We can determine if CaO is ionic or covalent by analyzing Ca and O. When one atom bonds to various atoms in a group, the bond strength typically decreases as we move down the group. We measure the strength of a covalent bond by the energy required to break it, that is, the energy necessary to separate the bonded atoms. Thus, the lattice energy of an ionic crystal increases rapidly as the charges of the ions increase and the sizes of the ions decrease. (a) [latex]\begin{array}{lll}\text{2 N-H bonds}\hfill & =\hfill & \hfill 2\left(390\right)\\ \text{1 N-O bond}\hfill & =\hfill & \hfill 200\\ \text{1 O-H bond}\hfill & =\hfill & \hfill \underline{464}\\ \hfill & \hfill & \hfill \text{1444 kJ}\end{array};[/latex], (b) [latex]\begin{array}{lll}\text{3 N-H bonds}\hfill & =\hfill & \hfill 3\left(390\right)\\ \text{1 N-O bond}\hfill & =\hfill & \hfill \underline{200}\\ \hfill & \hfill & \hfill \text{1370 kJ}\end{array};[/latex] The lower it is, the more exothermic the reaction will be. (a) [latex]\begin{array}{ll}\hfill DH\text{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma{D}}}_{\text{bonds formed}}\\ & =2{D}_{\text{Cl-Cl}}+3{D}_{\text{F-F}}-6{D}_{\text{Cl-F}}\\ & =-564\text{kJ}\end{array}\text{;}[/latex], (b) [latex]\begin{array}{ll}\hfill DH\text{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma{D}}}_{\text{bonds formed}}\\ & ={D}_{\text{C-C}}+4{D}_{\text{C-H}}+{D}_{\text{H-H}}-{D}_{\text{C-C}}-6{D}_{\text{C-H}}\\ & =611+4\left(415\right)+436-345-6\left(415\right)\\ & =-128\text{kJ}\end{array}\text{;}[/latex], (c) [latex]\begin{array}{ll}\hfill DH\text{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma{D}}}_{\text{bonds formed}}\\ & =2{D}_{\text{C-C}}+12{D}_{\text{C-H}}+7{D}_{\text{O-O}}-8{D}_{\text{C-O}}-12{D}_{\text{O-H}}\\ & =2\left(345\right)+12\left(415\right)+7\left(496\right)-8\left(741\right)-12\left(464\right)\\ & =-2354\text{kJ}\end{array}[/latex], 4. NaF crystallizes in the same structure as LiF but with a Na–F distance of 231 pm. Predict whether IBr is ionic or covalent, based on the location of their constituent atoms in the periodic table.
For ionic bonds, the lattice energy is the energy required to separate one mole of a compound into its gas phase ions. In these two ionic compounds, the charges Z+ and Z– are the same, so the difference in lattice energy will depend upon Ro. The energy required to break a specific covalent bond in one mole of gaseous molecules is called the bond energy or the bond dissociation energy. Predict whether CoBr 2 is ionic or covalent, based on the location of their constituent atoms in the periodic table. Standard Thermodynamic Properties for Selected Substances gives a value for the standard molar enthalpy of formation of HCl(g), [latex]\Delta{H}_{\text{f}}^{\textdegree },[/latex] of –92.307 kJ/mol. Identify the more polar bond in the following pair of bonds: HF or HCl. (a) [latex]\begin{array}{ll}\hfill \Delta{H}_{298}^{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma{D}}}_{\text{bonds formed}}\\ & ={D}_{\text{H-H}}+{D}_{\text{Br-Br}}-2{D}_{\text{H-Br}}\hfill \\ & =436+190-2\left(370\right)=-114\text{kJ}\end{array}\text{;}[/latex], (b) [latex]\begin{array}{ll}\hfill \Delta{H}_{298}^{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma}D}_{\text{bonds formed}}\\ & =4{D}_{\text{C-H}}+{D}_{\text{I-I}}-3{D}_{\text{C-H}}-{D}_{\text{C-I}}-{D}_{\text{H-I}}\hfill \\ & =4\left(415\right)+150-3\left(415\right)-240-295=30\text{kJ}\end{array}\text{;}[/latex], (c) [latex]\begin{array}{ll}\hfill \Delta{H}_{298}^{\textdegree }& ={\text{\Sigma{D}}}_{\text{bonds broken}}-{\text{\Sigma{D}}}_{\text{bonds formed}}\\ & ={D}_{\text{C}=\text{C}}+4{D}_{\text{C-H}}+3{D}_{\text{O}=\text{O}}-4{D}_{\text{C}=\text{O}}-4{D}_{\text{O-H}}\\ & =611+4\left(415\right)+3\left(498\right)-4\left(741\right)-4\left(464\right)\\ & =-1055\text{kJ}\end{array}\text{;}[/latex], 3. Then, [latex]{U}_{\text{NaF}}=\frac{-2054\text{kJ}\text{A}{\text{mol}}^{-1}\left(-1\right)}{2.31\text{A}}=889\text{kJ}{\text{mol}}^{-1}\text{or}890{\text{kJ mol}}^{-1}[/latex]. The precious gem ruby is aluminum oxide, Al2O3, containing traces of Cr3+. Hess’s law can also be used to show the relationship between the enthalpies of the individual steps and the enthalpy of formation. Find out more about how we use your information in our Privacy Policy and Cookie Policy. The enthalpy of a reaction can be estimated based on the energy input required to break bonds and the energy released when new bonds are formed.