# A quantitative measure of the strength of an acid is

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A quantitative measure of the strength of an acid is given by the acid dissociation constant, K a . Equilibrium Constant: The acid dissociation constant, K a , is the equilibrium constant for a dissociation reaction. Consider first a simple reaction and its equilibrium features: Kinetics gives the following rate equations:

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3 At equilibrium there is no change in the concentration of A or B; therefore: At equilibrium the reaction has not stopped. Rather, the rates of the forward reaction (k 1 [A]) and the reverse reaction (k 2 [B]) are equal, thus: At equilibrium, the fraction of A (f a ) and B (f b ) are given by: Some simple intuitive conclusions can be drawn from the above equations: If K eq >>1 then f b =1 and f a =1/K eq If K eq << 1 then f b =K eq and f a =1 A separate page, Ratios & Fractions in the Henderson-Hasselbalch Equation , relates the above principles to the specific case of problem solving using the Henderson-Hasselbalch Equation. 2.4 Self-Dissociation of Water The equilibrium constant for the ionization of water is: Since the concentration of water is very high (55.5 M) and practically constant, we can incorporate it into the equilibrium constant to give: This is the ion product constant for water.
4 In pure water [H + ] = [OH - ] = 10 -7 M because water has no net charge. Definition of pX: pX = - log[X] A specific example is: pApples = -log[Apples], or more important: pH = -log[H + ] Acid Base Equilibrium The equilibrium for ionization (dissociation) of an acid is given by: as above, assuming that the concentration of water is constant: Which gives rise to the Henderson-Hasselbalch equation: When the pH = pK a , there are equal amounts of [A - ] and [HA] in solution. When the pH is lower than the pK a , the [HA] > [A - ]. When the pH is higher than the pK a , then [HA] < [A - ]. This is a simple example of the application of Le Chatelier's principle : If we add acid (to decrease the pH), the system will respond by increasing the amount of [HA] to reduce the concentration of the added acid and return the system to its initial pH The acidity constant, K a , is a property of the acid. For example:

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• Spring '06
• clavijo

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