SCH 3U Key to More Suggested Practice (Review for Unit 1)
(Be sure to review all of the material in the boxes above even if a practice problem does not appear somewhere on this sheet
for the concept.)
1.
Models of the Atom – Review questions
Refer to the models of the atom below when answering the questions.
i)
In which order were the models shown in the diagrams below developed? (Answer: B, D, C, A, missing is the quantum
mechanical or Schrodinger model.)
ii) Identify the scientist associated with each model. (Answer: A = Bohr ; B = Dalton; C = Rutherford; D = Thomson)
iii) Briefly describe each model. Explain how it differs from the one that came before it. (See page 11 in text.)
iv) Sketch one other model of the atom, identify the scientist associated with it and explain how it differs from the
other models. (See page 11 of McGraw – Hill Ryerson, Chemistry 11 text.)
A.
B.
C.
D.
2.
(Answers: All of these questions have been answered in your notes. Please ask your teacher if there is something you
need help with.)
Intramolecular and intermolecular forces; effect of intermolecular forces on properties of molecules; comparing
properties of ionic compounds and molecules
1a) Draw the Lewis dot diagrams for ammonia, NH3, methane, CH4 , carbon monoxide, CO and lithium fluoride, LiF. Add
in partial charges and bond dipoles where applicable. What type of bonding occurs WITHIN each of the substances
in this list? Explain how you know. Show any calculations you do.
b) Is ammonia a polar molecule or is it a non – polar molecule? Explain how you know you are correct. Repeat this
question for methane and then again for carbon monoxide.
c) Is NaCl a molecule? What kind of structure is it most likely to have? Explain your answers.
d) What type of attractive force(s) exist(s) BETWEEN neighboring ammonia molecules? Explain how you know. Repeat
this question for neighboring molecules of methane and then again for carbon monoxide neighbors.
e) Rank the 4 substances listed in part 1a) from the one that likely has the highest melting and boiling points to the
one that has the lowest. Explain your reasoning.
f) Rank the 4 substances from the one that likely is the most water soluble to the one that is the least water soluble.
Explain your reasoning.
g) Which (if any) of the substances listed in 1a) would be expected to be electrolytes? Explain your answer.
Answer:
NH3
CH4
CO
LiF
Molecule or ionic
Molecule
Molecule
Molecule
Ionic compound
compound?
(2 non – metals)
(metal + non – metal)
Lewis dot diagram
δ[Li]1+ [F]1-
3.
δ+
++
Type of bonding
within the substance
Polarity of substance
(See handout with
Table 3 pg. 85 on it
from Nelson,
Chemistry 11 text.)
Force between
neighboring
molecules
(intramolecular
force)
Relative boiling point
δ+
δ+
δ-
δ-
Polar covalent bonds
(ΔEN = 3.0 – 2.2)
Polar molecule
Slightly polar covalent
bonds (ΔEN = 2.6 – 2.2)
Non – polar molecule
Polar covalent bond
(ΔEN = 3.4 – 2.6)
Polar molecule
Hydrogen bonding
(H – bonding can only
occur between two
molecules if INSIDE
each molecule there is
at least one of the
following bonds: H – F;
H – O; H – N.)
(London dispersion
forces are there, too,
but are not the
predominant force.)
Second from the
highest as hydrogen
bonding is the
strongest of the
intermolecular forces
and thus requires the
most energy to
overcome when
changing state from
liquid to vapour.
London dispersion forces
only
Dipole – dipole forces
(London dispersion
forces are there, too,
but are not the
predominant force.)
Lowest – London forces
between molecules are
the easiest to overcome
when moving from liquid
 vapour during
“boiling”.
Second from last –
dipole – dipole forces
are stronger and
harder to overcome
than London forces
but not as strong or
as hard to overcome
as H – bonding.
Ionic bond
(ΔEN = 4.0 – 1.0)
Ionic compound – full
charges are present
so this compound is
more polar than any
polar molecule.
This is an ionic
compound held
together by ionic
bonds – the
attraction between
positive and negative
ions. It does not have
intermolecular forces
– it is not a molecule.
Highest – ionic bonds
must be overcome in
order to melt the
ionic compound and
release the ions from
the crystal lattice
structure.
NH3
Polar molecules are
attracted to polar
water molecules and
so are usually water
soluble.
No. The only
molecules that are
electrolytes are acids.
Relative water
solubility
Electrolyte?
4.
CH4
Non – polar molecules
are not attracted to
polar water molecules
and so are not usually
water soluble
No. (See box to the
left.)
CO
Polar molecule so
likely water soluble.
No. (See box to the
left.)
LiF
Ionic compounds are
attracted to polar
water molecules and
so tend to be water
soluble.
Yes. When ionic
substances are
placed into water,
they dissolve so the
crystal lattice
structure falls apart
and frees the ions so
that they can carry
an electric charge.
Any substance that
does this in water is
called an electrolyte.
Note that ionic
compounds can also
conduct electricity if
molten.
Write out Lewis dot structures to show the formation of NH3 and MgCl2 from their elements.
+
3

2
 Mg  +
6
2

Mg
gg
a) Define the term electrolyte. Identify the substances in the following list that are generally described as
electrolytes. List: Non – polar molecules, slightly polar molecules, polar molecules, acids, bases, ionic
solids (Answer: acids, bases, ionic solids)
b) Under what conditions will ionic solids conduct an electric charge? Explain why these conditions are
necessary. (Answer: when either in aqueous solution or when molten – because under these conditions, the ions are
released from the crystal lattice so they are free to move around and carry and electric charge)
7. Identify which of the following will be held together by ionic bonds and which will be held together with covalent
bonds. Be prepared to state your reasoning.
Na2SO4(s), SO42-, H2SO4, HCl, C6H12O6
1+
(Answer: Na2SO4 is an ionic compound. The Na ions and the SO42- ion are held together by ionic bonds. The atoms within
the SO42- ion are non – metals, held together by covalent bonds.
The H2SO4 is an acid. It is a molecule entirely held together by covalent bonds but when it is placed in water, it will
react with water to produce ions in solution. The same is true of HCl. It, too, is a molecule held together by covalent
bonds. Again, though, when placed into water, a reaction with water releases ions into aqueous solution! (Acids are kind
of weird.) C6H12O6 is also a molecule (glucose to be exact) held together by covalent bonds - all non – metals.)