AI Transcription, Pages 201-225
Page 202
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O X Y G E N
48. OCCURENCE. Oxygen is the most abundant element in nature. It
occurs both in the free state and combined with other elements.
1. In the free state; it is found in air in the ratio of 21 %
by volume. It is also found dissolved in water in a proportion
little more than that present in air.
2. In the combined state; it constitutes about one half of the
earth crust, eight ninths the weight of water, and about 70 %
of the living bodies.
49. METHODS OF PREPARATION
1. Laboratory methods:-
(a) From potassium chlorate. Oxygen is readily prepared in the
lab., by gently heating a mixture of potassium chlorate
(K Cl O₃) and manganese dioxide (Mn O₂). The evolved gas
is collected by an upward displacement of water as shown
in (Fig.3). The equation for the reaction is:
2 K Cl O₃ ——Mn O₂——> 2 K Cl + 3O₂↑
⟦Δ⟧
potassium potassium oxygen
chlorate chloride
K Cl O₃ + Mn O₂
Burner
delivery
Tube
oxygen
water
trough
⟦Δ⟧
2 K Cl O₃ ——————> 2 K Cl + 3O₂↑
Fig. 3
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47. AIR IS NECESSARY FOR BURNING AND RUSTING.
When some subtances are heated in the air their weights
increase. For example, if we leave a piece of iron for a few
days in a damp place it will rust and increase in weight. In
such cases a certain part of air disappears during rusting or
burning and rusting is accompanied by an increase in weight.
Lavoisier, a French scientist heated some mercury for a
few days in a retort the neck of which extends into an air
jacket converted in a trough containing mercury. He found
that mercury rose to about one-fifth of the jacket and that
the mercury in the retort became covered by a red powder
which differed from mercury in every detail. He then took a
burning piece of wood and introduced it into the jacket and
found that it was extinguished. Therefore he said that only
one fifth of the air is necessary for burning. This one
fifth of air was later called Oxygen. It is the part which
has disappeared from the air jacket and combined with the
mercury in the retort to form the red powder.
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Page 14.
A I R
WHAT IS AIR ?
Is it a mixture or a compound ?
What are the gases which constitute it ?
What is the importance of each constituent for living bodies ?
-----------------
44. THE CONSTITUENTS OF AIR.
Air is a mixture of different gases and not a compound. It is
a mixture of the following gases:-
| 1. | Oxygen | 21 % |
| 2. | Nitrogen | 78 % |
| 3. | Carbon Dioxide | 0.04% approximately |
| 4. | Water vapour | variable proportions |
| 5. | Rare gases (Argon, Neon., Krepton & Helium) | 1 % approximately |
| 6. | Dust. | |
45. AIR IS A MIXTURE.
Experiments proved that air is a mixture and not a compound
because:-
1. Every gas which is a part of air retains its physical and chemical
properties.
2. The proportions of the constituents of air are variable.
3. Its constituents can be separated easily as when liquid air is
evaporated, nitrogen is liberated first leaving Oxygen.
46. ALL THE CONSTITUENTS OF AIR ARE NECESSARY FOR LIFE.
1. Oxygen: It is necessary for respiration in both plants and animals.
2. Nitrogen: It dilutes oxygen and if only oxygen was found in the air,
the lungs would have been burnt. Nitrogen is also taken
by certain bacteria which live in the roots of some plants
and is changed into proteins, (e.g. beans roots).
3. Carbon dioxide: It is taken through the leaves of plants and changed
into starch or sugar in the presence of sunlight and
chlorophyll.
4. Water vapour: It prevents much evaporation from the bodies of
plants and animals.
5. Rare gases: They are inactive gases and are of no importance
for life.
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47. AIR IS NECESSARY FOR BURNING AND RUSTING.
When some subtances are heated in the air their weights
increase. For example, if we leave a piece of iron for a few
days in a damp place it will rust and increase in weight. In
such cases a certain part of air disappears during rusting or
burning and rusting is accompanied by an increase in weight.
