Balance of exothermic chemical reactions is displaced towards the final products at withdrawal of the marked-out heat from reagents. This circumstance is widely used in chemical technology: cooling the reactor, it is possible to receive the final product of high purity.
The nature does not love changes
Josiah Uillard Gibbs entered fundamental concepts of entropy and an enthalpy into science, having generalized property of inertia on everything in general the phenomena in the nature. Their essence is as follows: everything in the nature resists any influences therefore the world in general strives for balance and chaos. But because of the same inertia the balance cannot instantly be established, and chaos pieces, interacting among themselves, generate certain structures, that is islands of an order. As a result the world dvuyedin, both is chaotic, and is ordered at the same time.
Le Chatelie's principle
From Gibbs's principles the principle of maintenance of balance of chemical reactions formulated in 1894 by Henri-Louis Le Chatelie directly follows: the system which is in chemical balance at any impact on it itself changes the state so that to parry (to compensate) influence.
What is the chemical balance
"Balance" does not mean that in a system (for example, mixes of hydrogen and vapors of iodine in the closed vessel) nothing occurs. In this case there all the time there are two reactions: H2 + I2 = 2HI and 2HI = H2 + I2. Chemists designate such process by one formula in which the equal-sign is replaced with a bidirectional arrow or two opposite directed shooters: H2 + I2 2HI. Such reactions are called reversible. Le Chatelie's principle is fair only for them. (From right to left) and the return (from left to right) reactions are equal in the equilibrium system of speed of a straight line, concentration of initial substances – iodine and hydrogen – and a product of reaction, iodide hydrogen, remain invariable. But their atoms and molecules rush about all the time, facing among themselves and changing partners. In a system maybe not one, and several couples of reacting substances. There can also be difficult reactions when three and more reagents, and reactions catalytic interact. In that case the system will be equilibrium when concentration of all substances in it do not change. It means that speeds of all direct reactions are equal to speeds corresponding to them the return.
Exothermic and endothermic reactions
The majority of chemical reactions proceeds or with allocation of energy which turns into warmth, or with absorption of warmth from the environment and use of its energy for reaction. Therefore the equation given above will be correct to write so: H2 + I2 2HI + Q where Q is amount of the energy (warmth) participating in reaction. For exact calculations the amount of energy is specified directly in joules, for example: FeO (t) + SO(g) Fe (t) + SO2(g) + 17 kJ. Letters in brackets (t), (g) or () say in what phase – firm, liquid or gaseous – there is a reagent.
Balance constant
The main parameter of a chemical system is its constant of balance of Kc. It is equal to the relation of a square of concentration (share) of the final product to the work of concentration initial a component. Concentration of substance can be designated by the front index with or (that is more evident), to conclude its designation in square brackets. For an example we will receive expression of Kc = [HI] ^2 / ([H2]*[I2]) above. At 20 degrees Celsius (293 K) and atmospheric pressure the corresponding sizes will be: [H2] = 0.025, [I2] = 0.005 and [HI] = 0.09. From here we receive, under the set conditions, Kc = 64.8. It is necessary to substitute HI, but not 2HI as molecules of iodide hydrogen do not communicate with each other, and exist everyone in itself.
Reaction conditions
It is not without reason told above "under the set conditions". The constant of balance depends on a combination of factors at which there is a reaction. In usual conditions three of all possible prove: concentration of substances, pressure (if at least one of reagents participates in reaction in a gas phase) and temperature.
Concentration
Let's say we mixed initial substances A and B in a vessel (reactor) (Poses. 1a in the drawing). If continuously to delete a reaction product With (Poses. 1b), balance will not turn out: reaction will go, slowing down until And yes In completely turn into S. Khimik will tell: we displaced balance to the right, to the final product. Shift of chemical balance means shift to initial substances to the left. If to do nothing, then at some, so-called equilibrium, concentration About process, it seems, will stop (Poses. 1v): speeds of direct and return reactions will even out. This circumstance complicates chemical production as it is very difficult to receive clean, without residues of raw materials, a ready-made product.
Pressure
Now we will present that And yes In to us (g), and With – (). Then, if pressure in the reactor does not change (it is admissible, it very big, Poses. 2b), reaction will take place up to the end, as on Poses. 1b. If pressure because of allocation With increases, then sooner or later there will come balance (Poses. 2v). It also prevents chemical production, but it is already easier to cope with difficulties as With it is possible to pump out. However, if final gas turns out less, than initial (2 NO (g) + O2(g) 2NO2(g) + 113 kJ, for example), then again we meet difficulties. In this case – initial substances it is necessary generally 3 asking, and the final product it turns out 2 asking. It is possible to carry out reaction, supporting pressure in the reactor, but it is technically difficult, and there is a problem of purity of a product.
Temperature
At last, suppose, that our reaction exothermic. If continuously to take away the marked-out heat, as on Poses. 3b, in principle, it is possible to force to react And yes In completely and ideally clean S. However, of time for this purpose infinitely is required to receive it is a lot of, but if reaction exothermic, technical ways it is possible to receive the final product of any beforehand of the set purity. Therefore chemical engineers try to choose initial substances such that reaction was exothermic. And here if to impose thermal isolation on the reactor (Poses. 3v), reaction will quickly come to balance. If it endothermic, then for the best purity About the reactor it is necessary to warm up. This method is also widely used in chemical technology.
What it is important to know
The balance constant does not depend on thermal effect of reaction and existence of the catalyst in any way. Warming up/cooling the reactor or entering into it the catalyst, it is possible to accelerate achievement of balance only. But purity of the final product is provided with the ways considered above.