{"id":505,"date":"2016-01-11T20:00:26","date_gmt":"2016-01-11T20:00:26","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/other-aspects-of-covalent-bonds-2\/"},"modified":"2020-07-30T17:18:19","modified_gmt":"2020-07-30T17:18:19","slug":"other-aspects-of-covalent-bonds","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/other-aspects-of-covalent-bonds\/","title":{"raw":"Other Aspects of Covalent Bonds","rendered":"Other Aspects of Covalent Bonds"},"content":{"raw":"
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Learning Objectives<\/h3>\r\n
    \r\n \t
  1. Describe a nonpolar bond and a polar bond.<\/li>\r\n \t
  2. Use electronegativity to determine whether a bond between two elements will be nonpolar covalent, polar covalent, or ionic.<\/li>\r\n \t
  3. Describe the bond energy of a covalent bond.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n

    Consider the H2<\/sub> molecule:<\/p>\r\n

    \"H-H\"<\/a><\/p>\r\n\r\n

    <\/div>\r\n

    Because the nuclei of each H atom contain protons, the electrons in the bond are attracted to the nuclei (opposite charges attract). But because the two atoms involved in the covalent bond are both H atoms, each nucleus attracts the electrons by the same amount. Thus the electron pair is equally shared by the two atoms. The equal sharing of electrons in a covalent bond is called a nonpolar covalent bond<\/a>.<\/p>\r\n

    Now consider the HF molecule:<\/p>\r\n

    \"H-F\"<\/a><\/p>\r\n\r\n

    <\/div>\r\n

    There are two different atoms involved in the covalent bond. The H atom has one proton in its nucleus that is attracting the bonding pair of electrons. However, the F atom has nine protons in its nucleus, with nine times the attraction of the H atom. The F atom attracts the electrons so much more strongly that the electrons remain closer to the F atom than to the H atom; the electrons are no longer equally balanced between the two nuclei.<\/p>\r\n\r\n

    Because the electrons in the bond are nearer to the F atom, this side of the molecule takes on a partial negative charge, which is represented by \u03b4\u2212 (\u03b4 is the lowercase Greek letter delta). The other side of the molecule, the H atom, adopts a partial positive charge, which is represented by \u03b4+:<\/span><\/div>\r\n
    <\/div>\r\n
    \"H-F-Charge\"<\/a><\/div>\r\n
    <\/div>\r\n

    A covalent bond between different atoms that attract the shared electrons by different amounts and cause an imbalance of electron distribution is called a polar covalent bond<\/a>.<\/p>\r\nTechnically, any covalent bond between two different elements is polar. However, the degree of polarity is important. A covalent bond between two different elements may be so slightly imbalanced that the bond is, essentially, nonpolar. A bond may be so polar that an electron actually transfers from one atom to another, forming a true ionic bond. How do we judge the degree of polarity?\r\n

    Scientists have devised a scale called electronegativity,<\/a> a\u00a0scale for judging how much atoms of any element attract electrons.\u00a0Electronegativity is a unitless number; the higher the number, the more an atom attracts electrons. A common scale for electronegativity is shown in Figure 9.3 \"Electronegativities of the Elements.\"<\/a><\/span><\/p>\r\n\r\n

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    Figure 9.3<\/span>\u00a0Electronegativities of the Elements<\/a><\/p>\r\n\r\n[caption id=\"attachment_4319\" align=\"alignnone\" width=\"400\"]\"Figure<\/a> Figure 9.3 Electronegativities by Elements. By Joanjoc at ca.wikipedia [Public domain], from Wikimedia Commons[\/caption]\r\n

    Electronegativities are used to determine the polarity of covalent bonds.<\/p>\r\n\r\n<\/div>\r\n

    The polarity of a covalent bond can be judged by determining the difference<\/em>\u00a0between the electronegativities of the two atoms involved in the covalent bond, as summarized in Table 9.2 \"Electronegativities of Bond Types.\"<\/p>\r\n

    Table 9.2 Electronegativities of Bond Types<\/p>\r\n\r\n

    \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Electronegativity Difference<\/th>\r\nBond Type<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    0<\/td>\r\nnonpolar covalent<\/td>\r\n<\/tr>\r\n
    0\u20130.4<\/td>\r\nslightly polar covalent<\/td>\r\n<\/tr>\r\n
    0.4\u20131.9<\/td>\r\ndefinitely polar covalent<\/td>\r\n<\/tr>\r\n
    >1.9<\/td>\r\nlikely ionic<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\nThe unequal sharing of electrons in a covalent bond is usually indicated by partial charge notation as seen earlier, or by a dipole arrow. Dipole arrows depict the unequal sharing by showing the flow of electron density. Dipole arrows have an end with a \u201c+ sign\u201d depicting an electropositive area from which electron density is being pulled, and an end with an arrowhead\u00a0pointing to the\u00a0more electronegative atom toward which electron density is being pulled.\"H-F_dipole_arrow\"<\/a>\r\n
    \r\n

