Question 1.
What would be the electron dot structure of carbon dioxide which has the
formula CO
2?
Answer.
The atomic number (Z) for carbon is six and
its electronic configuration is 2, 4. Carbon has four valence electrons. Each
oxygen atom (Z = 8) has six valence electrons (2, 6). In order to complete its
octet, the carbon atom shares its four valence electrons with the four
electrons of two oxygen atoms as follows:
Question 2.
What would be the electron dot structure of
a molecule of sulphur which is made up of eight atoms of sulphur?
Answer.
The atomic number (Z) of sulphur is sixteen
and its electronic configuration is 2, 8, 6. The sulphur atom has six valence
electrons. The chemical formula of sulphur molecule is S8. Each sulphur atom
is linked to similar atoms on either sides by single covalent bonds and thus,
completes its octet. The molecule is in the form of a ring also represented by
a crown shape.
Question 3.
How many structural isomers can you draw
for pentane?
Answer.
Pentane (C
5H
12) has a skeleton of five carbon atoms. It can exist as a straight chain as
well as two branched chains. There are three structural isomers for the
hydrocarbon which is an alkane.
Question 4.
What are the two properties of carbon which
lead to the huge number of carbon compounds we see around us?
[2013]
Answer.
(a)
Catenation: Carbon
has the unique property of self-linking which is known as catenation. In fact,
any number of carbon atoms can be linked to one another by covalent bonds.
This is on account of the stability of the C—C bonds since the size of the
carbon atom is quite small.
(b)
Linking of carbon with other atoms: Carbon is tetravalent in nature and can readily unite with atoms such as
hydrogen, oxygen, nitrogen, sulphur, etc. by electron sharing.
Question 5.
What will be the formula and electron dot
structure of cyclopentane?
Answer.
Cyclopentane is a cyclic compound with the
formula C
5H
12. The structure of the compound may be represented as:
Question 6.
Draw the structufes of the following
compounds:
(a) Ethanoic
acid
(b) Bromopentane
(c) Butanone
(d) Hexanal
Are structural isomers possible for
bromopentane?
Bromopentane has a chain of five carbon atoms. It can exist in a number of
forms which are structural isosmers.
- The structural isomers (a), (b) and (c) which differ in the position of the Br atom are known as position isomers.
- The structural isomers (d), (e) and (f) which differ in the arrangement of carbon atoms in the chain are called chain isomers.
- In writing the IUPAC name, the prefix bromo is written before methyl. In fact, alphabetical order is followed while naming the different prefixes.
Question 7.
How would you name the following compounds?
Answer.
(a) Bromoethane
(b) Hex-1-yne
(c) Methanal
Question 8.
Why is the conversion of ethanol into
ethanoic acid an oxidation reaction?
Answer.
Ethanoic acid (CH
3COOH) has one oxygen atom more and two hydrogen atoms less than ethanol (C
3H
5OH). In general,
- Loss of hydrogen is known as oxidation.
- Gain of oxygen is known as oxidation.
Therefore, it is an oxidation reaction.
Question 9.
A mixture of ethyne and oxygen is used for
welding. Can you tell why a mixture of ethyne and air is not used?
Answer.
When ethyne is burnt in oxygen, a large
quantity of heat along with light is produced. The heat evolved can be used
for gas welding which is usually carried to weld small broken pieces of
articles made up of iron.
Air mainly contains of a mixture of nitrogen (4 parts) and oxygen (1 part). As we know, nitrogen gas does not support combustion. This means that in air, only oxygen will help in the combustion of ethyne. Therefore, it is always better to use oxygen for the combustion of ethyne.
Question 10.
How would you distinguish experimentally
between an alcohol and a carboxylic acid?
Answer.
The distinction can be made by the
following, tests:
(a) Dip a
strip of blue litmus paper separately in both alcohol and carboxylic acid
taken in two glass tubes. The colour will change to red in the tube containing
carboxylic acid and not in the tube which contains alcohol.
(b) Add a
small amount of solid sodium hydrogen carbonate (NaHCO
3) in both the tubes. A brisk effervescence accompanied by bubbles will be
noticed in the tube containing carboxylic acid and not in the tube containing
alcohol.
Question 11.
