CHEMISTRY

CHAPTER 8
Creative Question-Answers:
1. (a)

In a chemical bond, the energy by which an atom remains joined with another atom is
called bond energy.

(b)

Every substance is made up of atoms and molecules. There they are added with each
other by an attractive force. This attraction force is called bond energy. During chemical
reactions old bonds are broken and new bonds are formed. For breaking old bonds and
formation of new bonds, changes of heat occurs in a chemical reaction.

All substances have a specific energy at a specific temperature. This energy is called
internal energy. If we express the total internal energy of reactants in a reaction with E1
and the total internal energy of products with E2, then the change of heat in that
reaction is:

Changes of heat = Total energy of products – Total energy of reactants

H = E2 – E1

E2 E1
Energy 

Internal energy of products Internal energy of reactants

Energy 


 Heat absorbed  Heat evolved



E1 E2
Internal energy of reactants Internal energy of products

Reaction co-ordinates  Reaction co-ordinates 

 (c)

Given balanced reaction is:

C3H8 + 4Cl2  C3H4Cl4 + 4HCl

𝐻𝐻𝐻 𝐶𝑙 𝐶𝑙 𝐶𝑙
Or, (H- 𝐶 - 𝐶 - 𝐶 -H) + 4 (Cl-Cl)  (H- 𝐶 - 𝐶 - 𝐶 -Cl) + 4 (H-Cl)
𝐻𝐻𝐻 𝐻 𝐻 𝐻
Here, 4 mole (C-H) bonds and 4 mole (Cl-Cl) bonds have been broken. On the other hand,
4 mole (C-Cl) bonds and 4 mole (H-Cl) bonds have been formed.

We know,

Changes of heat in a reaction = Total energy absorbed during breaking old bonds – Total
energy released during formation of new bonds

Or, H = {4 (C-H) + 4 (Cl-Cl)} - {4 (C-Cl) + 4 (H-Cl)}

Or, H = {4 x 414 + 4 x 244} - {4 x 326 + 4 x 431}

 H = - 396 Kjmol-1

(d)

Mentioned balanced reaction (i) is:

CH4 + 2O2  CO2 + 2H2O

𝐻
Or, (H- 𝐶 -H) + 2 (O=O)  (O=C=O) + 2 ( O )
/ \
𝐻 H H
Here, 4 mole (C-H) bonds and 2 mole (O=O) bonds have been broken. On the other hand,
2 mole (C=O) bonds and 4 mole (O-H) bonds have been formed.

We know,

Changes of heat in a reaction = Total energy absorbed during breaking old bonds – Total
energy released during formation of new bonds

Or, H = {4 (C-H) + 2 (O=O)} - {2 (C=O) + 4 (O-H)}

Or, H = {4 x 414 + 2 x 498} - {2 x 724 + 4 x 464}

 H = - 652 Kjmol-1

Now,

Mass of 1 mole CH4 = {(1 x 12) + (4 x 1)} g

= 16 g

 16 g CH4 evolves 652 Kj heat

652
 1 g CH4 evolves Kj heat
16

= 40.75 Kj heat

Again,

Mentioned balanced reaction (ii) is:

C2H6 + O2  CO2 + H2O

7
Or, C2H6 + O2  2CO2 + 3H2O
2

𝐻𝐻
7
Or, (H- 𝐶 - 𝐶 -H) + (O=O)  2 (O=C=O) + 3 ( O )
2
𝐻𝐻 H H
7
Here, 6 mole (C-H) bonds, 1 mole (C-C) bond and mole (O=O) bonds have been broken.
2
On the other hand, 4 mole (C=O) bonds and 6 mole (O-H) bonds have been formed.

