Hydrogen iodide, equilibrium

This equilibrium is established when hydrogen iodide is heated, hydrogen-iodine bonds being broken. 

Properties—Hydrogen iodide is a colourless gas. It is very soluble in water and fumes in moist air (cf. hydrogen chloride), to give hydriodic acid. Its solution forms a constant boiling mixture (cf. hydrochloric and hydrobromic acids). Because it attacks mercury so readily, hydrogen iodide is difficult to study as a gas, but the dissociation equilibrium has been investigated. 

Comparable results are not obtained with the less reactive iodine, because the hydrogen iodide formed tends to reduce the iodo compound and a condition of equilibrium is produced ... 

However, if an oxidising agent (fuming nitric acid or sodium persulphate) is present to destroy the hydrogen iodide as it is formed, the equilibrium is displaced and the iodo compound may be conveniently prepared, for example ... 

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). 

There are two main issues concerning the chemistry of the reaction and the separation. One is how to separate the hydriodic acid and sulfuric acid produced by the Bunsen reaction. The other is how to carry out the hydrogen iodide (HI) decomposition section, where the presence of azeotrope in the vapor-liquid equilibrium of the hydriodic acid makes the energy-efficient separation of HI from its aqueous solution difficult, and also, the unfavorable reaction equilibrium limits the attainable conversion ratio of HI to a low level, around 20%. 

Reaction (9) generates methyl iodide for the oxidative addition, and reaction (10) converts the reductive elimination product acetyl iodide into the product and it regenerates hydrogen iodide. There are, however, a few distinct differences [2,9] between the two processes. The thermodynamics of the acetic anhydride formation are less favourable and the process is operated much closer to equilibrium. (Thus, before studying the catalysis of carbonylations and carboxylations it is always worthwhile to look up the thermodynamic data ) Under standard conditions the AG values are approximately ... 

Hydrogen iodide (Hi) decomposes into iodine (T) and hydrogen (H2) at 300° C and establishes the following equilibrium ... 

Bimolecular reaction.—In the bimolecular reaction A+B M+N, let C A and CB respectively denote the concentrations of the substances A and B, expressed in mol. per litre. Similarly, let CM and CN respectively denote the concentrations of M and N. It has previously been shown that the speed of the reaction is equal to the product of the affinity or the driving force of the reaction, k, and the concentrations of the reacting substances, that is, the velocity of the reaction A+B is equal to kC CB. If A and B are the same, so that 2Av M+N, the speed of the -> reaction at any instant will be represented by kCA2. When hydrogen iodide dissociates 2HI H2+I2. The speed of the - reaction at any instant will be represented by kCBI2 and the speed of the <- reaction by k CiCB. When equilibrium occurs, the speeds of these two reactions are the same, and therefore the condition o equilibrium is kCrr —k C Ci, or K—kjV—C HCi/C Hi2. At 440°, when the system is in equilibrium, nearly 20 per cent, of the hydrogen iodide will have dissociated. Hence, at 440° (80 per cent.) 2HIt H2- -I2(20 per cent.). This means that if 100... 

A catalytic agent can alter the speed of a chemical action, bat it cannot alter the condition of equilibrium.—Although the speed of a chemical reaction is modified by the presence of a catalytic agent, the final state of equilibrium is not affected. If otherwise, J. H. van t Hoff showed that we could allow these substances to react alternately with and without the catalytic agent this would involve a change in the quantity combined, and the energy thus obtained could be made to do work. This would lead to perpetual motion, which is assumed to be impossible. This deduction has been confirmed experimentally with hydrogen iodide with and without platinum black. Hence, adds W. Nernst, the catalyst must always affect... 

When heated with hydrogen iodide or concentrated aqueous hydriodie acid, sulphur is reduced to hydrogen sulphide, but once more the reaction is incomplete, leading only to an equilibrium mixture ... 

Equation 6.9 gives the molar fractions of gaseous molecular hydrogen and hydrogen iodide xHi = 0.558 and xm = 0.442, respectively, in the reaction equilibrium at the standard state. 

Reactions in which a molecule dissociates into two different or equal fragments are very common, although many are so fast that they are practically at equilibrium (e.g., see dissociation of I2 in the hydrogen-iodide reaction in Section 4.2). 

In conclusion, the relevant points about this reaction, which obviously needs to be studied further to be understood more clearly in terms of mechanism, are the fact that the addition onto the double bond is rather fast and reaches hi equilibrium conversions and the ease with wdiich the diiodides generate hydrogen iodide in the presence of iodine. These points are the basic prerequisites in understanding the cationic initiation of certain monomers, as will be discussed in Sect. III-E-13. 

Hydrogen iodide iodinates trialkjisilanes in good yield in boiling carbon tetrachloride with no aluminum hahde present (116). This can perhaps be explained on the basis that some free iodine is always present in equilibrium with hydrogen iodide. 

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