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8/18/2019 capechemstry2012u1p2solutionandexplanations-130917211906-phpapp02
1/1
CAPE CHEMISTRY MAY/JUNE 2012 UNIT 1 PAPER 2 ‐ QUESTION 1 SOLUTION AND EXPLANATIONS
(a) A dative covalent or coordinate covalent bond is one in which one of the atoms supplies both the shared electrons to the covalent
bond. A dative covalent bond is written A→B, where the direction of the arrow shows the direction in which the electron pair is
donated. An example of this is the ammonium ion (NH4+) which is formed when ammonia (NH3)bonds with a proton (H
+), with the
bonding electrons coming from NH3. The ammonium ion has three covalent bonds and one dative covalent bond.
NH3 NH4+
The involvement of the electron pair as a dative
covalent bond is often shown as an arrow. Note
that once
NH4+ has
formed,
all
four
bonds
are
equivalent and you cannot tell which was formed
by a dative covalent bond
‐
electrical interactions among dipoles on neighboring molecules. The
strength of a given dipole‐dipole interaction depends on the sizes of the
dipole moments involved. The more polar the substance, the greater the
strength of its dipole‐dipole interactions. Butane, for instance, is a non‐polar
molecule with a relative molecular mass of 58 and a boiling point of ‐0.5 °C,
while propanone has the same molecular mass yet boils 57°C higher
because it is polar. The table which follows, lists all the different
intermolecular
forces.
Ion‐
dipole
forces
are
not
relevant
to
this
question.
Butane (C4H10) Propanone (C3H6O)
Alcohols and hydrogen bonding – Alcohols have
higher boiling points than other organic molecules
of similar molecular mass, between which
hydrogen bonding is not present. The boiling point
of propan‐1‐ol (rmm of 60), is expected to be
higher than both butane and propanone. Indeed, it
is 97 °C.
(c) Factors affecting solubility ‐ There is an old saying in science that says, “like dissolves like.” It means that substances that are similar
should form a solution. It also implies that substances that are not similar should not form a solution. As a general rule of thumb, “like
dissolves
like”
works
pretty
well.
The
term
“like”
refers
to
the
overall
polarity
of
the
solvent
molecule
(whether
polar
or
non‐
polar)
and
the overall polarity of the solute (whether polar, non‐polar, or an ionic species that is accommodated by polar solvents).
Polar species: Molecules with a permanent dipole and/or the ability to engage in hydrogen bonding . Water is a perfect example of a
. , . ,
sugars can engage in strong hydrogen bonding. Water, because of its polarity, is uniquely suited to accommodate positive and
negative ions in solution, making it an excellent solvent for many ionic compounds.
Non‐polar species: Molecules that do not have a permanent dipole and do not have the ability to engage in hydrogen bonding. Many
covalent, organic liquids fall into this category: oils, solvents derived from petroleum, carbon tetrachloride (CCl4), and so on. Polar and
ionic compounds are more likely to dissolve in a polar solvent, like H2O, than in a non‐polar solvent like CCl4 or oil. Polar water
molecules are not soluble in oil or carbon tetrachloride, both of which are non‐polar liquids. However, non‐polar compounds, like oils,
are soluble in non‐polar solvents but are not soluble in polar solvents like water. The likelihood of forming a solution using solvents
and solutes of differin olarit is summarized in the followin table. Kee in mind that olar ionic‐non‐ olar com arisons have some
limitations. Not all ionic compounds are soluble in water, but those that are do not dissolve in non‐polar solvents.
When
propane
dissolves
in water, it
forms
hydrogen
In liquids of high polarity such as water,
there are
strong
water
‐ water
attractions.
These are considerably stronger than either
iodine‐iodine attractions or iodine‐water
attractions. Consequently iodine molecules
cannot penetrate the water structure and
In non‐polar liquids such as benzene and
tetrachloromethane, there
are
weak
intermolecular forces. The benzene‐benzene
attractions are similar in strength to iodine‐
benzene and iodine‐iodine attractions. Thus
it is easy for benzene molecule to penetrate
with water
molecules.
to solvate uncharged iodine molecules.
Iodine is therefore almost insoluble in water.
molecules. Consequently iodine dissolves
easily in benzene.
Propanone and water Iodine and water Iodine and non‐polar solvent
Put Together By Denison At Global In
Cunupia. 739‐2656.