Thermo_ISM_ch10

Thermo_ISM_ch10 - Questions on Concepts Q10.1 Tabulated...

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174 Questions on Concepts Q10.1) Tabulated values of standard entropies of some aqueous ionic species are negative. Why is this statement consistent with the third law of thermodynamics? This is possible due to the choice of standard state as ( ) + H, a q 0 f S o . Although absolute entropies of neutral species can be determined, this is not possible for ionic species because the solution must be electrically neutral. Therefore, it is necessary to choose a reference value, but as generally only differences in entropies (and other thermodynamic quantities) are desired, it is not necessary to know the absolute value. Q10.2) Why is the value for the dielectric constant for water in the solvation shell around ions less than that for bulk water? Water molecules immediately around the ion are ordered more than those in the bulk. Thus, they cannot respond to the electric field due to solvated ions. Therefore, they are less effective in screening the electric field due to solvated ions, leading to a smaller value of ε r . Q10.3) Why is it possible to formulate a theory for the activity coefficient for electrolyte solutions, but not for neutral solutes? The dissolved species in electrolyte solutions have a universal form for their dominant interaction with the solvent – the Coulomb interaction. Non-electrolytes interact with much weaker, system specific potentials. The universality of the electrolyte- electrolyte interaction allows a general theory to be developed. Q10.4) Why are activity coefficients calculated using the Debye–Hückel limiting law always less than one? The activity coefficients are less than one because the charge on an electrolyte lowers the chemical potential of the electrolyte when compared with an analogous solution of uncharged solute molecules. This occurs because the charges interact attractively with the solvent, lowering the energy. Q10.5) Why does an increase in the ionic strength in the range where the Debye–Hückel law is valid lead to an increase in the solubility of a weakly soluble salt? In this regime, ln γ ± decreases with increasing I (as I ). The chemical potential of the electrolyte in solution becomes lower with increasing ionic strength, thus increasing solubility. Physically, increasing I leads to increased screening of repulsion between like-charged solute ions.
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175 Q10.6) Discuss how the Debye–Hückel screening length changes as the (a) temperature, (b) dielectric constant, and (c) ionic strength of an electrolyte solution are increased. (a) Increasing T increases the screening length. The random thermal motion spreads out the cloud of screening ions. (b) Increasing ε r increases screening length. Increased r makes the potential attracting the counterion cloud weaker, causing it to spread out more.
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This note was uploaded on 06/10/2010 for the course CHEM 127 taught by Professor Nefzi during the Spring '03 term at UCSD.

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Thermo_ISM_ch10 - Questions on Concepts Q10.1 Tabulated...

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