the group 1a and group 2a elements.pdf

7/23/2019 the group 1a and group 2a elements.pdf

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Cotton chapter

10,11



Group 1A



Qualitative alkali

metal

analysis



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



First ionization

energy



Soda production



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.



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)



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



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.



Diagonal

Relationships

The

elements

in

each encircled pair have

several similar properties.



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.



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



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.



Complexation of

Alakali

Metals

The binding constants for 18‐crown‐6 : Li<Na,Cs<Rb<K



Biological transporters

for

Na

and

K

Valinomycin and Nonactin: small cyclic polypeptides are the carriers of alkaline metals across membranes.



Reaction summary



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



Be structure



2 e‐ 3

center

bond



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.



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.



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



Mg production

At 2000 degrees

Ca, Sr, Ba are made on a small scale from the Cl salts by

reduction

with

Na.



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.



Group trends

the group 1a and group 2a elements.pdf

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