WBBSE Notes For Class 8 School Science Chapter 2 Element Compound And Chemical Reaction

Chapter 2 Element Compound And Chemical Reaction

Reactant And Reaction

We have already learnt that a substance can be identified or distinguished from others by its chemical properties.

The chemical properties of a substance are the properties it exhibits when it undergoes a complete change of its initial atomic composition and arrangement due to some action with other substances or influenced by heat, pressure or electricity or any other factor(s).

The process is called the chemical process or chemical reaction. So chemical reaction means a permanent rearrangement or regrouping.

Between the atoms or radicals of the combining substances (i.e., the reactants) to form one or more substances having different properties.

Chemicals participating in a reaction are called reactants. Reactants undergo chemical – reactions and form new substance(s) different ‘ from reactants, called products.

Generally, only mixing two or more reactants simply does not lead to the formation of products. We need some other factors, such as heat, light, pressure, etc., which induce, influence and regulate chemical changes.

Let us now discuss the factors which “affect” a chemical reaction.

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Factors Affecting A Chemical Reaction

1. Physical contact

A chemical reaction takes place only when the reactants are brought in contact with each other.

⇒ \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2\)

Calcium hydroxide is formed only when calcium oxide comes in contact with water.

2. Heat

Many chemical reactions are initiated when the reaction mixture (i.e., the mixture of reactants) is heated together. For example,

1. Heating of blue crystals of hydrated cupric nitrate will lead to the evolution of brown fumes of nitrogen dioxide (NO2 ) and a black residue of cupric oxide (CuO) is left. In absence of heating no such reaction occurs.

2. Mercury, when heated in air or oxygen, produces red mercuric oxide. Here the heated mercury combines with oxygen to produce mercuric oxide.

⇒ \(2 \mathrm{Hg}+\mathrm{O}_2 \stackrel{\Delta}{\longrightarrow} 2 \mathrm{HgO}\)

3. Burning of any fuel (say coal> petrol, diesel, kerosene, etc.), requires the presence of oxygen and is only initiated at high temperatures. High temperature is attained by heating.

Heat is a form of energy. So when heated, the reactants gain energy. These energized substances react at a much faster rate.

3. Light

Like heat, light is also a form of energy. Some substances can absorb light. Thus they gain energy and the energized substance(s) undergo a chemical reaction. Chemical reaction induced/initiated by light is commonly known as Photochemical Reaction.

The most important photochemical reaction known to us is photosynthesis. Here, green plants can utilize solar energy (in the form of visible light) to produce glucose from carbon dioxide and water. The process can be represented in a simplified way as follows:

⇒ \(6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \stackrel{\text { light }}{\longrightarrow} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}\)

In film cameras, photographic films are used. Films are coated with various substances which remain unchanged when stored in dark.

When exposed to light, the real image of an object is cast on the photographic film, and the photo-sensitive substances undergo photochemical reactions.

4. Pressure

Some chemical reactions are initiated only when pressure is applied. For example,

1. In crackers, a mixture of potassium chlorate and sulphur explodes when pressure is applied to them.

2. When the cap in a toy pistol is hit with a sudden blow (i.e., when pressure is applied) it explodes.

In industry, ammonia gas is produced from nitrogen and hydrogen gas at a pressure as high as 200 times the normal atmospheric pressure.

⇒ \(\mathrm{N}_2+3 \mathrm{H}_2 \stackrel{\text { Pressure }}{\longrightarrow} 2 \mathrm{NH}_3\)

3. When high pressure is applied, lead and sulphur react to form lead sulphide (PbS).

⇒ \(\mathrm{Pb}+\mathrm{S} \stackrel{\text { Pressure }}{\longrightarrow} \mathrm{PbS}\)

Pressure provides sufficient mechanical energy so that the reaction occurs at a much faster rate.

5. Presence Of Solvent

The presence of solvent is very important for many of the reactions. For example, when dry baking soda (NaHCO3) and dry crystals of tartaric acid are mixed, no visible change occurs, indicating no reaction is taking place.

