Chuong 8-9-Group IA-IIA

THE S-BLOCK ELEMENTS

Introduction
Modern chemical knowledge is based largely on principles that answer the “why” as well as the “how-to.”
In the remaining chapters, we will emphasize facts and applications, but we will refer to underlying principles repeatedly.
Four of the 14 elements that comprise the s-block elements are somewhat unusual cases, although not all for the same reason.
Hydrogen is the simplest element, with one proton and one electron, and its behavior is rather special.
Helium lies in the s-block but its electron configuration fits with the noble gases, and it will be considered in the next chapter.
Francium and radium are highly radioactive and we will not consider them further.
GROUP IA: THE ALKALI METALS
The Alkali Metals
Discoveries are recent.
Sodium and potassium (1807) by electrolysis.
Cesium (1860) and rubidium (1861) from emission spectra.
Francium (1939) from actinium radioactive decay.
Most salts are water soluble.
Natural brines are good sources.
Natural deposits allow mining of solids.
Flame Colors
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.
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, & Reactions of the Alkali Metals
Sodium and potassium are isolated primarily from brines (solutions of NaCl and KCl).
Lithium is obtained mostly from the mineral spodumene, 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.
Liquid sodium is used as a heat transfer medium in some types of nuclear reactors and in automobile engine valves, and its vapor is used in lamps for outdoor lighting.
Potassium is used in making KO2, used as an oxygen source for miner’s “self-rescuers” and similar devices:
Lithium is used in lightweight batteries of the type found in heart pacemakers, cellular telephones, digital cameras, and portable computers.
Occurrence, Preparation, Use, & Reactions of the Alkali Metals
Group I Compounds
Halides
NaCl 50 million
tons/year in U.S.
Preservative, used
on roads, water
softener regeneration,
feedstock for other chemicals
KCl from natural brines.
Plant fertilizers, feedstock.
Important Compounds of Lithium, Sodium, and Potassium
Lithium carbonate is the usual starting material for making other lithium compounds:
Li2CO3(aq) + Ca(OH)2(aq)  CaCO3(s) + 2 LiOH(aq)
One use of LiOH is to remove CO2 from expired air in submarines and space vehicles:
2 LiOH(s) + CO2(g)  Li2CO3(aq) + H2O
NaCl is the most important industrial sodium compound (50 million tons/yr).
It is used to prepare a number of other chemicals and consumer products, including plastics, paper, bleach, soap, and laundry detergent.
Carbonates
Li2CO3 is unstable relative to the oxide.
Used to treat manic depression (1-2 g/day).

Na2CO3 primarily used to manufacture glass.
Currently mined from rich U.S. resources but can be manufactured by the Solvay process (E. Solvay, Belgium, 1838-1922).
Production and Use
2 NaCl(l) → 2 Na(l) + Cl2(g)
Electrolysis:
KCl(l) + Na(l) → 2 NaCl(l) + K(g)
Sodium as a reducing agent:
TiCl4 + 4 Na → Ti + 4 NaCl
Preparation of Sodium Compounds from NaCl
The methods of preparation suggested by this diagram are not necessarily the preferred industrial methods.
The Solvay Process—One Way to Diagram an Industrial Process
The Alkali Metals and Living Matter
Hydrogen, oxygen, carbon, and nitrogen are the most abundant elements in the human body, in the order listed.
Sodium and potassium ions are in a second tier of seven elements that account for about 0.9% of the atoms.
Sodium ions are found primarily in fluids outside cells and potassium ions are abundant in fluids within cells.
Because most alkali metal compounds are water soluble, many drugs that are weak acids are administered in the form of their sodium or potassium salts.
Lithium carbonate is used in medicine to level out the dangerous manic “highs” that occur in manic-depressive psychoses.
Sodium Sulfate
H2SO4(conc. aq) + NaCl(s) → NaHSO4(s) + HCl(g)
NaHSO4(s) + NaCl(s) → Na2SO4(s) + HCl(g)
In the Kraft Process for making paper:
Na2SO4(s) + 4 C(s) → Na2S(s) + 4 CO(g)
45 kg/ton paper
Oxides and Hydroxides
Reaction with oxygen produces several ionic oxides.



