Electrochemistry - CH9 Electrochemistry Ions in solution...

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1 CH9. Electrochemistry Ions in solution 9.1 The Debye–Hückel theory 9.2 The migration of ions Electrochemical cells 9.3 Half-reactions and electrodes 9.4 Reactions at electrodes 9.5 Varieties of cell 9.6 The cell reaction 9.7 The electromotive force 9.8 Cells at equilibrium 9.9 Standard potentials 9.10 The variation of potential with pH 9.11 The determination of pH Applications of standard potentials 9.12 The electrochemical series 9.13 The determination of thermodynamic functions
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General Properties of aq. solutions Ionic Compounds in Water Although water is electrically neutral, it is a very effective solvent for ionic compounds. • One end of the water molecule, the O atom, is negative while the other end containing the H atoms is positive. • Positive ions (cations) are attracted by the negative end of H 2 O, and negative ions are attracted by the positive end of H 2 O. • This process helps to stabilize the ions in solution as well as prevent the ions from recombining.
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Activities of electrolytes Electrolytes have to be treated in a different way from nonelectrolytes because they dissociate, but the ions cannot be studied separately because the condition of electroneutrality applies = i i i z n 0 where n i is the amount of ions of charge z i e in the phase. The charge number (zi) is positive for cations and negative for anions; e is the charge on a proton 1.6023 x 10 -19 C. The chemical potential of a solute in a real solution is related to activity (a) by μ μ = + ringoperator RT a ln
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Solvent activity
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Solute activity
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Activities of electrolytes a b b = γ / ringoperator b θ = 1 mol kg –1 where the activity coefficient depends on the composition, the molality, and the temperature of the solution. As the solution approaches ideality, i.e. obeys Henry’slaw at low molalities,the activity coefficient tends toward 1: γ 1 0 a ak a b b b / ringoperator μ μ γ μ γ = + + = + ringoperator RT b RT RT ln ln ln ideal
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Activities of electrolytes
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Activities of electrolytes
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Debye-Hückel Theory Electrolytes containing ions with multiple charges have greater effects on the activity coefficients of ions than electrolytes containing only singly charge ions. To express electrolyte concentrations in a way that takes this into account, G. N. Lewis introduced ionic strength ( I ) defined by where z i is the charge (signed) of the ion in units of charge on the proton, and m is the molal concentration.
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