The
Chalcogens
Group 16
The Chalcogens – (Group 16)
•What is a chalcogen?
•Any element in group 16
•Nonmetal (O, S, Se), metalloid (Te) or
radioactive metal (Po)
•Has multiple allotropes (oxygen is O2 or O3;
sulfur has many allotropes – most often S8;
selenium can be Se8 or polymeric)
•Most form compounds with strong unpleasant odours
•Forms one monoatomic anion (-2); Po2+ and Po4+ also exist
•Has six valence electrons (valence electron configuration
[N.G.] ns2 np4) and a large negative ΔHelectronic attraction
Melting
Point
Boiling
Point
State
(at 20 °C)
Density
(at 20 °C)
-192.5 °C
-111 °C
Gas
0.00143 g/cm3
Sulfur
113 °C
445 °C
Solid
2.07 g/cm3
Selenium
221 °C
685 °C
Solid
4.79 g/cm3
Tellurium
450 °C
1390 °C
Solid
6.24 g/cm3
Polonium
254 °C
962 °C
Solid
9.32 g/cm3
Oxygen
• Pertengahan abad 17 → udara mengandung komponen yg terkait dengan
pembakaran dan pernapasan → tidak diisolasi sampai 1774.
• Joseph Priestley menemukan OKSIGEN (kemudian dinamai oleh
LAVOISIER → Pembentuk asam)
• Oksigen ditemukan dalam gabungan dengan unsur lain dalam berbagai
macam senyawa seperti air, silika, alumina, dan oksida besi, Fe3O4 adalah
contoh yang jelas.
• Paling melimpah di kerak bumi dan tubuh manusia → sebagai agen
pengoksidasi untuk metabolisme makanan dan sangat penting untuk
kehidupan
The Chalcogens – Oxygen and Ozone
•When we refer to “oxygen”, we are typically referring to diatomic
oxygen (O2), a colourless, odourless, tasteless gas which, when cooled
to a low enough temperature, condenses to a pale blue liquid that
packs a powerful punch! Solid O2 is also pale blue.
•Diatomic oxygen is a strong oxidizing agent. As we have seen, it
reacts spontaneously with the metals of groups 1, 2 and 13 (as well as
some transition metals). Some of these reactions are quite slow
unless heat or a catalyst is added – or unless liquid O2 is used instead
of gaseous O2! Why might that be?
•Another way to increase the reactivity of gaseous O2 is to add water.
Iron rusts much more quickly in damp air than in dry air!
The Chalcogens – Oxygen and Ozone
•The other common allotrope of oxygen is ozone (O3). Ozone is a
significant component of “smog”; however it is also an essential component
of the upper atmosphere.
•Ozone is formed from O2 in an endothermic reaction:
•This reaction requires a large input of energy, such as passing an electric
current through a sample of oxygen (as done in a lab if a reaction requires
ozone) or electromagnetic radiation from the sun (as in the upper
atmosphere).
•Ozone is unstable, decomposing
to oxygen (O2). In order to
maintain a constant amount of
ozone, it must continually be
regenerated.
Sifat Oksigen
• Memiliki 2 alotrop
• Pada temperature ruang, O2 tidak berwarna dan tidak berbau
• Mengalami kondensasi menjadi cair pada -183 oC dan membeku pada -218 oC
• Sedikit larut dalam air (0,04 g/L atau 0,001 M pada 25oC)
• Konfigurasi electron? → bagaimana supaya octet?
• Ikatan pada O2 sangat kuat (entalpi ikatan = 495 kJ/mol) , membentuk ikatan
yang kuat pula dengan unsur yg lain (banyak senyawa mengandung oksigen
lebih stabil dibanding O2 secara termodinamika)
• Reaksi O2 memiliki energi aktivasi yang tinggi sehingga memerlukan suhu
tinggi
Pembuatan Oksigen
• Oksigen diperoleh dari udara
• Titik didih normal O2 = -183 oC dan N2 = -196 oC → ketika udara dicairkan dan dibiarkan hangat, N2
mendidih, tersisa cairan O2 yang sedikit terkontaminasi N2 dan Ar
• Di laboratorium, O2 dapat diperoleh dengan cara memanaskan H2O2 atau padatan KClO3 dengan MnO2
sebagai katalis.
Reaksi
: KClO3(s) → 2KCl(s) + 3O2(g)
• Proses fotosintesis
Kegunaan Oksigen
• Oksigen adalah satu dari bahan kimia industry yang paling
banyak digunakan → H2SO4 dan N2 Sekitar 30 juta ton per tahun
(US)
• Industri baja → Digunakan untuk menghilangkan ketidakmurnian
pada baja
• Industri kertas → memutihkan bubur kertas (mengoksidasi
senyawa yang berwarna menjadi tidak berwarna)
• Industri kesehatan → memudahkan kesulitan bernafas
• Digunakan bersama asetilen untuk pengelasan oksiasetilen
(Reaksi Eksoterm : Suhu 3000 oC)
2 C2H2(g) + 5 O2(g) → 4 CO2 (g) + 2 H2O(g) ΔHo = - 2510 kJ
Ozone
• Ozon adalah gas beracun yang berwarna biru pucat dengan bau tajam dan
menyebabkan iritasi
• Kebanyakan orang dapat mendeteksi sekitar 0,01 ppm di udara
• Paparan 0,1 hingga 1 ppm dapat menyebabkan sakit kepala, mata terbakar,
dan iritasi pada saluran pernapasan
O3(g) → O2(g) + O(g) ΔHo = 105 kJ
• Ozon merupakan oksidator yang lebih kuat dibanding O2. Ozone membentuk
oksida dengan banyak unsur yang lain dibawah kondisi dimana O2 tidak
dapat bereaksi.
