{"id":152,"date":"2016-01-11T19:59:34","date_gmt":"2016-01-11T19:59:34","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/neutralization-reactions-2\/"},"modified":"2020-05-06T20:08:56","modified_gmt":"2020-05-06T20:08:56","slug":"neutralization-reactions","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/neutralization-reactions\/","title":{"raw":"Neutralization Reactions","rendered":"Neutralization Reactions"},"content":{"raw":"
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Learning Objectives<\/p>\r\n\r\n<\/header>\r\n

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  1. Identify an acid and a base.<\/li>\r\n \t
  2. Identify a neutralization reaction and predict its products.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\nIn Chapter 3 \"Atoms, Molecules, and Ions\"<\/a>, in the section called \"Acids\"<\/a>, we defined an acid as an ionic compound that contains H+<\/sup> as the cation. This is slightly incorrect, but until additional concepts were developed, a better definition needed to wait. Now we can redefine an acid: an [pb_glossary id=\"1229\"]acid[\/pb_glossary]\u00a0is any compound that increases the amount of hydrogen ion (H+<\/sup>) in an aqueous solution. The chemical opposite of an acid is a base. The equivalent definition of a base is that a [pb_glossary id=\"1319\"]base[\/pb_glossary]\u00a0is a compound that increases the amount of hydroxide ion (OH\u2212<\/sup>) in an aqueous solution. These original definitions were proposed by Arrhenius (the same person who proposed ion dissociation) in 1884, so they are referred to as the Arrhenius definitions of an acid and a base, respectively.\r\n\r\nYou may recognize that, based on the description of a hydrogen atom, an H+<\/sup> ion is a hydrogen atom that has lost its lone electron; that is, H+<\/sup> is simply a proton. Do we really have bare protons moving about in aqueous solution? No. What is more likely is that the H+<\/sup> ion has attached itself to one (or more) water molecule(s). To represent this chemically, we define the [pb_glossary id=\"1320\"]hydronium ion[\/pb_glossary]\u00a0H3<\/sub>O+<\/sup>(aq), a water molecule with an extra hydrogen ion attached to it, as H3<\/sub>O+<\/sup>, which represents an additional proton attached to a water molecule. We use the hydronium ion as the more logical way a hydrogen ion appears in an aqueous solution, although in many chemical reactions H+<\/sup> and H3<\/sub>O+<\/sup> are treated equivalently.\r\n\r\nThe reaction of an acid and a base is called a [pb_glossary id=\"1321\"]neutralization reaction[\/pb_glossary]. Although acids and bases have their own unique chemistries, the acid and base cancel each other\u2019s chemistry to produce a rather innocuous substance\u2014water. In fact, the general reaction between an acid and a base is\r\n

    acid +\u00a0base \u2192\u00a0water +\u00a0salt<\/p>\r\nwhere the term salt\u00a0is generally used to define any ionic compound (soluble or insoluble) that is formed from a reaction between an acid and a base. (In chemistry, the word salt<\/em> refers to more than just table salt.) For example, the balanced chemical equation for the reaction between HCl(aq) and KOH(aq) is\r\n

    HCl(aq) +\u00a0KOH(aq) \u2192\u00a0H2<\/sub>O(\u2113) +\u00a0KCl(aq)<\/p>\r\nwhere the salt is KCl. By counting the number of atoms of each element, we find that only one water molecule is formed as a product. However, in the reaction between HCl(aq) and Mg(OH)2<\/sub>(aq), additional molecules of HCl and H2<\/sub>O are required to balance the chemical equation:\r\n

    2HCl(aq) + Mg(OH)2<\/sub>(aq) \u2192 2H2<\/sub>O(\u2113) +\u00a0MgCl2<\/sub>(aq)<\/p>\r\nHere, the salt is MgCl2<\/sub>. (This is one of several reactions that take place when a type of antacid\u2014a base\u2014is used to treat stomach acid.)\r\n

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    Example 4.13<\/p>\r\n\r\n<\/header>\r\n

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    Problems<\/h1>\r\nWrite the neutralization reactions between each acid and base.\r\n
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    1. HNO3<\/sub>(aq) and Ba(OH)2<\/sub>(aq)<\/li>\r\n \t
    2. H3<\/sub>PO4<\/sub>(aq) and Ca(OH)2<\/sub>(aq)<\/li>\r\n<\/ol>\r\n

      Solutions<\/h2>\r\nFirst, we will write the chemical equation with the formulas of the reactants and the expected products; then we will balance the equation.\r\n
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      1. The expected products are water and barium nitrate, so the initial chemical reaction is\r\nHNO3<\/sub>(aq) + Ba(OH)2<\/sub>(aq) \u2192 H2<\/sub>O(\u2113) + Ba(NO3<\/sub>)2<\/sub>(aq). To balance the equation, we need to realize that there will be two H2<\/sub>O molecules, so two HNO3<\/sub> molecules are required:\r\n

        2HNO3<\/sub>(aq) + Ba(OH)2<\/sub>(aq) \u2192 2H2<\/sub>O(\u2113) + Ba(NO3<\/sub>)2<\/sub>(aq)<\/p>\r\nThis chemical equation is now balanced.<\/li>\r\n \t

      2. The expected products are water and calcium phosphate, so the initial chemical equation is\r\nH3<\/sub>PO4<\/sub>(aq) + Ca(OH)2<\/sub>(aq) \u2192 H2<\/sub>O(\u2113) + Ca3<\/sub>(PO4<\/sub>)2<\/sub>(s). According to the solubility rules, Ca3<\/sub>(PO4<\/sub>)2<\/sub> is insoluble, so it has an (s) phase label. To balance this equation, we need two phosphate ions and three calcium ions; we end up with six water molecules to balance the equation:\r\n