Lavoisier, a French scientist heated some mercury for a
few days in a retort the neck of which extends into an air
jacket converted in a trough containing mercury. He found
that mercury rose to about one-fifth of the jacket and that
the mercury in the retort became covered by a red powder
which differed from mercury in every detail. He then took a
burning piece of wood and introduced it into the jacket and
found that it was extinguished. Therefore he said that only
one fifth of the air is necessary for burning. This one
fifth of air was later called Oxygen. It is the part which
has disappeared from the air jacket and combined with the
mercury in the retort to form the red powder.
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4- Amphoteric oxides:- Are those oxides which possess the proper-
ties of both acidic and basic Oxides. They combine with both
acids and bases to form salts.
Al2 O3 + 6H Cl ───────────→ 2 Al Cl3 + 3H2 O
Aluminium oxide hydrochloric Aluminium chloride
amphoteric oxide acid salt water
Al2 O3 + 2 Na OH ───────────→ 2 Na Al O2 + H2 O
Aluminium oxide Sod.hydroxide Sod. Aluminate
base salt water
Examples:- Zinc oxide (Zn O), Aluminium oxide (Al2O3), Lead
Monoxide (Pb O), Stannous oxide (Sn O).
5- Peroxides:- These are usually the oxides of metals containing
a greater percentage of oxygen than their basic oxides. They give
off oxygen on heating.
θ
2 Ba O2 ───────────→ 2 Ba O + O2 ↑
Barium peroxide Barium oxide oxygen
Examples:- Barium peroxide (Ba O2), Sodium peroxide (Na2 O2),
Zinc peroxide (Zn O2) & hydrogen peroxide (H2 O2).
A C I D S
59. DEFINITION: - An acid is a hydrogen compound whose hydrogen may
be replaced by a metal and whose water solution changes the
colour of litmus from blue to red.
Many compounds such as sugar and alcohol, contain hydrogen, but
are not acids, because their hydrogen atoms cannot be replaced
by a metal. The hydrogen of the acid is always accompanied by
a non-metallic element such as Cl,S or by a radical such as (N O3),
(SO4) and (P O4).
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60. PREPARATION OF ACIDS - There are two common methods for the
preparation of acids:
1- By the action of acidic oxides (acid anhydride) with water e.g.
S O3 + H2 O ------------> H2 S O4
C O2 + H2 O ------------> H2 C O3
2- By the action of sulphuric acid with a salt of the required
acid e.g.
H2 SO4 + 2 Na Cl ------------> 2 H Cl + Na2 SO4
61. PROPERTIES OF ACIDS:-
1- Acids taste sour.
2- They change ⟦litmus⟧ colour from blue to red.
3- All acids contain hydrogen which ⟦is⟧ replaceable by active
metals e.g.
Zn + 2H Cl ------------> Zn Cl2 + H2
4- They react with marble (Ca CO3) causing effervescence and
evolution of CO2.
2H Cl + Ca CO3 ------------> Ca Cl2 + C O2 + H2 O
5- Acids react with bases forming salt and water.
H Cl + Na O H ------------> Na Cl + H2 O
62. NAMING OF ACIDS:-
a- In case the acids do not contain oxygen i.e. when they are only
composed of ⟦hydrogen and a non-metal⟧ they are named by using the
full name of the non-metal with the prefix hydro - and ending
with - ic. Examples hydrochloric acid ( H Cl), hydrosulphuric acid
H2 S.
b- In case they contain enough oxygen, they are named by using the name
of non-metal ending with - ic. Examples, sulphuric acid ( H2 SO4 ),
carbonic acid ( H2CO3 ) and nitric acid ( H N O3 ).
c- When the acids contain one less atom of oxygen, they are named
by using the name of the non-metal ending with - ous. Examples,
sulphurous acid ( H2 SO3 ) and nitrous acid ( H N O2 )
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B A S E S
63. DEFINITION:- A base is a compound of a metallic element or a radical
with one or more hydroxyl ( O H) groups. The water solution of a base
change the colour of ⟦litmus⟧ from red to blue.
64. PREPARATION OF BASES:-
1- By the action of basic oxides with water:-
Ca O + H2 O ----------> Ca ( O H)2
Mg O + H2 O ----------> Mg ( O H)2
2- By the action between a base and a salt.
