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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク weak base ： ウィキペディア英語版 weak base In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As Brønsted–Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). pH has the formula: :$\backslash mbox\; =\; -\backslash log\_\; \backslash left(\backslash mbox^+\; \backslash right)$ Since bases are proton acceptors, the base receives a hydrogen ion from water, H2O, and the remaining H+ concentration in the solution determines pH. Weak bases will have a higher H+ concentration because they are less completely protonated than stronger bases and, therefore, more hydrogen ions remain in the solution. If you plug in a higher H+ concentration into the formula, a low pH results. However, pH of bases is usually calculated using the OH− concentration to find the pOH first. This is done because the H+ concentration is not a part of the reaction, while the OH− concentration is. :$\backslash mbox\; =\; -\backslash log\_\; \backslash left(\backslash mbox^-\; \backslash right)$ By multiplying a conjugate acid (such as NH4+) and a conjugate base (such as NH3) the following is given: :$K\_a\; \backslash times\; K\_b\; =\; \backslash times\; =\; ()\; ()$ Since $=\; ()\; ()$ then, ''$K\_a\; \backslash times\; K\_b\; =\; K\_w$'' By taking logarithms of both sides of the equation, the following is reached: :$logK\_a\; +\; logK\_b\; =\; logK\_w$ Finally, multiplying throughout the equation by -1, the equation turns into: :$pK\_a\; +\; pK\_b\; =\; pK\_w\; =\; 14.00$ After acquiring pOH from the previous pOH formula, pH can be calculated using the formula pH = pKw - pOH where pKw = 14.00. Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a base dissociation constant (Kb) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up: :$\backslash mathrm\{K\_b=\{()\; ()\backslash over()\}\}$ Bases that have a large Kb will ionize more completely and are thus stronger bases. As stated above, pH of the solution depends on the H+ concentration, which is related to the OH− concentration by the self-ionization constant (Kw = 1.0x10−14). A strong base has a lower H+ concentration because they are fully protonated and less hydrogen ions remain in the solution. A lower H+ concentration also means a higher OH− concentration and therefore, a larger Kb. NaOH (s) (sodium hydroxide) is a stronger base than (CH3CH2)2NH (l) (diethylamine) which is a stronger base than NH3 (g) (ammonia). As the bases get weaker, the smaller the Kb values become. ==Percentage protonated== As seen above, the strength of a base depends primarily on pH. To help describe the strengths of weak bases, it is helpful to know the percentage protonated-the percentage of base molecules that have been protonated. A lower percentage will correspond with a lower pH because both numbers result from the amount of protonation. A weak base is less protonated, leading to a lower pH and a lower percentage protonated. The typical proton transfer equilibrium appears as such: :$B(aq)\; +\; H\_2O(l)\; \backslash leftrightarrow\; HB^+(aq)\; +\; OH^-(aq)$ B represents the base. :$Percentage\backslash \; protonated\; =\; \backslash times\; 100\backslash \%\; =$ In this formula, ()initial is the initial molar concentration of the base, assuming that no protonation has occurred. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「weak base」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.