What are oxidising agents?
Answer.
Oxidising agents are the substances which
either on their own or on reacting with another substance release oxygen in
order to carry oxidation reactions. The commonly used oxidising agents are:
ozone, bromine water, a mixture of potassium dichromate and sulphuric acid or
a mixture of potassium permanganate and sulphuric acid, etc.
Question 12.
Will you be able to check if water is hard
by using a detergent?
Answer.
No, it is not possible. Detergents produce
foam in any type of water; whether hard or soft Therefore, a distinction
between the two cannot be made. However, soaps can be used for this purpose.
Question 13.
People use different methods to wash
clothes. Usually after adding the soap, they beat the clothes on a stone or
beat them with a paddle, scrub with a brush or the mixture is agitated in a
washing machine. Why is this agitation necessary to get clean clothes?
Answer.
The purpose of soap or detergent in washing
is to reduce friction between the oil drops carrying dirt particles and the
water so that they may mix with each other. All the methods that have been
suggested loosen the bonds between the dust or oil particles and fabrics of
clothes. The agitation helps in washing the clothes.
Chapter End Questions
Question 1.
Ethane, with the molecular formula C
2H
6 has:
(a) 6
covalent bonds
(b) 7
covalent bonds
(c) 8
covalent bonds
(d) 9
covalent bonds
Answer.
(b) 7
covalent bonds
Question 2.
Butanone is a four carbon compound with the
functional group
(a) carboxylic acid
(b) aldehyde
(c) ketone
(d) alcohol
Answer.
(c) ketone
Question 3.
While cooking, if the bottom of the vessel
is blackened on the outside, it means that
(a) the food
is not cooked completely
(b) the fuel
is not burning completely
(c) the fuel
is wet
(d) the fuel
is burning completely.
Answer.
(b) the fuel
is not burning completely
Question 4.
Explain the nature of the covalent bond
using the bond formation in CH
3Cl.
Answer.
The molecule of chloromethane (CH
3Cl) consists of three elements, i.e. carbon (Z = 6), hydrogen (Z = 1) and
chlorine (Z = 17). Carbon atom has four valence electrons (2, 4); hydrogen has
one while chlorine has seven electrons in the valence shell (2, 8, 7). In
order to complete its octet, carbon shares three valence electrons with three
hydrogen atoms while one is shared with the electron of chlorine atom. The
structure of covalent molecule may be written as follows:
Question 5.
Draw the electron dot structures for
(a) ethanoic acid
(b) H
2S
(c) propanone
(d) F
2
Question 6.
What is a homologous series? Explain with
an example.
Answer.
A homologous series is a series of carbon
compounds that have different numbers of carbon atoms but contain the same
functional group. It can be represented by the same general formula. Compounds
of homologous series differ by CH
2 from their consecutive members. All the compounds of a homologous
series show similar chemical and physical properties. For example, alkanes
such as methane, ethane, propane, butane, etc. belong to the same homologous
series. Similarly, all alkenes belong to a particular homologous series and
all alkynes belong to another homologous series.
Question 7.
How can ethanol and ethanoic acid be
differentiated on the basis of their physical and chemical properties?
[2011,2014]
Answer.
Distinction based on physical properties
are as:
(a)
Smell: Ethanol has a characteristic smell known as alcoholic smell which is
pleasant. Ethanoic acid has a vinegar-like smell.
(b)
Boiling point: Boiling point of ethanol (351 K) is less than that of ethanoic add (391
K).
(c)
Litmus test: Ethanol is neutral in nature and does ijot change in the colour of
litmus whether blue or red. Ethanoic acid is acidic and changes the colour of
a blue litmus strip to red.
Distinction
based on chemical properties:
(a)
Action with sodium hydrogen carbonate: On adding a small amount of sodium hydrogen carbonate to ethanoic acid,
carbon dioxide gas is evolved with brisk effervescence. However, no such
reaction is noticed in case of ethanol.
(b) Action with caustic alkali: Ethanoic add reacts with both sodium hydroxide (NaOH) and postassium
hydroxide (KOH) to form corresponding salt and water. Ethanol fails to react
with either of these.
Question 8.
Why does micelle formation take place when
soap is added to water? Will a micelle be formed in other solvents such as
ethanol also?
