Download Solutions Tutorial 8

School of Chemistry and
Physics
General Principles of
Chemistry - CHEM 110
Westville Campus, Durban
General Principles of Chemistry – CHEM110
Tutorial 8 – 9th and 11th April 2014
Inorganic Chemistry
Atomic Radii
1.
Arrange the following atoms in order of increasing atomic radius: Ca, Mg, Sr, Ba.
Answer:
Mg < Ca < Sr < Ba
Atomic radius increases down a group.
Ionic Radii
2.
Arrange the ions N3-, O2-, Mg2+, Na+, and F- in order of increasing ionic radius?
The more + the charge, the smaller the ionic radius. Also, when an atom loses electrons, it clings
ever more tightly to the ones it has left, further reducing the ionic radius.
Note that – charge means adding electrons. These electrons go in the outermost shells.
Mg2+< Na+< F- < O2- < N3-
3.
3+
Which of the following has the largest radius: Al , Al, B?
Answer: Al
Al is below B in the periodic table, Al is larger than B. Cations are smaller than their parent atoms,
3+
and so the Al ion is smaller than the Al atom.
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School of Chemistry and
Physics
General Principles of
Chemistry - CHEM 110
Westville Campus, Durban
Ionic Radii in an Isoelectronic Series
4.
Arrange the following isoelectronic series in order of decreasing size.
Rb+, Sr2+, Br-, As3-, Se2-
It's based on the atomic number, the larger the atomic number the smaller the radius
First, we note that this is an isoelectronic series of ions, with all ions having 36 electrons. In such a
series, size decreases as the nuclear charge (atomic number) of the ion increases. The atomic
numbers of the ions are As (33), Se (34), Br (35), Rb (37), and Sr (38).
Reasoning : Consider the trends within ions
For isoelectronic cations: the greater the positive charge, the smaller the ion
For isoelectronic anions: the greater the negative charge, the larger the ion
Thus, the ions decrease in size in the order As3- > Se2- > Br-
> Rb+ > Sr2+
Largest: --------------------------- Smallest:
Periodic Trends in Ionisation Energy
5.
Referring to a periodic table, arrange the following atoms in order of increasing first
ionisation energy: N, Be, Mg, Ar, Ne
Ionisation energy increases as we move left to right across a row.
It decreases as we move from the top of a group to the bottom.
Because Be, N and Ne are in the same row of the periodic table, we expect IE to vary in the
order Be < N < Ne.
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General Principles of
Chemistry - CHEM 110
School of Chemistry and
Physics
Westville Campus, Durban
Because Ne is above Ar in group 8A, we expect Ne to have the greater first ionization
energy: Ar < Ne.
Similarly, Mg is below Be in group 2A, and so we expect IE for Mg to be less than that of
Be: Mg < Be.
From these observations, we conclude that the ionization energies follow the order
Mg < Be < N < Ar < Ne
Oxides
6.
Classify the oxides: BaO, CO2, SnO, BeO, Li2O, GeO, and SO2 into:
(i)
Acidic
(ii)
Basic and
(iii)
Amphoteric
oxides:
i)
Acidic - CO2 and SO2
ii)
Basic - : BaO and Li2O
iii)
Amphoteric – BeO, SnO, GeO
A resultant borderline between basic and acidic oxides occurs along a diagonal.
Properties of s- and p-Block Elements
Li
Be
B
C
N
O
F
Na
Mg
Al
Si
P
S
Cl
K
Ca
Ga
Ge
As
Se
Br
Rb
Sr
In
Sn
Sb
Te
I
Cs
Ba
Tl
Pb
Bi
Po
At
Basic Oxides
Amphoteric Oxides
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Acidic Oxides
General Principles of
Chemistry - CHEM 110
School of Chemistry and
Physics
Westville Campus, Durban
7.
Complete and balance the following equations. Also identify the product as an oxide,
superoxide, peroxide or hydroxide. Include the phases of the products.
(i)
K(s) + O2(g) (Excess)
(ii)
Sr(s) + 2H2O (g)
(iii)
Heat
Heat
Sr(OH)2(aq) + H2(g) (hydroxide)
Heat
2Na + O2(g)(Excess)
KO2(s) (superoxide)
Na2O2(s) (Peroxide)
Additional Questions
8.
Write chemical equations to illustrate the amphoteric nature of
(i)
H2O
(ii)
PbO
(i) H2O is amphoteric as it reacts in both basic and acidic media.
Reactions with acid:
H2O + HCl → H3O+ + Cl-
Reactions with base:
H2O + NH3 → OH- + NH4+(ii) PbO reacts with both acids and with bases:
In acid:
PbO + 2HCl → PbCl2 + H2O
In base:
PbO + 2NaOH + H2O → Na2[Pb(OH)4]
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School of Chemistry and
Physics
General Principles of
Chemistry - CHEM 110
Westville Campus, Durban
Electronegativity
9.
Explain the trends in Electronegativity in the Periodic Table
In general, electronegativities of the elements in the same Period increases as you go from
left to right across the period.
In general, the electronegativities of the elements in a Group decrease as you go down the
group from top to bottom.
10.
Put the following in order of INCREASING electronegativity:
Cs, F, Na, K
These elements are from different Groups and Periods. However, their electronegativity order can
be deduced as follows: F is the most electronegative atom of all elements. Therefore it ranks
highest. The remaining elements are all in the same Group (Group 1) and so their order will
decrease down the Group i.e. Na > K > Cs.
Combining this with the knowledge that F is the most electronegative of all elements, the order is
Cs < K < Na < F.
Acid Strength
11.
Determine which is the stronger acid in each of the following pairs? Give your reasoning.
(i) H3PO4 and H3AsO4
(ii) H2SO4 or H2SeO4
i) H3PO4 and H3AsO4
Since electronegativity decreases as one goes down a group in the Periodic Table, the strengths of
the oxoacids will decrease as you go down a group.
H3PO4 > H3AsO4
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School of Chemistry and
Physics
General Principles of
Chemistry - CHEM 110
Westville Campus, Durban
ii)
In case of oxoacids, for different central element, but same number of oxygen atoms, the
more electronegative central atom – stronger acid.
H2SO4 > H2SeO4
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