Main Question or Discussion Point
The bond enthaply of halogens decreases down the group so why is the reactivity of halogens with hydrogens decreases down the group?
X-X + H-H -->2H-XBond Energy trends are a function of electronegativity in series and of ionic size in groups. In series (consider Series II => Li, Be, B, C, N, O, F & Ne). Electronegativity increases with increasing atomic number due to increasing number of electrons and protons making up the atomic and ionic structures.
The effect of increasing electronegativity of neutral elements in series functions to decrease atomic radius b/c of increased force of electrostatic attraction. Ionizations follow the same trends in series for the Group I - III elements, but C, N, O & F tend to gain electrons causing a large in crease in ionic radius after Boron because of e-/e-repulsion, but electronegative effects still increase from Carbon through Fluoride. The order of increasing electronegativity is C <N<O<F. Applying this concept to chemical bonds (Covalent bonds) consider the C - H bond 1st. The low electronegativity of Carbon does not distort the chemical bond as much as the same bonds in N, O and F leaving a high electron cloud density in the Carbon to Hydrogen bond and results in a reasonably strong bond as compared to N - H, O - H or F - H which are polar covalent in nature. Distortion of the bonds by the highly electronegative N, O and F ions weakens the covalent bonds in these systems.
In groups or families of elements, size of central element plays a greater factor in strength of bond. This is especially true in the halogen family. Considering the compounds H - F, H - Cl, H - Br and H - I, bond strength decreases with increasing atomic number in the order (H - F) > (H - Cl) > (H - Br) > (H - I) due to the addition of principle energy levels (rings) creating a 'shielding effect', 'Shielding' decreases the electrostatic influence of the nucleus on the bond between the valence of the anion and the attached substrate. The acidity of the Hydrogen - Halide group is also referred to as the 'Hydrogen Halide Paradox'
So is this reasoning correct?I couldn't structure the energy flow diagram directly, so I typed it out in word and posted in jpeg. Hope this helps.
View attachment 99982
Not exactly ... 'Down' the group the strength of H-X bonds decrease and form weaker bonds giving up 'less' energy on formation... The bonding energies of the H-X series are 'size of halide ion' dependent, not 'electronegativity' dependent as in series substances. Such is why the trend is called 'The Hydrogen Halide Paradox'. As atomic number increases additional energy levels are present causing a shielding effect that reduces the influence of the nucleus on the valence level electrons. As a result the bond energy trends, from strongest to weakest follow H-F > H-Cl > H-Br > H-I ... The numerical values (supported by many online references) are ...So is this reasoning correct?
Down the group the strength of H-X bonds increases and to form a strong bond more energy is required. As a result due to difference in energy requirements the reactivity of H-X is higher up the group