Na2 CO3 + Ca ( O H)2 ----------> 2 Na O H + Ca CO3 ↓
Fe Cl3 + Na OH ----------> 3 Na Cl + Fe ( O H)3 ↓
3- By the action of an active metal and water:
2 Na + H2 O ----------> 2 Na OH + H2 ↑
65. PROPERTIES OF BASES.-
1- In general bases have a bitter taste.
2- Bases turn red litmus into blue.
3- Bases contain hydroxyl ( O H) group.
4- Bases do not react with marble.
5- Bases ⟦neutralize⟧ acids forming salt and water.
66. NAMING OF BASES: Bases are named ⟦by placing⟧ the name of the metal
before the word " hydroxide ".
Examples: Sodium hydroxide ( Na OH), Calcium hydroxide (Ca ( O H)2 ).
Strong soluble bases such as ( Na OH) and ( K O H) are often called
alkalies.
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( H Y D R O G E N )
72. OCCURENCE Hydrogen is not as abundant as oxygen. It occurs both
free and in the combined state in nature. In the combined state it
occurs nearly everywhere, but in the free state, it occurs in natural
gas and in volcanic gases. Traces of hydrogen are found in the lower
strata of the earth's atmosphere and gradually its quantity increases
at high altitudes. Hydrogen is known to occur abundantly in the
sun's atmosphere.
In the combined state hydrogen constitutes about 1% of the earth crust
and one ninth of the weight of water. It occurs in all acids, in
animal and vegetable matter, such as butter, starch, sugar, in
hydrocarbons such as kerosene, gasoline, etc..
73. METHODS OF PREPARATION .-
1- LABORATORY METHODS.
a) by the action of certain metals on non oxidizing acids.
Metals such as zinc and iron react with certain dilute acids,
notably hydrochloric acid and sulphuric acid.
Zinc is put in a flask closed by a rubber stopper, through
which a thistle tube extends to the bottom of the flask, and a
delivery tube also extends to a trough containing water, over which
a large test tube filled with water is inverted. The dilute acid is
poured through the thistle tube until it covers its lower end.
Zinc replaces the hydrogen of the acid.
thistle tube
delivery tube
hydrogen gas
test tube
water
trough
HCl
Zn
Zn + 2H Cl ---------------------> Zn Cl₂ + H₂
Fig. 5
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and liberates this hydrogen as a gas, which passes through the
delivery tube and is collected by an upward displacement of water,
at the same time forming a new salt compound called zinc chloride.
Zn + 2 H Cl ⟦----------⟧> Zn Cl2 + H2
Fe + H2 SO4 ⟦----------⟧> Fe SO4 + H2
dil.
Ag + H Cl ⟦----------⟧> no reaction
b- By the action of certain metals on water:-
Various active metals (above hydrogen) react with water under
certain conditions liberating hydrogen and forming either a hydro-
xide or an oxide of the metal as follows:-
1. With cold water: as potassium, sodium & calcium
2 Na + H2 O ----------> 2 Na OH + H2 vigorously
sodium water sodium hydro-
xide
Ca + 2 H2 O ----------> Ca (OH)2 + H2 slowly
calcium + water calcium hydro-
xide
2. With hot water: as Magnesium, using Na OH as a catalyst.
Mg + 2 H2 O ----------> Mg (OH)2 + H2 slowly
Na OH
magnesium water Magnesium hydro-
xide
3. With steam of water: as red hot iron
3 Fe + 4 H2 O ----------> Fe3 O4 + 4 H2 Rapid
iron water magnetic oxide hydrogen
Metals below hydrogen do not react with water
Cu + H2 O ----------> no reaction
Note: The above reactions demonstrate the order of activity
of metals.
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( REPLACEMENT SERIES )
| 1 | Potassium | K ) | Highest |
| 2 | Sodium | Na ) | activity |
| 3 | Calcium | Ca ) | decreasing |
| 4 | Magnesium | Mg ) | to |
| 5 | Aluminum | Al ) | |
| 6 | Zinc | Zn ) | |
| 7 | Iron | Fe ) | |
| 8 | Nickle | Ni ) | |
| 9 | Tin | Sn ) | |
| 10 | Lead | Pb ) | |
| 11 | HYDROGEN | H ) | lowest |
| 12 | Copper | Cu ) | activity |
| 13 | Mercury | Mg ) | |
| 14 | Silver | Ag ) | |
| 15 | Platinum | Pt ) | |
| 16 | Gold | Au ) | |
Note: metals above hydrogen in the replacement series replace it from
dilute acids, while metals below hydrogen do not replace it
from acids.
