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    In terms of intermolecular forces why does Hydrogen iodide requires more heat energy for melting than hydrogen chloride does ?
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    (Original post by KKP109)
    In terms of intermolecular forces why does Hydrogen iodide requires more heat energy for melting than hydrogen chloride does ?
    Hydrogen iodide requires more energy then hydrogen chloride because it has a higher london dispersion forces ( also known as van der waal forces) then hydrogen chloride.

    The higher the number of electrons, the stronger the london dispersion forces hence more heat energy is required to break the bond
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    Hydrogen iodide is a larger molecule than hydrogen chloride and so has more electrons in its structure. This means that larger instantaneous dipoles can form. These result in stronger instantaneously induced dipole - (induced) dipole forces (london forces) in HI than HCl that require more energy to overcome
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    The lower down group 7, the lower the electronegativity and the greater the masses/sizes. They are therefore more stable and form more fleeting dipole-dipole interactions/Van der Waals forces. It takes more thermal energy to separate their molecules.
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    (Original post by tea&scrabble)
    The lower down group 7, the lower the electronegativity and the greater the masses/sizes. They are therefore more stable and form more fleeting dipole-dipole interactions/Van der Waals forces. It takes more thermal energy to separate their molecules.
    What do you mean by all this?
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    (Original post by tea&scrabble)
    The lower down group 7, the lower the electronegativity and the greater the masses/sizes. They are therefore more stable and form more fleeting dipole-dipole interactions/Van der Waals forces. It takes more thermal energy to separate their molecules.
    Don't talk about electronegativity because H-Cl bonds are more polar than H-I and so the dipole is larger and you might argue that there are dipole-dipole interactions that would imply the opposite. in fact the strong dipole outweighed by the instantaneous interactions due to having more electrons so it can be ignored.
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    Both molecules are polar, so the strongest intermolecular bonds they form are permanent dipole - permanent dipole bonds. Chlorine is more electronegative that Iodine, so the H-Cl bond is more polar than the H-I bond. Therefore, the strength of the intermolecular forces between HCl molecules is greater, and so they require more energy (i.e. a higher temperature) to break.
    EDIT: oops, that appears to imply the opposite answer. I must therefore agree with whoever posted above about the Instantaneous dipole- induced dipole forces being more significant in this case.
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    (Original post by Goods)
    Don't talk about electronegativity because H-Cl bonds are more polar than H-I and so the dipole is larger and you might argue that there are dipole-dipole interactions that would imply the opposite. in fact the strong dipole outweighed by the instantaneous interactions due to having more electrons so it can be ignored.
    If you can't ignore it until you've take it into account, are you really ignoring it? The greater permanent dipole interactions in HCl are important and should be noted in any discussion of the relative strength of internuclear forces.
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    (Original post by dragonkeeper999)
    Both molecules are polar, so the strongest intermolecular bonds they form are permanent dipole - permanent dipole bonds. Chlorine is more electronegative that Iodine, so the H-Cl bond is more polar than the H-I bond. Therefore, the strength of the intermolecular forces between HCl molecules is greater, and so they require more energy (i.e. a higher temperature) to break.
    This is wrong; the melting and boiling points of HI are higher than the melting and boiling points of HCl. As others have already said, the greater permanent dipole interactions associated with HCl are outweighed by the greater London forces of HI.
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    The london dispersion forces are stronger as the hydrogen iodide molecule is more 'polarisable'. This feature of the molecule relates to the availability of low lying excited states in the atom/molecule, which are closer in energy for HI due to the I atom.
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    (Original post by BJack)
    If you can't ignore it until you've take it into account, are you really ignoring it? The greater permanent dipole interactions in HCl are important and should be noted in any discussion of the relative strength of internuclear forces.
    touché
 
 
 
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