Reversible Reactions

REVERSIBLE REACTIONS (equilibrium), chemical reactions which under certain conditions of temperature, pressure, and concentration proceed in one direction, while under changed conditions proceed in the opposite direction. As a result, under given conditions a state of equilibrium arises, i.e., the system contains both final and initial substances in a certain ratio of concentrations. Let us have the reaction: mA + nB + ... ⇌ pM + qN + ... where A, B... are initial, M, N... are final substances, and m, n, p, q... are stoichiometric coefficients (for example in the reaction Na + 3H₂ ⇌ 2NH₃, m=1, n=3, p=2, q=0). The reaction under some conditions proceeds from left to right, and under others from right to left, which is depicted by the sign ⇌. The established equilibrium is "dynamic equilibrium" (van't Hoff): as many molecules form per unit of time, so many decompose, as a result of which a stationary state is established. At equilibrium, the rates of the forward and reverse processes are equal. - When considering molecular processes, the postulate known as the "principle of detailed balance" plays a large role. It states that to every direct molecular process there must correspond exactly the same reverse process, and the molecules pass through exactly the same stages, but only in the reverse direction. Temperature greatly affects reversible reactions. If the reaction is accompanied by the release of heat (exothermic reaction), then an increase in temperature favors the reverse reaction. Conversely, in endothermic reactions, an increase in temperature favors the forward reaction. An example can be the dissociation of iodine vapor, J₂ ⇌ 2J - 21,000 cal. At comparatively low temperature, iodine vapor consists almost exclusively of molecules and has a violet color. At higher temperature, it consists almost exclusively of atomic iodine and is colorless. - Pressure also affects reversible reactions. If the products of the reaction occupy a larger volume than the initial substances, then the application of increased pressure favors the reverse reaction, and conversely, if the reaction is accompanied by a decrease in volume, then pressure favors the forward reaction. Thus, in the synthesis of methyl alcohol from carbon monoxide and hydrogen: CO + 2H₂ ⇌ CH₃OH, one molecule is obtained from three, and pressure favors the synthesis reaction. - Special external features are presented by cases when one of the forming substances is removed from the sphere of reaction, for example in the form of a precipitate or gas if the reaction takes place in solution. Thus, when evaporating NaCl with H₂SO₄, sodium sulfate is obtained quantitatively because hydrogen chloride constantly escapes. Such reactions are called irreversible. Generally, reactions proceed in the direction of removal of reaction products. This occurs on the basis of the law of mass action (see): If the forming substance, for example M, leaves the sphere of reaction, then its concentration in the system is very small; the denominator is small, and for the constancy of K to be maintained, the numerator must decrease, i.e., the concentration of initial substances, and thus the reaction tends to go to completion. In principle all reactions can be considered reversible, only often the equilibrium is shifted too far to the right or to the left, so that the equilibrium concentration of some substances is almost equal to zero. Thus, for example, water is little dissociated into ions, the equilibrium is strongly shifted toward the formation of undissociated molecules. Hydrogen at ordinary temperature is negligibly dissociated into atoms, but still a little atomic hydrogen is contained in hydrogen even at ordinary temperature. That this is so can be judged on the basis that when temperature increases, the number of atoms continuously increases, and hydrogen passes almost completely into atomic state at 3,000°. The equilibrium state is strongly distorted if subsequent reactions proceed after the main reaction or if side reactions occur. Reversibility seems to have completely disappeared. Thus, for example, it is difficult to imagine equilibrium in the combustion of protein, because the probability is too small that from the products of combustion (CO₂, water, nitrogen, sulfur dioxide, etc.) protein would be formed again. The chain of successive reactions is too complex and there are too few chances that these simple molecules, upon colliding, would arrange themselves into the complex molecule of protein. However, we know a similar reaction, the reaction reverse to the combustion of carbohydrates - their synthesis from CO₂ and water with the release of oxygen in the green parts of plants; only it proceeds under different energy conditions than combustion (photochemical reaction). There exists a huge number of reactions which, although possible under given conditions of temperature and pressure, nevertheless do not proceed due to the presence of passive resistances. Therefore, for the realization of reversible reactions or equilibrium reactions, the presence of catalysts (see Catalysis) that reduce passive resistances is often very important. They give reactions the possibility to proceed and allow approaching equilibrium from both sides. In this case, the selective action of catalysts is manifested, which often from very complex reaction possibilities allow one certain equilibrium to be established. Thus, alcohols upon heating decompose, giving a complex mixture of aldehydes, olefins, water, hydrogen, carbon monoxide, saturated hydrocarbons, etc. Acting on the vapors of ethyl alcohol with a catalyst - finely divided copper (for example reduced from oxide) - at elevated temperature (about 200°), we have the possibility to realize in almost pure form the equilibrium reaction: CH₃CH₂OH ⇌ CH₃CHO + H₂ and at higher temperature: CH₃CHO ⇌ CH₄ + CO. Acting on the same alcohol with aluminum oxide, we realize another reversible reaction: CH₃CH₂OH ⇌ CH₂=CH₂ + H₂O. When temperature decreases, the equilibrium shifts from right to left. Catalysts can be used in the study of reversible reactions because they do not shift the position of equilibrium, but only accelerate the advancement of the system toward it. The indications found in literature on the shifting of equilibrium upon closer examination always turn out to be erroneous. The apparent shifting is due to side reasons, mainly the poisoning of catalysts. The state of equilibrium is accessible to exact thermodynamic calculation. The consideration of reversible reactions as a special case of reversible processes played a large role in chemistry, making it possible to establish quantitatively the magnitude of affinity. Until the question of affinity was thermodynamically developed, many chemists, along with Berthelot, considered that the direction of the reaction is determined by the sign of the thermal effect of the reaction: such reactions proceed in which heat is released. From this point of view, reversible reactions remained completely incomprehensible, because if the forward reaction is exothermic, then the reverse will necessarily be endothermic. The merit of van't Hoff and Nernst was that instead of the thermal effect of Berthelot, they put maximum work. Mathematical analysis shows that, if one starts from the same initial substances or from the same final ones, then the reaction is accompanied by a decrease in free energy and positive maximum work until equilibrium is reached. The thermal effect and affinity (i.e., standard maximum work) are connected by the equation F=U - TS, where F is the difference in free energies of both parts of the chemical reaction equation, U is the same difference in total energy, T is absolute temperature, and S is the change in entropy. The change in free energy is numerically equal to the maximum work or affinity. At low temperatures, the TS term is small, and affinity and thermal effect differ little from each other. Similarly, at large reaction heats, U is significantly greater than TS and ΔU. Thus, the principle of Berthelot is approximately valid only in special cases and is completely inapplicable at high temperatures or with small thermal effects.