    Example 9.7<\/h3>\r\n

    What is the polarity of each of the following bonds?<\/p>\r\n\r\n

      \r\n \t
    1. C\u2013H<\/li>\r\n \t
    2. O\u2013H<\/li>\r\n<\/ol>\r\n

      Solution<\/p>\r\n

      Using Figure 9.2 \"Electronegativities of the Elements,\"<\/a> we can calculate the electronegativity differences of the atoms involved in the bond.<\/p>\r\n\r\n

        \r\n \t
      1. For the C\u2013H bond, the difference in electronegativities is 2.5 \u2212 2.1 = 0.4. Thus we predict that this bond will be slightly polar covalent.<\/li>\r\n \t
      2. For the O\u2013H bond, the difference in electronegativities is 3.5 \u2212 2.1 = 1.4, so we predict that this bond will be definitely polar covalent.<\/li>\r\n<\/ol>\r\n

        Test Yourself<\/em><\/p>\r\n

        What is the polarity of each of these bonds?<\/p>\r\n\r\n

          \r\n \t
        1. Rb\u2013F<\/li>\r\n \t
        2. P\u2013Cl<\/li>\r\n<\/ol>\r\n

          Answers<\/em><\/p>\r\n\r\n

            \r\n \t
          1. likely ionic<\/li>\r\n \t
          2. polar covalent<\/li>\r\n<\/ol>\r\n<\/div>\r\n

            The polarity of a covalent bond can have significant influence on the properties of the substance. If the overall molecule is polar, the substance may have a higher melting point and boiling point than expected; also, it may or may not be soluble in various other substances, such as water or hexane.<\/p>\r\n

            It should be obvious that covalent bonds are stable because molecules exist. However, the bonds can be broken if enough energy is supplied to a molecule. To break most covalent bonds between any two given atoms, a certain amount of energy must be supplied. Although the exact amount of energy depends on the molecule, the approximate amount of energy to be supplied is similar if the atoms in the bond are the same. The approximate amount of energy needed to break a covalent bond is called the bond energy<\/a>\u00a0of the covalent bond. Table 9.3 \"Bond Energies of Covalent Bonds\"<\/a> lists the bond energies of some covalent bonds.<\/p>\r\n\r\n

            \r\n

            Table 9.3<\/span> Bond Energies of Covalent Bonds<\/p>\r\nBondEnergy (kJ\/mol)\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
            Bond<\/th>\r\nEnergy (kJ\/mol)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
            C\u2013C<\/td>\r\n348<\/td>\r\n<\/td>\r\nN\u2013N<\/td>\r\n163<\/td>\r\n<\/tr>\r\n
            C=C<\/td>\r\n611<\/td>\r\nN=N<\/td>\r\n418<\/td>\r\n<\/tr>\r\n
            C\u2261C<\/td>\r\n837<\/td>\r\nN\u2261N<\/td>\r\n946<\/td>\r\n<\/tr>\r\n
            C\u2013O<\/td>\r\n351<\/td>\r\nN\u2013H<\/td>\r\n389<\/td>\r\n<\/tr>\r\n
            C=O<\/td>\r\n799<\/td>\r\nO\u2013O<\/td>\r\n146<\/td>\r\n<\/tr>\r\n
            C\u2013Cl<\/td>\r\n328<\/td>\r\nO=O<\/td>\r\n498<\/td>\r\n<\/tr>\r\n
            C\u2013H<\/td>\r\n414<\/td>\r\nO\u2013H<\/td>\r\n463<\/td>\r\n<\/tr>\r\n
            F\u2013F<\/td>\r\n159<\/td>\r\nS\u2013H<\/td>\r\n339<\/td>\r\n<\/tr>\r\n
            H\u2013Cl<\/td>\r\n431<\/td>\r\nS=O<\/td>\r\n523<\/td>\r\n<\/tr>\r\n
            H\u2013F<\/td>\r\n569<\/td>\r\nSi\u2013H<\/td>\r\n293<\/td>\r\n<\/tr>\r\n
            H\u2013H<\/td>\r\n436<\/td>\r\nSi\u2013O<\/td>\r\n368<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            A few trends are obvious from Table 9.3 \"Bond Energies of Covalent Bonds.\"<\/a>\u00a0For bonds that involve the same two elements, a double bond is stronger than a single bond, and a triple bond is stronger than a double bond. The energies of multiple bonds are not exact multiples of the single-bond energy; for carbon-carbon bonds, the energy increases somewhat less than double or triple the C\u2013C bond energy, while for nitrogen-nitrogen bonds the bond energy increases at a rate greater than the multiple of the N\u2013N single bond energy. The bond energies in Table 9.3 \"Bond Energies of Covalent Bonds\"<\/a> are average values; the exact value of the covalent bond energy will vary slightly among molecules with these bonds but should be close to these values.<\/p>\r\n

