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17 Nov 2019
Activity reflects ion behavior in solution rather than the number of ions present. Activity is set by ionic strength of the medium. Here does addition of an inert electrolyte, NaCl alter the extent of dissociation for weak acids with an analytical concentration of c*_A = 1.000 times 10^-3 M. Consider HA an acid added to solution at c*_A that dissociates as HA + H_2O H_3O^+ + A^-. For a monoprotic weak acid, write the thermodynamic equilibrium constant expression K' in terms of the activities and then in terms of concentrations and activity coefficients. For 0.1000 M NaCl, calculate ionic strength. It is not necessary to consider the effect of the ions of the acid on the ionic strength. Use the Extended Debye Huckel (EDH) equation to calculate the needed activity coefficients for the acid. Allow the ion size parameter alpha for the acid anion to be 400 pm. Substitute these activity coefficients into the expression for K' and solve for the practical equilibrium constant, K = [H_3O^+] [A^-]/[HA]. Find pK in terms of pK'. Is the acid more or less dissociated with salt present? why? The Davies equation provides good estimates of the activity coefficient up to ionic strengths 0.5 M. Note the Davies equation relies on charge and ionic strength, so for a monoprotic acid, gamma_H^+ = gamma_A^- log gamma_i = -0-51 z_i^2 (Squareroot mu/1 + Squareroot mu - 0.30 mu) (Davies Equation (DE)) For mu = 0.1000 M, repeat the calculation of K in terms of K', and pK relative to pK'. How does the result compare to EDH? For mu = 0.5000 M (the rough upper ionic strength for the Davies equation), repeat the calculation of K, K/K', pK relative to pK'. What is the impact of more inert salt?
Activity reflects ion behavior in solution rather than the number of ions present. Activity is set by ionic strength of the medium. Here does addition of an inert electrolyte, NaCl alter the extent of dissociation for weak acids with an analytical concentration of c*_A = 1.000 times 10^-3 M. Consider HA an acid added to solution at c*_A that dissociates as HA + H_2O H_3O^+ + A^-. For a monoprotic weak acid, write the thermodynamic equilibrium constant expression K' in terms of the activities and then in terms of concentrations and activity coefficients. For 0.1000 M NaCl, calculate ionic strength. It is not necessary to consider the effect of the ions of the acid on the ionic strength. Use the Extended Debye Huckel (EDH) equation to calculate the needed activity coefficients for the acid. Allow the ion size parameter alpha for the acid anion to be 400 pm. Substitute these activity coefficients into the expression for K' and solve for the practical equilibrium constant, K = [H_3O^+] [A^-]/[HA]. Find pK in terms of pK'. Is the acid more or less dissociated with salt present? why? The Davies equation provides good estimates of the activity coefficient up to ionic strengths 0.5 M. Note the Davies equation relies on charge and ionic strength, so for a monoprotic acid, gamma_H^+ = gamma_A^- log gamma_i = -0-51 z_i^2 (Squareroot mu/1 + Squareroot mu - 0.30 mu) (Davies Equation (DE)) For mu = 0.1000 M, repeat the calculation of K in terms of K', and pK relative to pK'. How does the result compare to EDH? For mu = 0.5000 M (the rough upper ionic strength for the Davies equation), repeat the calculation of K, K/K', pK relative to pK'. What is the impact of more inert salt?
Elin HesselLv2
1 Aug 2019