| Activity (chemistry) |
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Information AboutActivity (chemistry) |
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It is given by the equation where is the Activity Coefficient , which must be measured for different substances, and is the Mole Fraction . The activity coefficient of Ion s in solution can be estimated with the Debye-Hückel Equation , the Davies Equation or the Pitzer Equation . Activity effects are the result of interactions between ions both electrostatic and covalent. The activity of an ion is influenced by its surroundings. The reactivity of an ion in a cage of water molecules is different from that in the middle of a counterion cloud. This type of activity is relevant throughout chemistry from Reaction Rate s to Equilibrium Constant s. For instance large deviations can exist between the calculated hydrogen ion concentration of a strong acid in solution and the hydrogen activity derived from a PH Meter or a PH Indicator . Two examples serve to illustrate this point. In a solution of Potassium Hydrogen Iodate at 0.02 M the activity is 40% lower than the calculated hydrogen ion concentration resulting in a much higher PH than expected. When to a 0.01M Hydrochloric Acid solution containing Malachite Green indicator is added a 5M solution of Lithium Chloride the color changes from green to yellow indicating increasing acidity when in fact the solution is diluted. Although at low ionic strength (<0.1M) the activation coefficient decreases with increasing ionic strength this coefficient can actually increase with ionic strength in a high ionic strength regime. For hydrochloric acid solutions the minimum is around 0.4M. EXTERNAL LINKS |
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