{"id":7629,"date":"2021-06-08T21:57:05","date_gmt":"2021-06-08T21:57:05","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/bronsted-lowry-acids-and-bases\/"},"modified":"2021-10-06T23:03:12","modified_gmt":"2021-10-06T23:03:12","slug":"bronsted-lowry-acids-and-bases","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistry\/chapter\/bronsted-lowry-acids-and-bases\/","title":{"raw":"Br\u00f8nsted-Lowry Acids and Bases","rendered":"Br\u00f8nsted-Lowry Acids and Bases"},"content":{"raw":"[latexpage]\r\n
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Learning Objectives<\/p>\r\n\r\n<\/header>\r\n

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  1. Identify a Br\u00f8nsted-Lowry acid and a Br\u00f8nsted-Lowry base.<\/li>\r\n \t
  2. Identify conjugate acid-base pairs in an acid-base reaction.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nThe Arrhenius definition of acid and base is limited to aqueous (that is, water) solutions. Although this is useful because water is a common solvent, it is limited to the relationship between the H+<\/sup> ion and the OH\u2212<\/sup> ion. What would be useful is a more general definition that would be more applicable to other chemical reactions and, importantly, independent of H2<\/sub>O.\r\n\r\nIn 1923, Danish chemist Johannes Br\u00f8nsted and English chemist Thomas Lowry independently proposed new definitions for acids and bases, ones that focus on proton transfer. A Br\u00f8nsted-Lowry acid<\/strong>\u00a0is any species that can donate a proton (H+<\/sup>) to another molecule. A Br\u00f8nsted-Lowry base<\/strong>\u00a0is any species that can accept a proton from another molecule. In short, a Br\u00f8nsted-Lowry acid is a proton donor (PD), while a Br\u00f8nsted-Lowry base is a proton acceptor (PA).\r\n\r\nIt is easy to see that the Br\u00f8nsted-Lowry definition covers the Arrhenius definition of acids and bases. Consider the prototypical Arrhenius acid-base reaction:\r\n

    [latex]\\begin{array}{ccccc}\r\n\\ce{H+(aq)}&+&\\ce{OH-(aq)}&\\rightarrow&\\ce{H2O(\\ell)} \\\\ \\\\\r\n\\text{(acid)}&&\\text{(base)}&&\r\n\\end{array}[\/latex]<\/p>\r\nThe acid species and base species are marked. The proton, however, is (by definition) a proton donor (labelled PD), while the OH\u2212<\/sup> ion is acting as the proton acceptor (labelled PA):\r\n

    [latex]\\begin{array}{ccccc}\r\n\\ce{H+(aq)}&+&\\ce{OH-(aq)}&\\rightarrow&\\ce{H2O(\\ell)} \\\\ \\\\\r\n\\text{(PD)}&&\\text{(PA)}&&\r\n\\end{array}[\/latex]<\/p>\r\nThe proton donor is a Br\u00f8nsted-Lowry acid, and the proton acceptor is the Br\u00f8nsted-Lowry base:\r\n

    [latex]\\begin{array}{ccccc}\r\n\\ce{H+(aq)}&+&\\ce{OH-(aq)}&\\rightarrow&\\ce{H2O(\\ell)} \\\\ \\\\\r\n\\text{(BL acid)}&&\\text{(BL base)}&&\r\n\\end{array}[\/latex]<\/p>\r\nThus H+<\/sup> is an acid by both definitions, and OH\u2212<\/sup> is a base by both definitions.\r\n\r\nAmmonia (NH3<\/sub>) is a base even though it does not contain OH\u2212<\/sup> ions in its formula. Instead, it generates OH\u2212<\/sup> ions as the product of a proton-transfer reaction with H2<\/sub>O molecules; NH3<\/sub> acts like a Br\u00f8nsted-Lowry base, and H2<\/sub>O acts like a Br\u00f8nsted-Lowry acid:\r\n

    [latex]\\begin{array}{ccccc}\r\n\\chemfig{H-\\Lewis{6:,N}(-[:90]H)-H}&+&\\chemfig{H-[:30]O-[:-30]H}&\\rightarrow&\\chemfig{H-N^+(-[:90]H)(-[:-90]H)-H}+\\chemfig{^{-}O-H} \\\\\r\n\\text{proton acceptor}&&\\text{proton donor}&&\r\n\\end{array}[\/latex]<\/p>\r\nA reaction with water is called hydrolysis<\/strong>; we say that NH3<\/sub> hydrolyzes to make NH4<\/sub>+<\/sup> ions and OH\u2212<\/sup> ions.\r\n\r\nEven the dissolving of an Arrhenius acid in water can be considered a Br\u00f8nsted-Lowry acid-base reaction. Consider the process of dissolving HCl(g) in water to make an aqueous solution of hydrochloric acid. The process can be written as follows:\r\n