            To be broken, covalent bonds always require energy; that is, covalent-bond breaking<\/em> is always an endothermic<\/em> process. Thus the \u0394H<\/em> for this process is positive:<\/p>\r\nMolecule\u2013O\u2013H \u2192\u00a0Molecule\u2013O +\u00a0H \u00a0 \u00a0 \u00a0\u0394H<\/em> \u2248 +463 kJ\/mol<\/span><\/span>\r\n

            However, when making a covalent bond, energy is always given off; covalent-bond making<\/em> is always an exothermic<\/em> process. Thus \u0394H<\/em> for this process is negative:<\/p>\r\nMolecule\u2013S +\u00a0H \u2192\u00a0Molecule\u2013S\u2013H \u00a0 \u00a0 \u00a0\u0394H<\/em> \u2248 \u2212339 kJ\/mol<\/span><\/span>\r\n

            Bond energies can be used to estimate the energy change of a chemical reaction. When bonds are broken in the reactants, the energy change for this process is endothermic. When bonds are formed in the products, the energy change for this process is exothermic. We combine the positive energy change with the negative energy change to estimate the overall energy change of the reaction. For example, in<\/p>\r\n2H2<\/sub> +\u00a0O2<\/sub> \u2192\u00a02H2<\/sub>O<\/span><\/span>\r\n

            We can draw Lewis electron dot diagrams for each substance to see what bonds are broken and what bonds are formed:<\/p>\r\n

            \"H-2-O\"<\/a><\/p>\r\n\r\n

            <\/div>\r\n

            (The lone electron pairs on the O atoms are omitted for clarity.) We are breaking two H\u2013H bonds and one O\u2013O double bond and forming four O\u2013H single bonds. The energy required for breaking the bonds is as follows:<\/p>\r\n\r\n

            \r\n\r\n\r\n\r\n\r\n\r\n
            2 H\u2013H bonds:<\/td>\r\n2(+436 kJ\/mol)<\/td>\r\n<\/tr>\r\n
            1 O=O bond:<\/td>\r\n+498 kJ\/mol<\/td>\r\n<\/tr>\r\n
            Total:<\/td>\r\n+1,370 kJ\/mol<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            The energy given off during the formation of\u00a0the four O\u2013H bonds is as follows:<\/p>\r\n\r\n

            \r\n\r\n\r\n\r\n\r\n
            4 O\u2013H bonds:<\/td>\r\n4(\u2212463 kJ\/mol)<\/td>\r\n<\/tr>\r\n
            Total:<\/td>\r\n\u22121,852 kJ\/mol<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            Combining these two numbers:<\/p>\r\n\r\n

            \r\n\r\n\r\n\r\n\r\n
            <\/td>\r\n+1,370 kJ\/mol +\u00a0(\u22121,852 kJ\/mol)<\/td>\r\n<\/tr>\r\n
            Net Change:<\/td>\r\n\u2212482 kJ\/mol \u2248 \u0394H<\/em><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            The actual \u0394H<\/em> is \u2212572 kJ\/mol; we are off by about 16%\u2014although not ideal, a 16% difference is reasonable because we used estimated, not exact, bond energies.<\/p>\r\n\r\n

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            Example 9.8<\/h3>\r\n

            Estimate the energy change of this reaction.<\/p>\r\n

            \"C-2-H-6\"<\/a><\/p>\r\n\r\n

            <\/div>\r\n

            Solution<\/p>\r\n

            Here, we are breaking a C\u2013C double bond and an H\u2013H single bond and making a C\u2013C single bond and two C\u2013H single bonds. Bond breaking is endothermic, while bond making is exothermic. For the bond breaking:<\/p>\r\n\r\n

            \r\n\r\n\r\n\r\n\r\n\r\n
            1 C=C:<\/td>\r\n+611 kJ\/mol<\/td>\r\n<\/tr>\r\n
            1 H\u2013H:<\/td>\r\n+436 kJ\/mol<\/td>\r\n<\/tr>\r\n
            Total:<\/td>\r\n+1,047 kJ\/mol<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            For the bond making:<\/p>\r\n\r\n

            \r\n\r\n\r\n\r\n\r\n\r\n
            1 C\u2013C:<\/td>\r\n\u2212348 kJ\/mol<\/td>\r\n<\/tr>\r\n
            2 C\u2013H:<\/td>\r\n2(\u2212414 kJ\/mol)<\/td>\r\n<\/tr>\r\n
            Total<\/td>\r\n\u22121,176 kJ\/mol<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n

            Overall, the energy change is +1,047 +\u00a0(\u22121,176) = \u2212129 kJ\/mol.<\/p>\r\n

            Test Yourself<\/em><\/p>\r\n

            Estimate the energy change of this reaction.<\/p>\r\n

            \"C-2-H-6-2\"<\/a><\/p>\r\n\r\n

            <\/div>\r\n

            Answer<\/em><\/p>\r\n

            \u2212295 kJ\/mol<\/p>\r\n\r\n<\/div>\r\n

            \r\n
            \r\n

            Key Takeaways<\/h3>\r\n