We know,

Changes of heat in a reaction = Total energy absorbed during breaking old bonds – Total
energy released during formation of new bonds
7
Or, H = {6 (C-H) + 1 (C-C) + (O=O)} - {4 (C=O) + 6 (O-H)}
2

7
Or, H = {6 x 414 + 1 x 344 + x 498} – {4 x 724 – 6 x 464}
2

 H = - 1109 Kjmol-1

Now,

Mass of 1 mole C2H6 = {(12 x 2) + (6 x 1)} g

 = 30 g

 30 g C2H6 evolves 1109 Kj heat

1109
 1 g C2H6 evolves Kj heat
30

= 36.97 Kj heat

Since 1 g CH4 produces more heat than 1 g C2H6 when reacting with O2, therefore
between these two hydrocarbons CH4 is more effective as fuel.

(2)
(a)
The electrode in which oxidation reaction occurs is called anode.

(b)

When edible soda (baking soda) or sodium bicarbonate (NaHCO3) reacts with lemon
juice or vinegar, endothermic reaction occurs.

Baking soda (NaHCO3) is a strong base. Lemon juice contains citric acid (C6H8O7) and
vinegar is 4-10% aqueous solution of acetic acid or ethanoic acid (CH3COOH) and both
of these are weak organic acid. The reaction between strong base and weak acid is acid-
base neutralization reaction. In this reaction, sodium salt, water and carbon dioxide
oxide are produced, and heat is absorbed. Here the total internal heat of the reactants
is less than the total internal energy of products.

3NaHCO3 + C6H8O7  Na3C6H8O7 + H2O + CO2

NaHCO3 + CH3COOH  CH3COONa + H2O + CO2

(c)

In reaction (i) of stem, A is hydrogen gas (H2) and B is nitrogen gas (N2).
Mentioned complete reaction (i) is:
A2 (g) + B2 (g)  2BA3 (g); H = - 92 Kj

Or, 3H2 (g) + N2 (g)  2NH3 (g); H = - 92 Kj

_
 𝐻
Or, 3 (H-H) + (N=N)  2 (H-𝑁-H); H = - 92 Kj
.
Here, 3 mole (H-H) bonds and 1 mole (N-N) bond have been broken. On the other hand,
6 mole (N-H) bonds have been formed.

Given, bond energy of (H-H) and (N-H) are 435 Kjmol-1 and 391 Kjmol-1 respectively.

We know,

Changes of heat in a reaction = Total energy absorbed during breaking old bonds – Total
energy released during formation of new bonds
_
Or, H = {3 (H-H) + 1 (N=N)} - {6 (N-H)}

Or, - 92 = {3 x 435 + (NN)} – (6 x 391)

Or, - 92 = 1305 + (NN) – 2346

Or, - 92 – 1305 + 2346 = (NN)

 (NN) = 949 Kjmol-1

So, the bond energy of (NN) is 949 Kjmol-1.

(d)

In reaction (ii) of stem, B is nitrogen gas (N2) and C is oxygen gas (O2).

Mentioned complete reaction (ii) is:

B2 (g) + C2 (g)  BC (g)

Or, N2 (g) + O2 (g)  2NO (g)

Or, (NN) + (O=O)  2 (N=O)

Here, 1 mole (NN) bond and 1 mole (O=O) bond have been broken. On the other hand,
2 mole (N=O) bonds have been formed.

Given, bond energy of (NN), (O=O) and (N=O) are 520, 498 and 419 Kj/mol respectively.

We know,
Changes of heat in a reaction = Total energy absorbed during breaking old bonds – Total
energy released during formation of new bonds

Or, H = {(NN) + (O=O)} - {2 (N=O)}

Or, H = {520 + 498} - {2 x 419}

Or, H = 1018 – 838

 H = + 180 Kjmol-1

Therefore, the H of the above reaction is + 180 Kjmol-1.

We know,the chemical reactions in which heat is absorbed are called endothermic

reactions. Here, the value of H is positive. So, in the above reaction, the total internal
energy of reactants is less than the total internal energy of products. As a result, the
reactant will absorb heat in order to convert into product. Thus we can say that the
reaction is an endothermic reaction.

The changes of heat in an endothermic reaction is given below using energy diagram:
3.
(a)

The smoke of automobiles consists of carbon monoxide, nitrous oxide and unused
methane. In presence of sunlight, these react to create various poisonous gases. They
are known as photochemical smog.