    HCl(g) +\u00a0H2<\/sub>O(\u2113) \u2192\u00a0H3<\/sub>O+<\/sup>(aq) +\u00a0Cl\u2212<\/sup>(aq)<\/p>\r\nHCl(g) is the proton donor and therefore a Br\u00f8nsted-Lowry acid, while H2<\/sub>O is the proton acceptor and a Br\u00f8nsted-Lowry base. These two examples show that H2<\/sub>O can act as both a proton donor and a proton acceptor, depending on what other substance is in the chemical reaction. A substance that can act as a proton donor or a proton acceptor is called amphiprotic<\/strong>. Water is probably the most common amphiprotic substance we will encounter, but other substances are also amphiprotic.\r\n

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    Example 12.5<\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nIdentify the Br\u00f8nsted-Lowry acid and the Br\u00f8nsted-Lowry base in this chemical equation.\r\n

    C6<\/sub>H5<\/sub>OH + NH2<\/sub>\u2212<\/sup> \u2192 C6<\/sub>H5<\/sub>O\u2212<\/sup> + NH3<\/sub><\/p>\r\nSolution<\/em>\r\nThe C6<\/sub>H5<\/sub>OH molecule is losing an H+<\/sup>; it is the proton donor and the Br\u00f8nsted-Lowry acid. The NH2<\/sub>\u2212<\/sup> ion (called the amide ion) is accepting the H+<\/sup> ion to become NH3<\/sub>, so it is the Br\u00f8nsted-Lowry base.\r\n\r\nTest Yourself<\/em>\r\nIdentify the Br\u00f8nsted-Lowry acid and the Br\u00f8nsted-Lowry base in this chemical equation.\r\n

    Al(H2<\/sub>O)6<\/sub>3+<\/sup> + H2<\/sub>O \u2192 Al(H2<\/sub>O)5<\/sub>(OH)2+<\/sup> + H3<\/sub>O+<\/sup><\/p>\r\nAnswer<\/em>\r\nBr\u00f8nsted-Lowry acid: Al(H2<\/sub>O)6<\/sub>3+<\/sup>; Br\u00f8nsted-Lowry base: H2<\/sub>O\r\n\r\n<\/div>\r\n<\/div>\r\n

    Observe the reaction between NH3<\/sub> and H2<\/sub>O:<\/p>\r\n

    [latex]\\begin{array}{ccccc}\r\n\\chemfig{H-\\Lewis{6:,N}(-[:90]H)-H}&+&\\chemfig{H-[:30]O-[:-30]H} &\\rightarrow & \\chemfig{H-N^+(-[:90]H)(-[:-90]H)-H}+\\chemfig{O-H^{+}}}\r\n\\end{array}[\/latex]<\/p>\r\nThe chemical reaction does not go to completion; rather, the reverse process occurs as well, and eventually the two processes cancel out any additional change. At this point, we say the chemical reaction is at equilibrium<\/em>. Both processes still occur, but any net change by one process is countered by the same net change by the other process; it is a dynamic<\/em>, rather than a static<\/em>, equilibrium. Because both reactions are occurring, it makes sense to use a double arrow instead of a single arrow:\r\n

    [latex]\\begin{array}{ccccc}\r\n\\chemfig{H-\\Lewis{6:,N}(-[:90]H)-H}&+&\\chemfig{H-[:30]O-[:-30]H} &\\rightleftharpoons & \\chemfig{H-N^+(-[:90]H)(-[:-90]H)-H}+\\chemfig{O-H^{-}}}\r\n\\end{array}[\/latex]<\/p>\r\nWhat do you notice about the reverse reaction? The NH4<\/sub>+<\/sup> ion is donating a proton to the OH\u2212<\/sup> ion, which is accepting it. This means that the NH4<\/sub>+<\/sup> ion is acting as the proton donor, or Br\u00f8nsted-Lowry acid, while OH\u2212<\/sup> ion, the proton acceptor, is acting as a Br\u00f8nsted-Lowry base. The reverse reaction is also a Br\u00f8nsted-Lowry acid base reaction:\r\n

