Hence the ionization equilibrium lies virtually all the way to the right, as represented by a single arrow:. In contrast, acetic acid is a weak acid, and water is a weak base. Similarly, in the reaction of ammonia with water, the hydroxide ion is a strong base, and ammonia is a weak base, whereas the ammonium ion is a stronger acid than water.
Hence this equilibrium also lies to the left:. All acid—base equilibria favor the side with the weaker acid and base. A pH value around 7 is neutral neither acidic nor basic , a pH value less than 7 is acidic, and a pH value greater than 7 is basic.
Ka is the acid dissociation constant. A large Ka value indicates a strong acid because it means an acid largely dissociates into its ions. A large Ka value also means the reaction arrow favors the formation of production. In contrast, a small Ka value means only a small amount of acid dissociates, indicating a weak acid.
A small Ka value means the reaction favors the reactants rather than the products. For example, pKa is the -log of Ka. Because of the way the log function works, a smaller pKa means a larger Ka. If you know pH, you can calculate pOH. If you know an equilibrium constant, you can calculate the others. The pH scale ranges from 0 to A low pH value indicates acidity, a pH of 7 is neutral, and a high pH value indicates alkalinity. The pH value can tell you whether you're dealing with an acid or a base, but it offers limited value indicating the true strength of the acid of a base.
The formulas to calculate pH and pOH are:. Ka, pKa, Kb, and pKb are most helpful when predicting whether a species will donate or accept protons at a specific pH value. They describe the degree of ionization of an acid or base and are true indicators of acid or base strength because adding water to a solution will not change the equilibrium constant. Ka and pKa relate to acids, while Kb and pKb deal with bases. Like pH and pOH , these values also account for hydrogen ion or proton concentration for Ka and pKa or hydroxide ion concentration for Kb and pKb.
Ka and Kb are related to each other through the ion constant for water, Kw:. Ka is the acid dissociation constant. Similarly, Kb is the base dissociation constant, while pKb is the -log of the constant. Acids and bases dissociate according to general equations:. In the formulas, A stands for acid and B for base. In a problem that involves calculating the Kb from the Ka, you are usually given the Ka and the Kw.
For example, you may be asked to calculate the Kb of the chloride ion. The given Ka of the conjugate acid of the chloride ion, which is hydrogen chloride, is 1. The given Kw is 1. Solve the equation for Kb by dividing the Kw by the Ka. Put the values from the problem into the equation. The Kb is 1. Mara Pesacreta has been writing for over seven years.
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