Entropy hydrogen halides

Table 6.2). Entropy effects, although smaller, contribute in the same direction. It is easy to see that an explanation of the relative acid strengths of the hydrogen halides is not a trivial exercise. Moreover, electronegativity does not enter into the discussion one must exercise care because it is all too easy to conclude from electronegativity (see Table 1.7) that HF is expected to be the strongest acid in the series. 

The deciding factor on why HF is a weak acid and not a strong acid like the other hydrogen halides is entropy. What occurs when HF dissociates in water as compared to the other hydrogen halides ... 

In the case of the hydrogen halides the argument can be extended to give information about behaviour in aqueous solution, as shown in Table 9. In the first place the standard entropies of reaction in the gas phase can be calculated from statistical mechanics, leading to AGg. 

A carefully studied example is the transition which takes place in liquidheliumat about 2.2 K. Theheat capacity and density of liquid helium as functions of the temperature are shown in Fig. 22. In this instance at least it is known that the two phases are able to co-exist, not merely at a particular temperature and pressure, but along a p-T equilibrium curve, as in the ordinary phase transitions already discussed. Similar effects are known to occur in many solids, notably in alloys, in the crystalline ammonium salts, in polymers and in solidified methane and hydrogen halides. For example, in ammonium chloride there is a sharp break in the heat capacity curve at —30.4 GL Now in any given example, if it were knotm with certainty that the latent heat and volume change were vanishingly small, the entropy... 

Details about ILs properties are covered in this book in the contributions by Seddon, Chiappe and Scott. However, two features deserve a comment for their possible consequences on reactivity and catalysis. First, depending on a delicate balance of entropie and enthalpic factors, including the polarity of the transition state structures with respect to regents, a reaction can be either speeded up or decelerated when carried out in an ionic liquid medium compared to a molecular solvent. An elegant study by Welton shows that in S-,2 reactions, primary, secondary and tertiary amines are more reactive as nucleophiles in ionic liquids, while halides react faster in conventional molecular solvents such as CH2CI2. In particular in a series of [Bmim] salts the order of nucleophilicity of halides is determined by the anion partner. To the same direction moves a kinetic study by Dyson on a cationic Ru(II) complex-catalysed hydrogenation of styrene in ILs, where it is clearly demonstrated that both the cation and the anion of the IL can inhibit or accelerate the formation of the active catalytic species. ... 

The free energy, enthalpy, entropy, and volume of the hydrated electron are measurable in principle from the temperature and pressure dependencies of the forward and reverse rates of the unimolecular reaction of this species with water to form hydrogen atom and hydroxide ion. Data presently available determine values only for free energies of activation in both directions and for enthalpy and entropy of activation in one direction. Values for the other properties can be predicted if it is assumed that the enthalpy, entropy, and volume of the hydrated electron can be calculated by extrapolating measurements on halide ions to the radius (2.98 A.) necessary to fit the free energy data. The predictions for enthalpy and entropy are thought to be reasonably accurate, but the value for volume change is less reliable. 

The activation entropy for thermal decomposition of /ra j -[MnX(CO)3(PPh3)2] indicates that manganese-phosphorus bond breaking is rate-determining. The rate law for the decomposition of CoH(CO)4 to Co2(CO)8 plus hydrogen is second-order in cobalt compound. While this decomposition has previously been studied in the gas phase, this is the first report of decomposition kinetics in the liquid phase. The first step in reaction of Co2(CO)s with organomercury halides is solvent-induced disproportionation of the dimeric carbonyl. ... 

Step (1) causes some experimental difficulty. It is the reverse of the dissolution of gaseous HX in water to form solvated undissociated HX. Since HCl, HBr and HI are essentially fully dissociated in aqueous solution, measurement of enthalpy or entropy changes for step (1) must be estimated from somewhat unsatisfactory comparisons with noble gases and methyl halides. For HF, which is a weak acid in dilute aqueous solution, it might appear that values of 2sH° and AS° for step (1) could be obtained directly. However, IR spectroscopic data indicate that the species present in solution is the strongly hydrogen-bonded ion-pair F ""HOH2. ... 

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