shift in equilibrium position the same way as if a product or reactant were added or removed. 

---