the group 1a and group 2a elements.pdf
TRANSCRIPT
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Cotton chapter
10,11
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Group 1A
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Group 1A
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Qualitative alkali
metal
analysis
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Alkali Metals
The group 1A elements with their ns1 valence electron
configurations are very active metals. They lose their valence electrons very readily. They have low ionizationenergies and react with nonmetals to form ionic solids.
2Na(s) + Cl2(g) 2NaCl(s) The expected trend in reducing ability,
Cs>Rb>K>Na>Li
Alkali metals all react vigorously with water to releasehydrogen gas.
2M(s) + 2H2O(l) 2M+(aq) + 2OH‐
(aq) + H2(g)
Observed reducing abilities: Li>K>Na
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First ionization
energy
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Soda production
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Properties and
Trends
in
Group
1A
The Group 1A metals exhibit regular trends for a
number of properties. Irregular trends suggest that factors are working
against each other in determining a property (such as
the density “discrepancy” between sodium and potassium).
The alkali metals have two notable physical properties:
they are all soft and have low melting points. When freshly cut, the alkali metals are bright and
shiny—typical metallic properties. The metals quickly
tarnish, however,
as
they
react
with
oxygen
in
the
air.
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Alkali Metal
Oxides
In the presence of ample oxygen,
4Li + O2 → 2Li2O (regular oxide)
2Na + O2 → Na2O2 (peroxide)K + O2 → KO2 (superoxide)
Rb + O2 → RbO2 (superoxide)
Cs + O2 → CsO2 (superoxide)
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The oxides
of
Group
1A
Direct reaction of the alkali metals with O2 gives :Li ‐> oxide, peroxide (trace)Na ‐> peroxide , oxide (trace)
K,Rb,Cs ‐> superoxide
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Diagonal Relationships:
The Special
Case
of
Lithium
In some of its properties, lithium and its compounds
resemble magnesium and its compounds. Lithium carbonate, fluoride, hydroxide, and phosphate are
much less water soluble than those of other alkali metals.
Lithium is the only alkali metal that forms a nitride (Li3N).
When it burns in air, lithium forms a normal oxide (Li2O)
rather than a peroxide or a superoxide.
Lithium carbonate and lithium hydroxide decompose to form
the oxide on heating, while the carbonates and hydroxides of other Group 1A metals are thermally stable.
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Diagonal
Relationships
The
elements
in
each encircled pair have
several similar properties.
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Occurrence, Preparation, Use, and
Reactions of
the
Alkali
Metals
Sodium and potassium are isolated primarily from
brines
(solutions of NaCl and KCl). Lithium is obtained mostly from the mineralspodumene, LiAl(SiO3)2.
To convert an alkali metal ion into an alkali metal atom, the ion must take on an electron—a process of reduction.
This is not easy with the alkali metals; they are excellent reducing agents.
Potassium was the first alkali metal to be prepared by
electrolysis.
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The Hydroxides
of
Group
1A
The hydroxides are white crystalline solids
NaOH (318 C̊)
KOH (360 C̊)
Hydroxides are freely soluble exothermically in water and in alcohols
Strong alkali bases
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Ionic salts
Li has some special properties as compared to the
other elements of group 1
LiOH is less soluble then the other Hydroxides
Li3N is
stable
(the
only
stable
Nitrite
in
this
group)
Electrical conductive
Precipitation reactions: The larger the M‐ ion the more
salts are available. Na salts are very soluble, but the mixed salts with Zn
and Uranium can be used in quantitative analysis.
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Complexation of
Alakali
Metals
The binding constants for 18‐crown‐6 : Li<Na,Cs<Rb<K
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Biological transporters
for
Na
and
K
Valinomycin and Nonactin: small cyclic polypeptides are the carriers of alkaline metals across membranes.
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Reaction summary
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Reaction summary
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Group 2A
elements
Group 2A elements are very reactive
Valence electron configuration ns2
Lose 2 valence electrons to form noble gas configuration
Form ionic compounds containing M
2+
cations Oxides are basic:
MO(s) + H2O(l)M2+(aq) + 2OH‐
(aq)
Ca, Sr, and Ba react vigorously with water:
M(s) + 2H2O(l)M2+(aq) + 2OH‐
(aq) + H2(g)
Be form covalent bond because of small size and high
electronegativity
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Be structure
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2 e‐ 3
center
bond
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Mg Mg is intermediate in its behaviour between Be and
the other elements in this group.
Its chemistry is entirely ionic in nature.
Mg: high
polarization
ability
The Hydroxides of Be and Mg are only slightly soluble
in water.
The other hydroxides of this group are water soluble
and highly basic.
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Ca, Sr,
Ba,
Ra
They form a closely related group with the change in
chemical and physical parameters follow a systematic
change.
e.g.
Electropositive nature increases from Ca‐>Ra.
Hydration energy of salts increase.
Insolubility of most salts, especially sulfates increases in the same way.
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Be
and
its
compounds Be is obtained by Ca or Mg reduction of the BeCl2 or
BeF2 salts.
Be metal is relatively unreactive. It does not react with
water at red heat, and does not react with air below
600 deg C.
Reaction with air only as fine powder ‐> BeO, Be3N2.
Be salts do not lose their water molecules over strong
desiccants
(P2O5). Aqueous solutions of Be salts are acidic
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Mg production
At 2000 degrees
Ca, Sr, Ba are made on a small scale from the Cl salts by
reduction
with
Na.
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Ion exchange
Ca2+ and Mg2+ ions cause the hardness of water. These ions
of hard water interfere with the action of detergents andform precipitate with soap. Those ions are removed by ionexchange with ion exchange resins.
Ca2+ and Mg2+ are often removed during ion exchange,releasing Na+ into solution.
Ion exchange resin: large molecules that have many ionicsites.
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Group trends
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Group trends