(b)

The substances that let lectricity pass in their molten or aqueous solution state due their
presence of free ions are called electrolyte.

Potassium chloride (KCl) is an ionic compound. It has free ions in its molten or aqueous
solution state.

KCl → K+ + Cl-
H2O
KCl → K+ (aq) + Cl- (aq)

When we connect an outer electric source with molten or aqueous solution of KCl, it
conducts electricity. For this reason, KCl is an electrolyte.

(c)

The mentioned cell in the stem contains melted MgCl2 which is an ionic salt.

By applying heat, magnesium chloride (MgCl2) salt is melted. Then rhis molten solution
is taken into an electrolytic cell as electrolyte.

In molten solution of MgCl2, two ions are present, Mg+ and Cl-.

MgCl2 → Mg+ + 2Cl-

+ -

Anode (+) Cathode (-)

Cl- Mg+2

Fig: electrolysis of molten MgCl2

When electricity passes through this solution, anion (Cl-) is attracted towards the anode.
Here, Cl- is oxidized in anode and chlorine gas (Cl2) is produced.

Anode reaction: Cl-  Cl + e-

2Cl-  Cl2 + 2e-

On the other hand, cation (Mg+2) is attracted towards the cathode. Here, Mg+2 is
reduced in cathode and magnesium metal is produced.

Cathode reaction: Mg+2 + 2e-  Mg

Therefore, through oxidation reaction, chlorine gas (Cl2) is produced in anode and
through reduction reaction magnesium metal (Mg) is produced in cathode. So, it can be
said that, electrolysis is an oxidation and reduction process.

(d)

The mentioned cell in the stem is an electrolytic cell which contains melted MgCl2. In
order to give a coating of nickel on iron, a salt solution of nickel (Ni) has to be taken in
the cell instead of MgCl2 as electrolyte. The coating of nickel over iron can be done
through electroplating process.

The electrolysis process in which a layer of low reactive metal is givemn upon a highly
reactive metal is called electroplating. This process is used for increasing brightness of
metal or to prevent decaying of metal. For example, a layer of gold, silver or chromium
or nickel is given upon iron.

The process of electroplating upon iron by using Nickel is given below:

At first, the iron products have been taken as cathode, the nickel metal rod has been
taken as anode and aqueous soution of nickel sulphate (NiSO4) has been taken as
electrolyte.

Nickel

Ni+2

Fig: Electroplating of nickel over iron

Nickel metal from anode is oxidized into nickel ion (Ni+2) by donating electron and gets
dissolved in water.

Anode reaction : Ni  Ni+2 + 2e-

Again, this nickel ion is reduced into nickel metal and depoisited over iron product in
cathode.

Cathode reaction: Ni+2 + 2e-  Ni

In this way, a coating of nickel can be given upon iron.

4.
(a)

The electrolysis process in which a layer of low reactive metal is givemn upon a highly
reactive metal is called electroplating.

(b)

The differences between nuclear fission reaction and nuclear fusion are:

Nuclear fission Nuclear fusion

1. The nuclear reaction in which a large 1. The nuclear reaction in which atoms of
nucleus of an element breaks into small element is added with each other
nuclei of smaller elements is called and forms a big nucleus is called nuclear
nuclear fission reaction. fusion reaction.

2. It mainly occurs in the nuclear power 2. It mainly occurs in the sun.

plant.

3. It is a chain reaction. 3. It is not a chain reaction.

4. After reaction, huge amount of 4. After reaction, no nuclear wastes are

nuclear wastes are present. present.
 (c)

In the mentioned cell, copper purification process has been given.

To purify copper, electrolysis process is applied. To do this, a pure rod of copper is used
as cathode and an impure rod of copper is used as anode. Aqueous solution of copper
sulphate is used as electrolyte.

When electricity passes through this solution, then copper metal from anode is oxidized
into copper ion (Cu+2) by donating two electrons. These two electrons are accepted by
copper ion and deposited as copper metal upon cathode. The impurities of anode are
deposited as anode mud.

