# 3156 kj rxn h bonds broken h bonds formed h 3134 kj

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 = –3156 kJ = rxn H bonds broken H  + bonds formed H  = 3134 kJ + (–3156 kJ) = –22 kJ 9.50 Plan: To find the heat of reaction, add the energy required to break all the bonds in the reactants to the energy released to form all bonds in the product. Remember to use a negative sign for the energy of the bonds formed since bond formation is exothermic. The bond energy values are found in Table 9.2. Solution: H H H H C C H H H Br C C H H Br H + Reactant bonds broken: 1 x C=C = (1 mol)(614 kJ/mol) = 614 kJ 4 x C–H = (4 mol)(413 kJ/mol) = 1652 kJ 1 x H–Br = (1 mol)(363 kJ/mol) = 363 kJ bonds broken H  = 2629 kJ Product bonds formed: 5 x C–H = (5 mol)(–413 kJ/mol) = –2065 kJ 1 x C–C = (1 mol)(–347 kJ/mol) = –347 kJ 1 x C–Br = (1 mol)(–276 kJ/mol) = –276 kJ bonds formed H  = –2688 kJ rxn H = bonds broken H  + bonds formed H  = 2629 kJ + (–2688 kJ) = –59 kJ 9-9
9.51 Electronegativity increases from left to right across a period (except for the noble gases) and increases from bottom to top within a group. Fluorine (F) and oxygen (O) are the two most electronegative elements. Cesium (Cs) and francium (Fr) are the two least electronegative elements. 9.52 In general, electronegativity and ionization energies are directly related. Electronegativity relates the strength with which an atom attracts bonding electrons and the ionization energy measures the energy needed to remove an electron. Atoms that do not require much energy to have an electron removed do not have much attraction for bonding electrons. 9.53 Ionic bonds occur between two elements of very different electronegativity, generally a metal with low electronegativity and a nonmetal with high electronegativity. Although electron sharing occurs to a very small extent in some ionic bonds, the primary force in ionic bonds is attraction of opposite charges resulting from electron transfer between the atoms. A nonpolar covalent bond occurs between two atoms with identical electronegativity values where the sharing of bonding electrons is equal. A polar covalent bond is between two atoms (generally nonmetals) of different electronegativities so that the bonding electrons are unequally shared. The H–O bond in water is polar covalent . The bond is between two nonmetals so it is covalent and not ionic, but atoms with different electronegativity values are involved. 9.54 Electronegativity is the tendency of a bonded atom to hold the bonding electrons more strongly. Electron affinity is the energy involved when an atom acquires an electron. 9.55 The difference in EN is a reflection of how strongly one atom in a bond attracts bonding electrons. The greater this difference is, the more likely the bond will be ionic; the smaller the EN difference, the more covalent the bond. 9.56 Plan: Electronegativity increases from left to right across a period (except for the noble gases) and increases from bottom to top within a group.
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