4: Activity and Ionic Strength

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The parameters we have been studying so far help to characterize the energetics and spontaneity of reversible reactions. In general, if we assume that the likelihood of two molecules reacting with one another is based on the probability that they will 'collide,' then the 'reactivity' of that substance is directly proportional to its concentration - the more molecules of a substance that are present in a system, the more likely that collisions will take place between that substance and another reactant. However, there are situations where additional factors influence the reactivity of compounds. Either the substance itself is at such a high concentration that molecules of the same substance interact with one another to affect its reactivity, or molecules of one substance are interacting with molecules of one or more other substances to influence reactivity. In this section, we will examine terms such as 'activity,' define the reference state for quantifying activity, and then adjust activity using activity coefficients, as calculated using the ionic strength of a solution.

4.1: 4.1. Reactivity and The Standard State
Before examining something known as the electrochemical potential (E), it is useful to explore some aspects of what is known as the chemical energy that you have likely learned about before in general or physical chemistry.
4.2: Ionic Strength
The behavior of an electrolyte solution deviates considerably from that an ideal solution. Indeed, this is why we utilize the activity of the individual components and not the concentration to calculate deviations from ideal behavior. In 1923, Peter Debye and Erich Hückel developed a theory that would allow us to calculate the mean ionic activity coefficient of the solution and could explain how the behavior of ions in solution contribute to this constant.
4.3: Activity and Activity Coefficients
The bulk of the discussion in this chapter dealt with ideal solutions. However, real solutions will deviate from this kind of behavior. So much as in the case of gases, where fugacity was introduced to allow us to use the ideal models, activity is used to allow for the deviation of real solutes from limiting ideal behavior.