Anode reaction: Cu  Cu+2 + 2e-

Cathode reaction: Cu+2 + 2e-  Cu

In this way, copper is 99.99% pure. So, we can get almost 100% pure copper from the
mentioned ore.

(d)

In the stem, copper purification is done by using copper rods as electrode and aqueous
solution of copper sulphate as electrolyte. If we use granite or platinum rod instead of
carbon rod, the electrolysis process will be different. The electrolysis process of aqueous
copper sulphate (CuSO4) solution using platinum (Pt) as electrode is given below:

At first, CuSO4 is mixed with water and taken into a cell vessel. In dilute solution of
CuSO4, four ions are present: cations Cu+2 and H+, anions SO4-2 and OH-.

H2O  H+ + OH-

CuSO4  Cu+2 + SO4-2

When electricity is passed through this solution anion SO4-2 and OH- are attracted
towards the anode. Due to more reactivity, OH- reacting in anode will produce oxygen
(O2) gas and water (H2O) by oxidation reaction.

Anode reaction: OH-  OH + e-

OH + OH  H2O + O

O + O  O2

4OH-  2H2O + O2 + 4e-

On the other hand, cation Cu+2 and H+ are attracted towards the cathode. Due to higher
reactivity, Cu+2 reacting in cathode will produce copper (Cu) metal by reduction reaction.

Cathode reaction: Cu+2 + 2e-  Cu

2Cu+2 + 4e-  2Cu

In this way, through electrolysis of aqueous solution of CuSO4 using platinum rod, O2
and H2O will be produced in anode and copper metal will be produced in cathode.

5.
(a)

H means the amount of heat energy absorbed or evolved in a chemical reaction.

(b)

Hydrogen fuel cell is a type of cell where hydrogen is used as fuel.

Hydrogen fuel cell is used to produce electricity. In this type of cell, hydrogen molecule
is oxidized in its anode and oxygen molecule is reduced at the cathode to produce water.
As a result, electron is conducted from anode to cathode and electricity is produced.
This electrify can used to run cars even. All mobile phones, computers, calculators etc.
all over the world use batteries.

(c)

Electricity is produced due to the chemical reaction by using the reactant and product
in reaction (i).
Mentioned reaction (i) is:

Zn (s) + CuSO4 (aq)  ZnSO4 (aq) + Cu (s)

Electricity is produced by using the reactant Zn and product Cu in galvanic cell by taking
as anode and cathode in two different beakers. The cell where chemical elements
undergo reaction and produce electrical enery is called galvanic cell. The production of
electricity in galvanic cell Zn and Cu is given below:

In anode cell, a zinc rod and aqueous solution of zinc sulphate are taken as anode and
electrolyte. On the other hand, copper rod and aqueous solution of copper sulphate are
taken as cathode and electrolyte. Anode and cathode rods are connected with each
other by copper wire and bulb.

Since the reactivity of metal zinz is more than that of copper metal, zinc is oxidized in
anode by donating two electrons and dissolves as zinc cation (Zn+2). Here, zinc rod
becomes negatively charged and due to electrostatic attraction, copper rod becomes
positively charged.

Anode reaction: Zn  Zn+2 + 2e-

On the other hand, copper ion of cathode cell is reduced into copper metal by accepting
two electrons donated by zinc and deposited as copper metal upon cathode.

Cathode reaction: Cu+2 + 2e-  Cu

Due to the flow of electrons from anode to cathode, flow of electricity occurs and bulb
glows. But after some time, the flow of electricity stops due to the difference in ions
between anode and cathode cell. To balance the ions in anode and cathode cell, a U
shaped tube is filled with potassium chloride salt solution used between anode and
cathode cell.
 (d)

In the stem, mentioned tube ‘A’ is salt bridge.

The U shaped tube filled with potassium chloride salt solution used between anode and
cathode in galvanic cell is called salt bridge. A chemical substance named agar-agar is
mixed with KCl is put inside the U shaped tube. Is creates a gel-like substance which is
called salt bridge.