II. COMMERCIAL METHODS:-
a- By electrolysis of water.- When an electric current is passed
through acidified water, hydrogen is liberated at the cathode.
b- By the action of Hot Coke on steam:-
When steam is passed over white - hot coke (carbon), a mixture of
hydrogen and carbon monoxide gas is formed which is called
" water gas ".
H₂ O + C ----------> CO + H₂
When the mixture of both gases is cooled to a very low temperature,
the CO gas changes to a solid, leaving pure hydrogen. In other
cases, when the mixture of both hydrogen and carbon monoxide
gases is mixed with more steam and passed over a catalyst,
(divided Fe), CO will be changed into CO₂
CO + H₂ O + H₂ ----------> C O₂ + 2 H₂
Then C O₂ + H₂ are treated with water at a pressure of 3o atmos-
pheres when C O₂ dissolves leaving H₂
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To prove that hydrogen is a reducing agent, we pass the dry
gas (H2) over heated copper oxide as in the figure below.
drying tube
containing CaCl2
H2
CuO
drops of water
HCl
Zn
Preparation of Hydrogen
Fig. 6
The hydrogen combines with the oxygen present in the oxide,
forming water, the residue left behind in the tube is metallic
copper. Whenever reduction takes place, it is always accompa-
nied by oxidation, as can be observed by examining the equation
for this reaction:-
Cu O + H2 --------> Cu + H2 O
copper oxide hydrogen copper water
substance reduced reducing agent,
also oxidizing also substance
agent oxidized
76. TEST FOR HYDROGEN: Hydrogen may be identified by one chemical
test:- When hydrogen burns, the only product formed is water.
77. USES OF HYDROGEN:- Hydrogen is used:
1- To obtain a very high temperature, for welding metals.
2- For filling balloons and airships, but owing to its great
combustibility it is often substituted by helium (He).
3- For hydrogination:- Many liquid oils and fats, such as cotton
seed oil & coconut oil have disagreeable odours & tastes & can
not in their simple state be used for cooking. They are con-
verted into palatable solid fats fit for cooking, by passing
hydrogen in the presence of powdered nickel as a catalyst.
The product is called VEGETALINE.
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4- For the preparation of ammonia by Habber process.
5- It is used as a reducing agent.
6- It is a valuable constituent of some fuel gases such as
water gas and producer gas.
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W A T E R
78. OCCURRENCE. ⟦Water⟧ is ⟦present⟧ in nature, and it occupies about 3/4 of
the earth's surface. It is also present in many salts and animal and
vegetable bodies. For example, lean meat is about 60% water, while
tomatoes are about 95% water. The human body contains about 70%
water.
79. FORMATION. Being abundant in nature, water is not usually prepared
from other materials. However, it is a product of many chemical re-
⟦actions⟧ some of which are:
1. Direct union of oxygen and hydrogen by an electric spark.
2 H2 + O ----------> 2 H2O.
2. Oxidation of a compound of hydrogen.
CH4 + 2O2 ----------> CO2 + 2 H2O
3. Reduction of an oxide by hydrogen.
CuO + H2 ----------> Cu + H2O
4. Neutralization of acids and bases.
NaOH + HCl ----------> Na Cl + H2O
80. PROPERTIES.
a- Physical Properties:
1. Water is liquid at ordinary temperature. It is tasteless and
odourless. It has no colour when in thin layers, but it acquires
a bluish tinge when it is observed through great thickness.
2. Water freezes at 0°C (32°F), and boils at 100°C (212°F).
3. Its greatest density, namely 1, (i.e. 1cc weighs 1 gm) is reached
<del>at 4 degree centigrade.</del>
4. It dissolves almost all substances to some extent, and may there-
fore be considered as a universal solvent. The pleasant taste of
drinking water is due to dissolved air and minerals.