        2H3<\/sub>PO4<\/sub>(aq) + 3Ca(OH)2<\/sub>(aq) \u2192 6H2<\/sub>O(\u2113) + Ca3<\/sub>(PO4<\/sub>)2<\/sub>(s)<\/p>\r\nThis chemical equation is now balanced.<\/li>\r\n<\/ol>\r\n

        Test Yourself<\/h1>\r\nWrite the neutralization reaction between H2<\/sub>SO4<\/sub>(aq) and Sr(OH)2<\/sub>(aq).\r\n

        Answer<\/h2>\r\nH2<\/sub>SO4<\/sub>(aq) + Sr(OH)2<\/sub>(aq) \u2192 2H2<\/sub>O(\u2113) + SrSO4<\/sub>(aq)\r\n\r\n<\/div>\r\n<\/div>\r\nNeutralization reactions are one type of chemical reaction that proceeds even if one reactant is not in the aqueous phase. For example, the chemical reaction between HCl(aq) and Fe(OH)3<\/sub>(s) still proceeds according to the equation:\r\n

        3HCl(aq) +\u00a0Fe(OH)3<\/sub>(s) \u2192\u00a03H2<\/sub>O(\u2113) +\u00a0FeCl3<\/sub>(aq)<\/p>\r\neven though Fe(OH)3<\/sub> is not soluble. When one realizes that Fe(OH)3<\/sub>(s) is a component of rust, this explains why some cleaning solutions for rust stains contain acids\u2014the neutralization reaction produces products that are soluble and wash away. (Washing with acids like HCl is one way to remove rust and rust stains, but HCl must be used with caution!)\r\n\r\nComplete and net ionic reactions for neutralization reactions will depend on whether the reactants and products are soluble, even if the acid and base react. For example, in the reaction of HCl(aq) and NaOH(aq):\r\n

        HCl(aq) +\u00a0NaOH(aq) \u2192\u00a0H2<\/sub>O(\u2113) +\u00a0NaCl(aq)<\/p>\r\nThe complete ionic reaction is:\r\n

        H+<\/sup>(aq) +\u00a0Cl\u2212<\/sup>(aq) +\u00a0Na+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq) \u2192\u00a0H2<\/sub>O(\u2113) +\u00a0Na+<\/sup>(aq) +\u00a0Cl\u2212<\/sup>(aq)<\/p>\r\nThe Na+<\/sup>(aq) and Cl\u2212<\/sup>(aq) ions are spectator ions, so we can remove them to have:\r\n

        H+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq) \u2192\u00a0H2<\/sub>O(\u2113)<\/p>\r\nas the net ionic equation. If we wanted to write this in terms of the hydronium ion, H3<\/sub>O+<\/sup>(aq), we would write it as:\r\n

        H3<\/sub>O+<\/sup>(aq) +\u00a0OH\u2212<\/sup>(aq) \u2192\u00a02H2<\/sub>O(\u2113)<\/p>\r\nWith the exception of the introduction of an extra water molecule, these two net ionic equations are equivalent.\r\n\r\nHowever, for the reaction between HCl(aq) and Cr(OH)2<\/sub>(s), because chromium(II) hydroxide is insoluble, we cannot separate it into ions for the complete ionic equation:\r\n

        2H+<\/sup>(aq) +\u00a02Cl\u2212<\/sup>(aq) +\u00a0Cr(OH)2<\/sub>(s) \u2192\u00a02H2<\/sub>O(\u2113) +\u00a0Cr2+<\/sup>(aq) +\u00a02Cl\u2212<\/sup>(aq)<\/p>\r\nThe chloride ions are the only spectator ions here, so the net ionic equation is:\r\n

        2H+<\/sup>(aq) +\u00a0Cr(OH)2<\/sub>(s) \u2192\u00a02H2<\/sub>O(\u2113) +\u00a0Cr2+<\/sup>(aq)<\/p>\r\n\r\n

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        Example 4.14<\/p>\r\n\r\n<\/header>\r\n

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        Problem<\/h1>\r\nOxalic acid, H2<\/sub>C2<\/sub>O4<\/sub>(s), and Ca(OH)2<\/sub>(s) react very slowly. What is the net ionic equation between these two substances if the salt formed is insoluble? (The anion in oxalic acid is the oxalate ion, C2<\/sub>O4<\/sub>2\u2212<\/sup>.)\r\n

        Solution<\/h2>\r\nThe products of the neutralization reaction will be water and calcium oxalate:\r\n

        H2<\/sub>C2<\/sub>O4<\/sub>(s) + Ca(OH)2<\/sub>(s) \u2192 2H2<\/sub>O(\u2113) + CaC2<\/sub>O4<\/sub>(s)<\/p>\r\nBecause nothing is dissolved, there are no substances to separate into ions, so the net ionic equation is the equation of the three solids and one liquid.\r\n

        Test Yourself<\/h1>\r\nWhat is the net ionic equation between HNO3<\/sub>(aq) and Ti(OH)4<\/sub>(s)?\r\n

        Answer<\/h2>\r\n4H+<\/sup>(aq) + Ti(OH)4<\/sub>(s) \u2192 4H2<\/sub>O(\u2113) + Ti4+<\/sup>(aq)\r\n\r\n<\/div>\r\n<\/div>\r\n
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        Key Takeaways<\/p>\r\n\r\n<\/header>\r\n

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