Its function in a galvanic cell is given below:

In galvanic cell, during reaction in anode, lead metal is oxidized to lead ion (Pb+2), and
get dissolved in solution. On the other hand, in cathode cell, iron cation (Fe+2) from
copper sulphate (FeSO4) solution is reduced to iron metal and deposited over cathode.

In this way, positive lead ion (Pb+2) increases in anode cell and negative sulphate ion
(SO4-2) increases in cathode cell. For this reason, the solution becomes negative and
due to the difference in ions between anode and cathode cell, the flow of electrons
from anode to cathode stopsand procuction of electricity stops within a while.

Fe  Fe+2 + 2e-

Pb+2 + 2e-  Pb

To get continuous flow of electricity, we need to neutralize the excess ion produced in
anode and cathode. Chloride ion (Cl-) reacts with iron in anode cell and potassium ion
(K+) reacts with sulphate ion (SO4-2).

Reaction in anode cell : Fe+2 + 2Cl-  FeCl2

Reaction in cathode cell : 2K+ + SO4-2  K2SO4

In this way, excess ions are neutralized and production of electricity in galvanic cell
continues.
6.
(a)

Harmful gases produced for the burning of fossil fuels which fall on the earth’s surface
as different acids and mix with rain water, this rain is called acid rain.

(b)

The purity of fuel is an important factor.

If we burn impure fuels in insufficient air, carbon monoxide is produced alongside

carbon dioxide, which is harmful and dangerous for our health. Natural fuels need
refinement before being sent to the market as they contain compounds of nitrogen,
phosphorus, sulfur etc. in their oxide form. If they are burnt without refinement they
will end up causing acid rain which hampers the life of fishes, plants and other living
bodies. Again, the smoke of automobiles consists of carbon monoxide, nitrous oxide and
unused methane. In the presence of these react to create various harmful gases known
as photochemical smog. Elements of this smog harm the ozone layer of our atmosphere
which protects us from the ultraviolet rays of the sun. if this ozone layer is harmed, lives
of human beings will be in danger.

(c)

Above cell-1 is a dry cell. Electricity is produced in the dry cell by chemical reactions.

In a dry cell, a mixture of ammonium chloride (NH4Cl), charcoal powder manganese

dioxide thoroughly with little water added to make a paste. This mixture is taken in a
cylindrical shaped Zinc container. A carbon or graphite rod is introduced at the center
of the container in such a way that, it does not touch the zinc container. At the top of
the carbon rod there is a metal cup. The upper part of the cell is covered by a layer of
pitch.
The surface of the zinc cylinder acts as the negative electrode or as the anode. The
carbon rod with the metal cap acts as the positive electrode or cathode.

When we connect dry cell in an electric circuit then zinc from the cylinder oxidized by
donating two electron and converted into Zn+2 ion.

Reaction in anode: Zn  Zn+2 + 2e-

These two electrons come to the carbon rod through the connector and combine with
ammonium ion (NH4+) and manganese dioxide (MnO2) to produce ammonia gas (NH3),
di-manganese tri-oxide.

Reaction in cathode: 2NH₄ + 2MnO2 + 2e-  Mn2O3 + H2O + 2NH3

Total cell reaction: Zn + 2NH4+ 2MnO2  Zn+2 + Mn2O3 + H2O + 2NH3

When anode and cathode edges are connected to a bulb or any electronic item, the flow
of electricity begins or electricity is generated. Therefore, wherever electricity is
required, if we connect the dry cell there, the above reaction will occur and we will get
electricity.

(d)

Electrolysis of concentrated NaCl.

Identification of chlorine gas.

7.
(a)

Metallic conductors are metallic substances that conduct electricity due to flow of
electrons.

(b)

Ethanol or ethyl alcohol (CH3CH2COOH) is an organic substance. It is used as bio fuel.

Aqueous solution of ethanol can be used as an alternative to fossil fuel. Fossil fuels
contain sulphur, nitrogen, phosphorous as impurities. Due to burning the process, fossil
fuels produce various oxides. These oxides cause acid rain by reacting with water vapour
and pollutes the environment. On the other hand, ethanol doesn't contain any
impurities. So, if we use ethanol instead of fossil fuel, our environment can be pollution
free.