• Ozone mengoksidasi semua logam kecuali emas dan platinum
The Chalcogens – Oxygen and Ozone
•Ozone in the upper atmosphere absorbs a significant amount of UV radiation from
the sun, protecting organisms on Earth from a significant amount of biological
damage. In absorbing this radiation, the ozone is broken into O2 and free oxygen
atoms (which react with more ozone to form O2). This process is exothermic, and
helps regulate the Earth’s temperature:
•Needless to say, when it became widely recognized that ozone in the atmosphere was
disappearing, this was a cause for alarm! This news was first reported by chemists
Sherwood Rowland and Mario Molina in 1974. They won the Nobel Prize in Chemistry
in 1995.
•The “hole in the ozone layer” over the Antarctic was first reported in 1985 by British
scientists Joseph Farman, Brian Gardiner, and Jonathan Shanklin.
The Chalcogens – Oxygen and Ozone
•The “hole in the ozone layer” is not actually
a region with no ozone, but it is an area in
which the ozone has thinned substantially.
70% of the ozone over the Antarctic and
30% of the ozone over the Arctic had
disappeared by the early 1980s!
•In 2000, the “hole” over the Antarctic had
expanded to cover the southern tip of South
America.
The Chalcogens – Oxygen and Ozone
•Where is the ozone going?
•Certain pollutants – particularly chlorofluorocarbons (CFCs) used as
refrigerants – accelerate the decomposition of ozone to O2. The scheme
below is simplified but illustrates the point.
•First, the CFC absorbs UV radiation to give a free chlorine atom (a “free
radical”)
•This chlorine atom reacts with ozone to form O2 and ClO
•The ClO reacts with a free oxygen atom to form more O2 and regenerate the
chlorine atom
•This cycle continues until the chlorine atom finds something to react with
other than ozone. That means that one polluted CFC molecule can be
responsible for the destruction of millions of ozone molecules!
Pembuatan Ozon
• Ozon dapat disiapkan dengan melewatkan arus listrik melalui
O2 kering
listrik
3O2(g)
2 O3(g) ΔHo = 285 kJ
• Ozon tidak dapat disimpan dalam waktu lama karena mudah
mengalami dekomposisi membentuk O2
• Dekomposisi dikatalis oleh logam tertentu yaitu Ag, Pt, dan Pd
serta banyak oksida logam transisi lainnya.
Kegunaan Ozone
• Ozone digunakan untuk membunuh bakteri dan
mengoksidasi senyawa organic
• Cari kegunaan ozon yang lain!!!
OKSIDA, PEROKSIDA, dan SUPEROKSIDA
1. OKSIDA
1.
2.
3.
Pengertian
Oksida asam dan oksida basa, beserta contohnya
Oksida amphoteric, beserta contohnya
1.
2.
3.
Pengertian
Contoh (kapan membentuk peroksida), reaksi?
Kegunaan
1.
2.
3.
Pengertian
Contoh (kapan membentuk superoksida), reaksi?
kegunaan
2. PEROKSIDA
3. SUPEROKSIDA
Superoxides
The Chalcogens – Sulfur
•Sulfur exists as a wide range of allotropes:
S2 is violet
S3 is blue
S4 is red
S5 is red-orange
S6 is yellow-orange
S7 to S15 are all shades of yellow as are S18, S20 and S (a polymer)
•Some of these allotropes can be further divided into different forms based
on their crystal structure (e.g. -S8 and β-S8 have different densities and
melting points).
•Naturally occurring sulfur is -S8, eight sulfur
atoms in a puckered ring often called a crown.
This form of sulfur is insoluble in water.
•If sulfur is heated to 400 oC then rapidly cooled
(e.g. by pouring into cold water), we get
“plastic sulfur” which can be pulled into threads.
The Chalcogens – Sulfur
•Two methods of industrially obtaining S8 are the Frasch process
and the Claus process.
•The Frasch process is an extraction technique.
•In Texas and Louisiana, sulfur deposits 60-100 meters
thick are 400-800 meters underground.
•Superheated water (160oC liquid, 16 bar) melts the sulfur
then hot compressed air (20-25 bar) forces the molten
sulfur up a third pipe
•10-15 kg of water is needed
to extract 1 kg of S8
•The sulfur produced is
98-99.5% pure and can be
poured into moulds or
distributed as a liquid.
The Chalcogens – Sulfur
•The Claus process generates S8 from hydrogen sulfide (H2S).
•Recently, there has been pressure on industry to limit its emissions.
One compound of concern is H2S, a common impurity in oil and gas.
The Chalcogens – Sulfur
•The oxides of sulfur are strongly acidic. When sulfur trioxide is
dissolved in water, sulfuric acid is produced:
•If sulfur trioxide is bubbled through concentrated sulfuric acid, we
get “fuming sulfuric acid”:
•When power plants burn sulfur-containing coal or oils, they release
sulfur dioxide which reacts with another pollutant, nitrogen dioxide,
to produce sulfur trioxide and nitrogen monoxide:
•When the resulting sulfur trioxide dissolves in atmospheric water
vapour, we get acid rain!
The Chalcogens – Sulfur
•Like the halogens, sulfur forms oxoanions – notably sulfate and
sulfite. Sulfur also forms many more complicated oxoanions
with bridging sulfur atoms or sulfur atoms replacing one or
more oxygen atoms.
•Draw the Lewis structures of sulfur trioxide and sulfite.
•How do are these two species similar? How do they differ?
If you dissolved 1 mole of each in a liter of water, which would
give you a solution with a lower pH?