In limited oxygen supplies:
M2O (small amounts of Li2O2 from Li).
In excess oxygen:
Li and Na form the peroxide, M2O2.
K, Rb and Cs form the superoxide MO2.
Soaps and Detergents
A soap acts by dispersing grease and oil films into microscopic droplets.
The droplets detach themselves from the surfaces being cleaned, become suspended in water, and are removed by rinsing.
The alkali metal soaps are water soluble; the alkaline earth metal soaps are not.
A soap can function well in hard water only after a part of it is used up to precipitate all the alkaline earth metal ions present; in other words, the soap softens the water first.
Detergents and Soaps
Cleaning Action of a Soap
A soap has a hydrocarbon “tail” …
… and an ionic “head”
An oil droplet is attracted to the hydrocarbon tails, and the ionic ends permit the droplet to be solubilized in water.
GROUP 2: THE ALKALINE EARTH METALS
Emerald is based on the mineral beryl:
3BeO·Al2O3 ·6SiO2
Principle forms:
carbonates, sulfates and silicates
Oxides and hydroxides only sparingly soluble.
Basic or “alkaline”
Compounds do not decompose on heating.
Therefore named “earths”
Heavier elements compounds are more reactive and are similar to Group I (also in other respects).
Properties and Trends in Group 2A
Group 2A shows the same general trends of increasing atomic and ionic sizes and decreasing ionization energies from top to bottom as does group 1A.
The higher densities of the group 2A metals are mainly a consequence of the large differences in atomic sizes.
The group 2A metals are all good reducing agents.
Mg(OH)2 is virtually insoluble in water, however …
As the cation size increases from top to bottom on the periodic table, interionic attractions decrease in strength and the solubilities of the compounds in water increase.
Ba(OH)2 is sufficiently soluble to be used as a titrant in acid–base titrations.
ĐẶC ĐIỂM CHUNG
Là kim loại hoạt động, tăng dần từ BeRa. Kém so với KL kiềm do Z lớn, r nhỏ hơn
Dễ mất 2e trở thành M2+ trong hợp chất và trong dung dịch
Thế điện cực tương đương KL kiềm
Thể hơi chỉ bao gồm phân tử 1 nguyên tử
Ion không màu; nhiều hợp chất ít tan
Be khác nhiều hơn so với Li và các nguyên tố trong nhóm; Be giống nhiều Al, Mg giống nhiều Zn


. These were named alkaline earths because of their intermediate nature between the alkalis (oxides of the alkali metals) and the rare earths (oxides of rare earth metals).
The alkaline earth metals are silvery colored, soft, low-density metals, which react readily with halogens to form ionic salts, and with water, though not as rapidly as the alkali metals, to form strongly alkaline (basic) hydroxides. For example, where sodium and potassium react with water at room temperature, magnesium reacts only with steam and calcium with hot water.
Mg + 2H2O → Mg(OH)2 + H2
Beryllium is an exception: It does not react with water or steam, and its halides are covalent.
Reactions of Group 2A Metals
Reactivity with water increases from beryllium to barium:
Beryllium does not react with water.
Magnesium reacts with steam but not with cold water.
Calcium reacts slowly with cold water.
Strontium and barium react more rapidly with cold water.
All the alkaline earth metals react with dilute acids to displace hydrogen:
M(s) + 2 H+(aq)  M2+(aq) + H2(g).
The alkaline earth metals react with the halogens to form the corresponding halides, with oxygen to form the oxides, and with nitrogen to form the nitrides.
Beryllium
Unreactive toward air and water.
BeO does not react with water, all others from hydroxides.
Be and BeO dissolve in strongly basic solutions to form the BeO22- ion (therefore are acidic).
BeCl2 and BeF2 melts are poor conductors:
Therefore they are covalent rather than ionic solids.
The Special Case of Beryllium
Beryllium is somewhat different from the rest of group 2A.
BeO does not react with water, while the other group 2A metal oxides do so: MO + H2O  M(OH)2.
Be and BeO dissolve in strongly basic solutions to form the BeO22– ion. The oxide BeO has acidic properties. The other alkaline earth metal oxides are basic.
Molten BeF2 and BeCl2 are poor conductors of electricity; they are molecular substances (see below). The other group IIA compounds are almost entirely ionic.
Beryllium Chloride
Important Compounds of
Magnesium and Calcium
Several magnesium compounds occur naturally, either in mineral form or in brines. These include the carbonate, chloride, hydroxide, and sulfate.
Limestone is a naturally occurring form of calcium carbonate, containing clay and other impurities.
Portland cement is made by heating limestone, clay, and sand. When the cement is mixed with sand, gravel, and water, it solidifies into concrete.
Ordinary soda–lime glass is formed by heating limestone, sand, and sodium carbonate together.
Important Compounds of
Magnesium and Calcium (cont’d)
Limestone is used in the metallurgy of iron and steel to produce an easily liquefied mixture of calcium silicates called slag, which carries away impurities from the molten metal.
Precipitated (purified) calcium carbonate is used extensively as a filler in paint, plastics, printing inks, and rubber.
It is also used as a mild abrasive in toothpastes, food, cosmetics, and antacids.
Added to paper, calcium carbonate makes the paper bright, opaque, smooth, and capable of absorbing ink well.
Important Compounds of
Magnesium and Calcium (cont’d)
Quicklime (CaO) and slaked lime [Ca(OH)2] are the cheapest and most widely used bases, and are usually the first choice for neutralizing unwanted acids.
Slaked lime sees extensive agricultural use.
Quicklime is used to neutralize sulfur oxides formed when coal burns.
Gypsum has the formula CaSO4·2 H2O. Another hydrate of calcium sulfate is plaster of paris which has the formula CaSO4 ·½ H2O and is obtained by heating gypsum.
Gypsum is used to make the familiar “drywall” or “plaster board” wall material.

