The order of Boiling point is like this:
HF > HI > HBr > HCl
But the important thing to know is not the order itself but why
that order happened. Let's go to the basics; the periodic
table.
The halogens in the periodic table. Halogen literally means
'salt producing' since these compounds form many salts
If you notice, the elements present in the compounds you gave
belong to one family: the halogens. Halogens are a class of
elements that include Fluorine (F), Chlorine (Cl), Bromine (Br),
Iodine (I) and Astatine (At) in that order. When you progress it in
that order, new shells of electrons get added (and so does protons
and neutrons) and hence, they get 'heavier'. Hold this information
for now.
Since these halogens are bonded with hydrogen, they are known as
Hydrogen Halides. (Astatine is not usually
considered since it is very radioactive)
The hydrogen halide atoms arranged as HF > HCl > HBr >
HI from top to bottom
When the atomic number of the element increases, it must be certain
that their attractive force increases also. An analogy: a heavier
planet can attract more strongly than a lighter planet. Anyway,
when the atomic number of the element increases the number of
electrons present in them also increases. These electrons are
constantly moving around in the atom. At some point, there are more
electrons on one side of the atom than the other. This creates an
imbalance of charge, creating a small charge of δ+ and
δ−. These small charges induces its neighbours to create
additional charges of δ+ and δ− thus creating a
long chain of induced dipoles. A dipole is a pair of two opposite
but equal charges separated by a small distance. These effects are
temporary as the electrons can move about still. Since these
dipoles are induced (and hence can be reverted), they are also
known as induced dipole force or better known as London
Dispersion Forces.
London dispersion forces
A point to be noted is that they aren't really forces but rather
attraction between molecules.
Now, it must intuitively be certain that more the number of
electrons then more the amount of London dispersion
forces. This is true; the more the electrons the atom has,
the stronger the amount of London dispersion forces and hence
greater the attraction. The greater the attraction, the more energy
you have to put to break the bonds apart. If you see the trend
above, Iodine has more electrons than Bromine hence has a greater
attraction force between its atoms hence has a greater boiling
point. Similar for the case of Bromine and Chlorine. Hence, you can
place the order of their boiling points as:
I > Br > Cl
Hence,
HI > HBr > HCl
The boiling points of the hydrogen halides
However, you notice an anomaly; HF should have a boiling point
below that of HCl, then how come it has the highest boiling point?
After all, Fluorine has lesser electrons than Chlorine and hence
should have lesser London dispersion forces between them. This is
true but there is another reason behind that: Hydrogen
Bonding.
Picture fluorine; it sits right above the other halogens. It
belongs to Period 2 which does not have many shells to itself.
Since it has so less electrons, it must be very small. In fact, it
is. Fluorine is so small yet it has a really high charge density.
Why does it have a high charge density? Its outer electrons are not
as shielded by the inner electrons and it has 7 outer electrons.
(read more on: Shielding effect) These seven electrons face
tremendous attraction from its nucleus hence it 'shrinks'. This
causes the fluorine atom to shrink hence its charge density
increases. As such, it can attract other electrons to itself when
it is covalently bonded to another element. This property is called
electronegativity.
When the melting points of the Hydrogen halides are measured, it is found that of the...
HBr HI HCl HF List the above hydrogen halides in order from strongest acid to weakest acid.
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please help with this question for the circled substances:) TIA PART B-MELTING POINTS OF SOME ORGANIC SUBSTANCES 7. Cirele the pair of substances of which you measured the melting point range 2-naphthol (A) and naphthalene (B) benzophenone (W) and benzil (B Complete the following table: [6 marka) Hint Molecules containing only Cand H can be assumed to be non polar. Present in в Present in Stronger in which subatance? Why? Intermolecular Force A (yes or no) (yes or no) London...
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