    [latex]\\begin{array}{ccccc}\r\n\\chemfig{H-\\Lewis{6:,N}(-[:90]H)-H}&+&\\chemfig{H-[:30]O-[:-30]H} &\\rightleftharpoons & \\chemfig{H-N^+(-[:90]H)(-[:-90]H)-H}+\\chemfig{O-H^{-}}}\r\n\\end{array}[\/latex]<\/p>\r\nThis means that both reactions are acid-base reactions by the Br\u00f8nsted-Lowry definition. If you consider the species in this chemical reaction, two sets of similar species exist on both sides. Within each set, the two species differ by a proton in their formulas, and one member of the set is a Br\u00f8nsted-Lowry acid, while the other member is a Br\u00f8nsted-Lowry base. These sets are marked here:\r\n\r\n\"BL\r\n\r\nThe two sets \u2014 NH3<\/sub>\/NH4<\/sub>+<\/sup> and H2<\/sub>O\/OH\u2212<\/sup> \u2014 are called conjugate acid-base pairs<\/strong>. We say that NH4<\/sub>+<\/sup> is the conjugate acid of NH3<\/sub>, OH\u2212<\/sup> is the conjugate base of H2<\/sub>O, and so forth. Every Br\u00f8nsted-Lowry acid-base reaction can be labelled with two conjugate acid-base pairs.\r\n

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    Example 12.6<\/p>\r\n\r\n<\/header>\r\n

    \r\n\r\nIdentify the conjugate acid-base pairs in this equilibrium.\r\n

    (CH3<\/sub>)3<\/sub>N + H2<\/sub>O \u21c4 (CH3<\/sub>)3<\/sub>NH+ + OH\u2013<\/sup><\/p>\r\nSolution<\/em>\r\nOne pair is H2<\/sub>O and OH\u2212<\/sup>, where H2<\/sub>O has one more H+<\/sup> and is the conjugate acid, while OH\u2212<\/sup> has one less H+<\/sup> and is the conjugate base. The other pair consists of (CH3<\/sub>)3<\/sub>N and (CH3<\/sub>)3<\/sub>NH+<\/sup>, where (CH3<\/sub>)3<\/sub>NH+<\/sup> is the conjugate acid (it has an additional proton) and (CH3<\/sub>)3<\/sub>N is the conjugate base.\r\n\r\nTest Yourself<\/em>\r\nIdentify the conjugate acid-base pairs in this equilibrium.\r\n

    NH2<\/sub>\u2013 <\/sup>+ H2<\/sub>O \u21c4 NH3<\/sub> + OH\u2013<\/sup><\/p>\r\nAnswer<\/em>\r\nH2<\/sub>O (acid) and OH\u2212<\/sup> (base); NH2<\/sub>\u2212<\/sup> (base) and NH3<\/sub> (acid)\r\n\r\n<\/div>\r\n<\/div>\r\n

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    Chemistry Is Everywhere: Household Acids and Bases<\/h1>\r\nMany household products are acids or bases. For example, the owner of a swimming pool may use muriatic acid to clean the pool. Muriatic acid is another name for HCl(aq). In Chapter 4 \"Chemical Reactions and Equations\"<\/a>, in the section \"Neutralization Reactions\"<\/a>, vinegar was mentioned as a dilute solution of acetic acid [HC2<\/sub>H3<\/sub>O2<\/sub>(aq)]. In a medicine chest, one may find a bottle of vitamin C tablets; the chemical name of vitamin C is ascorbic acid (HC6<\/sub>H7<\/sub>O6<\/sub>).\r\n\r\nOne of the more familiar household bases is NH3<\/sub>, which is found in numerous cleaning products. NH3<\/sub> is a base because it increases the OH\u2212<\/sup> ion concentration by reacting with H2<\/sub>O:\r\n

    NH3<\/sub>(aq) +\u00a0H2<\/sub>O(\u2113) \u2192\u00a0NH4<\/sub>+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq)<\/p>\r\nMany soaps are also slightly basic because they contain compounds that act as Br\u00f8nsted-Lowry bases, accepting protons from H2<\/sub>O and forming excess OH\u2212<\/sup> ions. This is one explanation for why soap solutions are slippery.\r\n\r\nPerhaps the most dangerous household chemical is the lye-based drain cleaner. Lye is a common name for NaOH, although it is also used as a synonym for KOH. Lye is an extremely caustic chemical that can react with grease, hair, food particles, and other substances that may build up and clog a water pipe. Unfortunately, lye can also attack body tissues and other substances in our bodies. Thus when we use lye-based drain cleaners, we must be very careful not to touch any of the solid drain cleaner or spill the water it was poured into. Safer, nonlye drain cleaners (like the one in the accompanying figure) use peroxide compounds to react on the materials in the clog and clear the drain.\r\n\r\n<\/div>\r\n

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    Key Takeaways<\/p>\r\n\r\n<\/header>\r\n

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