This is the reason that the activity was initially created. Fugacity is the effective pressure for a non-ideal gas. The pressures of an ideal gas and a real gas are equivalent when the chemical potential is the same. The equation that relates the non-ideal to the ideal gas pressure is:.
Introduction Activities and concentrations can both be used to calculate equilibrium constants and reaction rates. Non-ideality in Gases Fugacity Fugacity is the effective pressure for a non-ideal gas. The activity of a substance abbreviated as a describes the effective concentration of that substance in the reaction mixture. Activity takes into account the non-ideality of the reaction mixture, including solvent-solvent, solvent-solute, and solute-solute interactions.
Thus, activity provides a more accurate description of how all of the particles act in solution. For very dilute solutions, the activities of the substances in the solution closely approach the formal concentration what the calculated concentration should be based on how much substance was measured out.
As solutions get more concentrated, the activities of all of the species tend to be smaller than the formal concentration. The decrease in activity as concentration increases is much more pronounced for ions than it is for neutral solutes. Activities are actually unitless ratios that compare an effective pressure or an effective concentration to a standard state pressure or concentration the correct term for the effective pressure is fugacity.
There are several ways to define standard states for the different components of a solution, but a common system is. Thus, when we discuss the activity of a gas, we actually are discussing the ratio of the effective pressure to the standard state pressure:. For all solids, the activity is a ratio of the concentration of a pure solid to the concentration of that same pure solid. For all liquids, the activity is a ratio of the concentration of a pure liquid to the concentration of that same pure liquid:.
For most experimental situations, solutions are assumed to be dilute with respect to the solvent. This assumption implies the solvent can be approximated with pure liquid.
According to Raoult's Law , the vapor pressure of the solvent in a solution is equal to the mole fraction of the solvent in the solution times the vapor pressure of the pure solvent:.
The mole fraction of solvent in a dilute solution is approximately 1, so the vapor pressure of the solution is essentially identical to the vapor pressure of the pure solvent. This means that the activity of a solvent in dilute solution will always has a value of 1, with no units.
Activity indicates how many particles "appear" to be present in the solution, which is different from how many actually are present. Hence, activity is a "fudge factor" to ideal solutions that correct the true concentration. The activity coefficient for a nonvolatile, neutral solute is often estimated by non-linear curve fitting, taking into account the molality of the solute and the activity of the solvent usually its vapor pressure.
In most situations, it is more practical to look up the values of the activity coefficient for a given solute than it is to carry out the curve fitting.
Estimating the activity coefficient of electrolytes solutes that dissolve or react with the solvent to form ions depends upon the number of ions formed by the dissociation of the solute in solution or the reaction of the solute with the solution, because each ion formed is dealt with individually.
In a theoretical, infinitely dilute ideal solution, an electrolyte would dissociate or react completely to form an integer number of independent ions. In reality, it is found that electrolytes almost always act as if they contain fewer moles of ions than expected based on the formal concentration.
An activity coefficient incorporates the particle interactions into a single term that modifies the formal concentration to give an estimate of the effective concentration, or activity, of each ion.
This means that the same limiting mean ionic activity coefficient is found for sodium chloride and potassium chloride and that also the values for the and salts sodium sulfate and calcium chloride are identical.
At higher electrolyte concentrations though, these values change very strongly and are usually modeled using empirical parameters regressed to the experimental data. The standard state for a liquid is the pure liquid, so the standard state of water is pure water, whose concentration is In dilute aqueous solutions, the concentration of water is very close to NOTE: In a cell, the total solute concentration is high, so the concentration of water is certainly lower than Nevertheless, biochemists commonly use 1.
But you gotta love 'em. Of course, if water is not in the balanced equation, it doesn't matter what the actual [H 2 0] is. Drop units.
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