[2011]
Answer.
Soap may be represented by the formula
RCOONa where R is an alkyl group which represents a long chain of carbon with
fifteen or more atoms. Now, oil drops containing dirt particles and water do
not mix. Soap helps in their mixing by reducing interfacial tension or
friction. Actually it forms a sort of bridge between oil drops and water in
which the alkyl portion (hydrophobic end) points towards oil drop while other
portion COON a (hydrophilic end) is directed towards water. This is known as
micelle formation. Thus, soap helps in the formation of a stable emulsion
between oil and water. Ethanol and other similar solvents which are of organic
nature do not help in micelle formation because soap is soluble in them.
Question 9.
Why are carbon and its compounds used as
fuels in most cases?
Answer.
Carbon burns in oxygen to form carbon
dioxide gas. The reaction is highly exothermic. That is why different forms of
coal are used as fuels. The most important compounds of carbon are
hydrocarbons. Just like carbon, hydrogen also readily burns in oxygen or air
to form water a heat. The hydrocarbon methane (CH
4) is the major constituent of natural gas. Propane (C
3H
8) and butane (C
4H
10) are present in liquid petroleum gas (LpG). Petrol and kerosene also contain
different hydrocarbons. Therefore, these are used as fuels.
Question 10.
Explain the formation of scum when hard
water is treated with soap.
[2011,2012]
Answer.
Soap is basically sodium or potassium salts
of higher fatty acids. Hard water contains Ca
2+ and Mg
2+ ions as their salts. When soap is added to hard water, the
corresponding calcium and magnesium salts are formed. These are in the form of
precipitates, also called scum.
where R is alkyl group having 15 to 17 carbon atoms.
Question 11.
What change will you observe by testing
soap with litmus paper (blue or red)?
Answer.
When soap is dissolved in water, the
solution is alkaline in nature due to the formation of alkali NaOH or KOH. The
solution changes the colour of red litmus to blue. However, the solution does
not change the colour of blue litmus.
Question 12.
What is hydrogenation? What is its
industrial application?
Answer.
Hydrogenation is a chemical reaction
between molecular hydrogen (H
2) and another compound or element, usually in the presence of a catalyst such
as nickel, palladium or platinum. The process is commonly employed to reduce
or saturate organic compounds. Catalytic hydrogenation has diverse industrial
uses. Most frequently, industrial hydrogenation relies on heterogeneous
catalysts.
In petrochemical processes,
hydrogenation is used to convert alkenes and aromatics into saturated alkanes
(paraffins) and cycloalkanes (naphthenes), which are less toxic and less
reactive. For example, mineral turpentine is usually hydrogenated, hydro
cracking of heavy residues into diesel, etc.
Question 13.
Which of the listed hydrocarbons undergo
addition reactions: C
2H
6, C
3H
8, C
3H
6, C
2H
2 and CH
4?
Answer.
C
3H
6 and C
2H
2 undergo addition reaction as they are unsaturated hydrocarbons. As
unsaturated hydrocarbons have double and triple bonds.
Question 14.
Give a test that can be used to
differentiate between butter and cooking oil.
Answer.
Butter is saturated in nature while cooking
oil is unsaturated. This means that cooking oil has at-least one C—C bond
present in the constituting compounds while butter does not have any such
bond. The distinction between them can be made by reacting with bromine water
or bromine dissolved in carbon tetrachloride. Cooking oil will discharge the
yellow colour of bromine while butter will not.
Question 15.
Explain the mechanism of cleansing action
of soap.
Answer.
Cleansing Action of Soap:
Soaps contain two chemically distinct
parts: a long hydrocarbon tail and the other negatively charged head. The long
hydrocarbon tail is nonpolar and water repelling, i.e. insoluble in water.
When a soap is dissolved in water, the molecules clump together in a spherical fashion as clusters called micelles. The tail stick inwards and the head outwards.
In cleaning, the long hydrocarbon tail attaches itself to oily or dirty part which is nonpolar and insoluble in water. The dirt is, thus, enveloped in the nonpolar end of soap micelles. The negatively charged heads of soap molecules make the micelles soluble in water. Thus, the dirt is washed away with soap.
