From Practical skills in Biology by Allan Jones et al.
At high concentrations of certain ionic solutes, $\gamma$ (activity coefficient) may increase to become greater than unity.
My questions : If activity coefficient $\gamma$ $=$ $ {a}/{[C]} $ where $a$ is activity which signifies effective concentration and $[C]$ is concentration, how can $a$ be greater than $[C]$ ?
Answer
A) Gamma < 1 means attractive forces dominate, and there is a decrease of vapour pressure of solute A relative to pure A. B) Gamma > 1 means repulsive forces dominate, and there is an increase of vapour pressure of solute A relative to pure A. You can think of activity coefficients as a qualifier for "escaping tendency" of the solute. With case A) above, the partial molar excess Gibbs energy of mixing is negative, meaning the solvent stabilises the solute, and energy is given away as a result of the mixing event (i.e. mixing is spontaneous). With case B) above, the partial molar excess Gibbs energy of mixing is positive meaning the solvent destabilises the solute, and energy is required in order to achieve mixing (i.e. mixing is NOT spontaneous). Think of activity as escaping tendency - a small activity means the solute remains in the solvent due to attractive forces whereas a large activity indicates a large escaping tendency of the solute because repulsive forces dominate. a = gamma x [conc.], right? So, if gamma > 1, then a is going to be larger than [conc.]. It might help you to consider fugacity, which is the analogue of activity for gases (which employs similarly the fugacity coefficient). Also consider an aqueous solution of ammonium hydroxide. You make it to be X M, right? But you can always smell ammonia since the solution you made is losing quantities of ammonia. Is the gamma for NH3 > or < than 1?
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