(c)

Mentioned complete reaction is:

Petroleum + O2 (g)  CO2 (g) + H2O (g) + Energy

Here, gas ‘A’ is CO2 or carbon dioxide. It is very harmful for environment. Fossil fuels are
mainly different hydrocarbons. During burning fossil fuels, CO2 and water vapour are
produced. The burning of fossil fuels produces 21.3 billion tons of CO2. The negative
effects of CO2 upon environment are given below:

i. CO2 reacts with rain water and produces carbonic acid (H2CO3). When it comes
down to earth with rain, we call that acid rain, which is very harmful for nature.
CO2 + H2O  H2CO3
ii. As a result of acid rain, the soil and water become acidic. The pH range of acid
rain is 5 to 6. But the best value of pH of fertile soil is 6 to 8. So, the fertility of soil
decreases and life of living beings is hampered.
iii. Since CO2 does not undergo any reaction with the other elements of air, amount
of CO2 is increasing usually in atmosphere day by day.
iv. Heat absorbing capacity of CO2 is high, that is, CO2 can absorb heat and can trap
the heat. Again, CO₂ gas stay nearer to the surface of the earth as it is heavy in
weight. That is why atmospheric temperature is increasing day by day, which is
called Global Warming.
v. The phenomenon of this temperature increase by CO2 gas is known as
Greenhouse Effect and CO2 is called the greenhouse gas. Due to global warming,
ice melts to water in earth poles and causes unexpected flood.

(d)

The complete reactions of stem are:

i. Petroleum + O2 (g)  CO2 (g) + H2O (g) + Energy

92U + 3 0n 
235 1 141 92 1
ii. 56Ba + 36Kr + 3 0n + Energy

In reaction (i), hydrocarbon petroleum reacts with oxygen and produces carbon dioxide,
water and energy. In reaction (ii), a 23592U nucleus is struck with a neutron at high
velocity, and the nucleus becomes almost equally divided into two parts and produces
141 92 1
56Ba and 36Kr nuclei, 3 neutrons ( 0n) and a huge amount of energy. Here energy is
produced by nuclear fission reaction. In this case, nuclear fission is more sustainable and
beneficial in terms of production of electricity.

The first reaction is a combustion reaction. Petroleum is a hydrocarbon and a fossil fuel.
This process is used in electricity production, transport system and in industries as a
source of energy. Fossil fuels generally satisfy the demands of our energy requirement.
During the combustion of petroleum, carbon hydrogen bonds of hydrocarbons are
broken down to from carbon – oxygen (CO2) and hydrogen – oxygen (H20) bonds. Due
to combustion of fuels, the internal energy of the products (H20 and CO2) are much less
than the internal energy of the hydrocarbon molecule. As a result, a large amount of
energy is being released as heat energy. This energy is transformed into electrical energy
by turbines and generator. But, the source of petroleum is decreasing because it is a
non-renewable fossil fuel. Soon it will run out. Its price is also really increasing. So,
production of electricity through this process is not very sustainable.

The second reaction is nuclear fission reaction. The nuclear process in which the nucleus
of a larger and heavy element is broken in the nucleus of smaller elements is called a
fission reaction. The reactor which helps control the chain reactions that occur during a
nuclear fission is called a nuclear reactor. These reactors are used as power generator.
When uranium- 235 is hit with high energy particle, it loses some mass, which turns into
energy according to E=mc2. One mule uranium produces massive amounts of energy.
The highly accelerated produced small elements bombard each other as well as the
reactor walls and produce huge amounts of heat energy. This heat is channeled through
to an elevator chamber where it is used for producing vapors, which are the channeled
to run turbines so that electricity can be produced. Though nuclear reaction plant is very
expensive and the production process is dangerous, the production of electricity
through this process can be very beneficial with proper management. Nuclear reactor
is used as power generator in many countries of the world. Bangladesh government has
taken the necessary preparation to set up a nuclear reaction generator facility.
Bangladesh will become very self-sufficient in electricity production, once the project is
complete.

So, electricity generation by the second reaction is more suitable.