Enthalpy of Cu

The second and third ionization enthalpies of Cu are very much lower than those of the alkalis and account in part for the transition metal character shown by the existence of colored paramagnetic ions and complexes in the II and III oxidation states. Even in the I oxidation state numerous transition-metal-like complexes are formed (e.g., those with olefins). 

Figure 4. Enthalpy of Cu ions binding in water (25°C) (A) heparin (O, dextran sulfate (M = 4 I0 and Td = 8 ICP) (A) cellulose sulfate (D.S. <= PO) (M) iso-carrageenan segments (( A) intact) polyelectrolyte concentration CNj ...

Quantities limiting partial enthalpy of R limiting partial enthalpy of Cu A /f heat of... 

Equations 20.176 and 20.179 emphasise the essentially thermodynamic nature of the standard equilibrium e.m.f. of a cell or the standard equilibrium potential of a half-reaction E, which may be evaluated directly from e.m.f. meeisurements of a reversible cell or indirectly from AG , which in turn must be evaluated from the enthalpy of the reaction and the entropies of the species involved (see equation 20.147). Thus for the equilibrium Cu -)-2e Cu, the standard electrode potential u2+/cu> hence can be determined by an e.m.f. method by harnessing the reaction... 

Other ordering systems show striking discrepancies with the predictions of the quasi-chemical theories. Cu-Pt,67 Co-Pt,38 and Pb-Tl36 are binaries the solid solutions of which exhibit a positive partial excess free energy for one of their components, as well as positive excess entropies of solution. Co-Pt goes even further in deviating from theory in that it has a positive enthalpy of solution,... 

Comparative studies [1028,1052,1053] of the decompositions of Ni, Co and Cu alkanoates from formate to valerate showed that both the cation present and the length of the alkane chain influenced the temperature and enthalpy of decomposition. No such relationship was found [1048], however, between chain length and temperature of reaction of a series of nickel salts between the propionate and the stearate in a study which included some qualitative identifications of the products. Mass... 

AH = 17.3 4.1 kcal.mole and ASl = —15 13 cal.deg . mole Thus the efficiency of Cu(II) as a catalyst is due to a more favourable entropy of activation. In fact the enthalpy of activation for the catalysed reaction is greater than that for the uncatalysed reaction . Recently, Espenson et have measured... 

Verevkin, S.P., Wandschneider, D., Heintz, A. (2000) Determination of vaporization enthalpies of selected linear and branched C7, C8, C9, Cu and Cu monoolefin hydrocarbons from transpiration and correlation gas-chromatography methods. J. Chem. Eng. Data 45, 618-625. 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

At present, the correlation contains one transition metal complex, Cu(Hfacac)2. The results on this complex are very interesting and somewhat unusual for a transition metal system in that enthalpies have been obtained in a poorly solvating solvent with nonionic donors (52), instead of the t5 ical stability constant study on a metal cation in some highly polar solvent. Data from this latter type of investigation have many practical uses, but are impossible to interpret and understand. The transition metal ion complex we have studied can be incorporated into the E and C scheme using the same base parameters that are used to correlate the enthalpies of formation of all the other Lewis acid-base adducts in the scheme. 

Another method to obtain enthalpies of formation of compounds is by solute-solvent drop calorimetry. This method was pioneered by Tickner and Bever (1952) where the heat formation of a compound could be measured by dissolving it in liquid Sn. The principle of the method is as follows. If the heat evolved in the dissolution of compound AB is measured, and the equivalent heat evolved in the dissolution of the equivalent amount of pure A and B is known or measured, the difference provides the enthalpy of formation of the compound AB. Kleppa (1962) used this method for determining enthalpies of formation of a number of Cu-, Ag- and Au-based binaries and further extended the use of the method to high-melting-point materials with a more generalised method. 

The unique behaviour of Cu in the first transition series, having a positive (M2 + /M) value, accounts for its inability to liberate H2 from dilute acids (/.< . those with a molar concentration of l mol dm-3). Only oxidizing acids (concentrated nitric and hot concentrated sulfuric) react with Cu, the acids being reduced. The reason for this behaviour is the high energy needed to transform Cu(s) to Cu2 + (g), which is not exceeded by the hydration enthalpy of the ion. 

Numerous studies of the EPR spectra of the Cu(R2 Dtc)2 complexes in the presence of bases have been reported. The interaction of Cu(n-Bu2Dtc)2 with Pip, Py, and w-hexylamine (Hex) was studied by variable-temperature EPR measurements (139). Evidence for the formation of 1 1 adducts was presented and thermodynamic parameters were reported. For the Pip, Hex, and Py adducts, respectively, equilibrium constants of 3.9(1), 2.1(1), and 0.40(2) 1 mole"1 were determined. In the same order, AH0 values of —7.5(4), —7.3(12), —5(2) kcal mole"1 and AS0 values of —22(1), —23(2), and —19(3) eu were reported. The rate of adduct formation is primarily limited by the entropy of activation, while the rate of dissociation is limited by the enthalpy of activation. 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

These points are well illustrated by comparing Cu, Ag and Au with respect to the relative stabilities of their oxidation states. Although few compounds formed by these elements can properly be described as ionic, the model can quite successfully rationalise the basic facts. The copper Group 1 Id is perhaps the untidiest in the Periodic Table. For Cu, II is the most common oxidation state Cu(I) compounds are quite numerous but have some tendency towards oxidation or disproportionation, and Cu(III) compounds are rare, being easily reduced. With silver, I is the dominant oxidation state the II oxidation state tends to disproportionate to I and III. For gold, III is the dominant state I tends to disproportionate and II is very rare. No clear trend can be discerned. The relevant quantities are the ionization energies Iu l2 and A the atomisation enthalpies of the metallic substances and the relative sizes of the atoms and their cations. These are collected below / and the atomisation enthalpies AH%tom are in kJ mol-1 and r, the metallic radii, are in pm. 

The instability of Cu+(aq) reflects the preference of copper for the II oxidation state, while silver prefers the I state. This is perhaps a little surprising, since other elements of the 4d series tend to favour higher oxidation states compared with their congeners in the 3d series. The enthalpy data given above help to rationalise this apparent anomaly. It might have been expected that the hydration enthalpy terms would favour the higher oxidation state for the smaller atom the difference in hydration enthalpy between M+ and M2+ should be greater for copper... 

Below 500 K heating of the solid salt results primarily in the vaporization of the covalent molecule as a monomer. In this temperature range the only thermal decomposition, into NOz and 02, is exhibited by the solid. The vapor is more stable. The vapor pressure of Cu(N03)2 was determined by Addison and Hathaway48 by extrapolating pressure-time curves to zero time in order to subtract the pressures of N02 and 02. These vapor pressures increased from 0.32 torr at 430 K to 3.59 torr at 405 K. A plot of log P vs. 1/T is linear and yields a sublimation enthalpy of 67.0 kJ. Above 500 K both the solid and the vapor phase decompose to N02 + 02. 

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