Decomposition of CaCO3 (lime)
Stalactites and Stalagmites
CO2 + H2O → H3O+ + HCO3-
Ka = 4.410-7
HCO3- + H2O → H3O+ + CO32-
Ka = 4.710-11
CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
Other Compounds
Gypsum, CaSO4·2H2O:
Plaster of paris CaSO4·½H2O by heating bypsum.
Used in drywall.
BaSO4 used in X-ray imaging .
Slaked lime used in mortar:
CaO absorbs water from the cement to form Ca(OH)2 which subsequently reacts with CO2 to form CaCO3.
Occurrence, Preparation, Uses, and Reactions of Group 2A Metals
Calcium and magnesium rank just ahead of sodium and potassium in abundance in the Earth’s crust.
Limestone is mainly CaCO3; dolomite is MgCO3· CaCO3.
Barium and strontium are found in the Earth’s crust at about 400 ppm, and beryllium is found at 2 ppm.
An important mineral source of beryllium is the mineral beryl, Be3Al2Si6O18.
Some familiar gemstones, including aquamarine and emerald, are beryl, distinctively colored by impurities.
To obtain beryllium metal, beryl is first converted to BeF2. Then the BeF2 is reduced to beryllium, using magnesium as the reducing agent.
Calcium is generally obtained by electrolysis of molten calcium chloride.
Strontium and barium can also be obtained by electrolysis, but are usually obtained by the high-temperature reduction of their oxides, using aluminum as the reducing agent.
Until recently, magnesium was obtained by the electrolysis of molten MgCl2, in the Dow process. Less expensive methods of obtaining magnesium are now available.
Dow Process for Production of Mg
Electrolysis of Molten MgCl2
Alloys of beryllium with other metals have many applications such as springs, clips, and lightweight structural materials.
Beryllium is nonsparking, and tools that must be used in flammable atmospheres are sometimes made of beryllium.
Magnesium has a lower density than any other structural metal and is an important metallurgical reducing agent. Magnesium is also used in batteries and fireworks.
Calcium is used to reduce the oxides or fluorides of less common metals to the free metals. Calcium is also alloyed with lead in lead–acid batteries, and is used to form other alloys with aluminum and silicon.
The Group 2A Metals
and Living Matter
Persons of average size have approximately 25 g of magnesium in their bodies.
The recommended daily intake of magnesium for adults is 350 mg.
Calcium is essential to all living matter. The human body typically contains from 1 to 1.5 kg of calcium.
Strontium is not essential to living matter, but it is of interest because of its chemical similarity to calcium.
Barium also has no known function in organisms; in fact, the Ba2+ ion is toxic.
Chemistry of Groundwater
Rainwater containing dissolved CO2 is acidic due to formation of H2CO3.
Acidic rainwater converts CaCO3 to Ca(HCO3)2:
As the water evaporates from the Ca(HCO3)2, the somewhat-soluble salt forms CaCO3 again. Deposited CaCO3 leads to stalactites, stalagmites, and other cave formations.
Hard Water and Water Softening
Hard water is groundwater that contains significant concentrations of ions (Ca2+, Mg2+, Fe2+) from natural sources.
Hard water tends to precipitate soaps, reducing their effectiveness.
If the primary anion is the hydrogen carbonate ion (HCO3-), the hardness is said to be temporary hardness.
If the primary anions are other than bicarbonate ion (Cl-, SO42-, HSO4-)then the hardness is called permanent hardness.
Soft: 0 - 20 mg/L as calcium
Moderately soft: 20 - 40 mg/L as calcium
Slightly hard: 40 - 60 mg/L as calcium
Moderately hard: 60 - 80 mg/L as calcium
Hard: 80 - 120 mg/L as calcium
Very Hard: >120 mg/L as calcium
Temporary Hard Water
Contains HCO3- ion.
When heated gives CO32-, CO2 and H2O.
The CO32- reacts with multivalent ions to form precipitates.
(for example CaCO3, MgCO3)
Soften water by precipitating the multivalent ions using slaked lime Ca(OH)2
Permanent Hard Water
Contains significant concentrations of anions other than carbonate.
For example SO42-, HSO4-.
Usually soften by precipitating the Ca2+ and Mg2+ using sodium carbonate leaving sodium salts in solution.

Bathtub ring is caused by
salts of Mg2+ and Ca2+ of
Palmitic acid
(a common soluble soap).
Water Softening
Ion exchange.
Undesirable cations, Mg2+ Ca2+ and Fe3+ are changed for ions that are not as undesirable, ex. Na+.
Resins or zeolites.
Water Softening
by Ion Exchange
Hard water with ions
An ion-exchange resin with acidic groups bound to Na+
As hard water passes through, hard-water cations are exchanged for Na+

Deionizing
Instead of replacing cations with Na+, they are replaced with H+.
Then the anions are replaced with OH-.

H+(aq) + OH-(aq) → H2O(l)
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.
A later review by František Kožíšek, M.D., Ph.D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and conversely to the WHO, sets some recommendations for the maximum and minimum levels of calcium (40-80 mg/L) and magnesium (20-30 mg/L) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Naturally very soft water is more likely to corrode (i.e. react chemically with) metal pipes in which it is carried, and as a result it may have elevated levels of cadmium, copper, lead and zinc.




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