5. Water when pure, does not conduct electricity.
b- Chemical Properties:
1. Water is extremely stable.
2. It reacts with various active metals such as Na, Ca, Fe, with
formation of hydr⟦o⟧gen.
3. It is decomposed by an electric current into hydrogen and oxygen.
4. It combines with certain metallic oxides forming bases:
Ca O + H2O ----------> Ca (OH)2
Calcium oxide + water Calcium hydroxide
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5. It combines with certain non-metallic oxides forming acids.
C O₂ + H₂O ---------> H₂C O₃
Carbon dioxide + water Carbonic acid
6. Water reacts with certain salts, forming both an acid and a
base. This process is called "hydrolysis".
Al₂(SO₄)₃ + 6 H₂O ---------> 2 Al(OH)₃ + 3 H₂SO₄
⟦Aluminium sulphate⟧ + water Aluminium + Sulphuric
hydroxide acid
7. Water combines with some compounds when they crystallize from
solution, forming hydrates, and this water is called water of
crystallization. For example:
Cu SO₄ + 5 H₂O ---------> Cu SO₄.5H₂O
⟦unhydrous⟧ copper sulphate Hydrated crystallized
plus water copper sulphate
8. Water acts as a catalyst. Many chemical reactions cannot
take place unless there is some moisture present, as for
example, rusting of iron.
81. WATER IS A COMPOUND. Water is a compound of oxygen and hydrogen.
in the ratio of 8:1 by weight and 1:2 by volume. One of the first
steps in the study of a chemical compound is to learn its composi-
tion, that is what elements it contains, and the exact amount of
each. There are two methods of obtaining this information.
1. Analysis. On passing an electric current in an acid water in
a voltametre (Hoffman apparatus), we get hydrogen at the Cathode
and oxygen at the anode. See pages 18 and 19.
2. Synthesis. On passing a spark through a mixture of definite
quantities of oxygen and hydrogen in an eudiometer, an explosion
occurs and drops of water form on the inner surface of the tube.
Spark wires connected to
gap induction coil
Mercury
Eudiometer
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b- Permanent Hardness: Is that hardness due to the presence of
soluble salts of Mg and Ca in water. These salts do not decompose
on heating. They are: Calcium sulphate (CaSO₄), Calcium chloride
(CaCl₂), Magnesium sulphate (MgSO₄) and Magnesium Chloride (MgCl₂).
In other words, they are Sulphate and Chloride salts of Magnesium
and Calcium.
Permanent hardness is removed by adding washing soda (Na₂CO₃)
as follows:
CaSO₄ + Na₂CO₃ ----------> Na₂SO₄ + CaCO₃
Calcium Sod. Sod. Cal.
sulphate Carbonate Sulphate Carbonate
MgCl₂ + Na₂CO₃ ----------> 2 NaCl + Mg CO₃
Magnesium Sodium Sodium Magnesium
chlordie Carbonate Chloride Carbonate
Sodium Chlordie and Sodium sulphate remain soluble and have no
action on soap lather.
--------------------
k.
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THE HALOGENS
84. NOMENCIATURE
The halogens refer to the four elements fluorine, chlorine, bromine,
and iodine. These four elements can combine with metals to form salts
such as common salt (NaCl) and hence the word halogen (meaning salt-
former) is given to this group of elements. These four elements are
grouped as one family because they resemble each other in properties
and in their chemical compounds. We shall choose the chief member of
this family, namely, Chlorine for our study as representative for the
others.
CHLORINE
[Marginalia] 85.
85. OCCURRENCE: This element is not present in nature in the free state
due to its great chemical activity. Therefore it exists in nature in
the combined form. The common compound of it is sodium chloride (NaCl)
which is common salt. Other Chlorides exist in nature such as those of
K, Mg and other elements.
86. PREPARATION:
Laboratory Methods
By oxidation of hydrochloric acid with an oxidizing agent such
as manganes dioxide:
4HCl + MnO₂ --------> MnCl₂ + 2H₂O + Cl₂
The apparatus commonly used for this preparation is illustrated
in the following diagram.
HCl
A
B
C
D
MnO₂
↓ Cl₂
Water
Sulphuric - acid
Fig. 7