But when an aqueous solution of baking soda and the aqueous solution of tartaric acid are mixed with each other, bubbles of carbon dioxide evolved from the reaction mixture.

There is no reaction between silver nitrate (AgNO3) and sodium chloride (NaCI) when they are in the solid state. The reaction takes place only when they are dissolved in water.

⇒ \(\mathrm{AgNO}_3+\mathrm{NaCl} \rightarrow \mathrm{AgCl}+\mathrm{NaNO}_3\)

Actually, a solvent plays a very crucial role during a chemical reaction. Primarily the reactant should be soluble in a solvent.

If the reactants are soluble in a solvent then they are intimately mixed and it allows sufficient “contact” (i.e., interaction) between the reactants to react chemically.

When the reactant is salt (i.e., NaCI, NaN03, etc.), dissolving it in water (solvent) produces ions and these ions react between themselves.

As we have just pointed out salts are soluble in water, but they are not soluble in solvents such as benzene, kerosene oil or turpentine oil (generally known as organic solvents).

So during a reaction involving salts (or reactants which produce ions when dissolved in water), we should not use such organic solvents.

In general, for a reactant to be soluble in the solvent, its chemical nature should be similar and we often use the phrase “like dissolves like”.

6. Electricity

If electricity is passed through acidulated water through two suitable electrodes (containing a little amount of sulphuric acid), then hydrogen and oxygen gas are separately evolved at two electrodes. In this case, the water undergoes electrolysis by the passage of an electric current.

⇒ \(2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_2+\mathrm{O}_2\)

Similarly, if an electric spark is passed through a mixture of hydrogen and oxygen gas, water will be formed. If no current is passed through it, no chemical change will take place.

⇒ \(2 \mathrm{H}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}\)

Sometimes, articles made of iron are coated with a thin film of another metal (say, nickel) to prevent it from rusting. This process of coating is done by electroplating.

Here also, the process is done utilizing electrical energy. So, electrical energy can also be utilized to initiate/induce and regulate chemical changes.

7. Catalyst.

Many chemical reactions occur very slowly. But the addition of a minute quantity of another substance can speed up the reaction.

The substances which can increase the rate of a chemical reaction are called catalysts. There are certain catalysts which slow down the rate of a particular chemical reaction.

They are termed negative catalysts. The chemical reaction which involves the use of a catalyst is called a catalytic reaction.

Catalyst is a substance which is present in small quantities and increases the rate of a chemical reaction without itself undergoing any permanent change. [ A negative catalyst however decreases the rate of the chemical reaction.] Examples Of Some Catalytic Reactions

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Catalytic reaction

 

Characteristics of a catalyst

  1. At a particular temperature, the addition of a catalyst increases the rate of a chemical reaction.
  2. A catalyst cannot initiate a reaction. This means that a catalyst cannot “start” a reaction which otherwise does not occur. It only influences the rate of a reaction.
  3. A catalyst participates in the reaction it catalyzes but is regenerated at the end of the reaction.
  4. A catalyst present in a very small amount is able to influence the rate of a reaction significantly.
  5. There is no universal catalyst which can enhance the rate of all the reactions. A suitable catalyst for a specific reaction must be found only by proper experimentation.

How Does A Catalyst Work

For various reactions, the mechanism by which a specific catalyst works is known. But this discussion is beyond the scope of this book.

At this point, we can only say that a catalyst helps a reaction to occur by a different route, which requires relatively less energy.

So more reactants can participate in a reaction, producing more products. Let us consider a situation where a catalyst is in the solid state within a reaction mixture which is in a liquid state (or gaseous state).

Generally in such cases, the solid catalyst provides a surface where at least one of the reactants can “seat” and only those reactants which are “seated” on the solid surface can react with other reactants which are also seated or which are not seated but remain very close to the solid surface.

So we realize that work more the surface area of a solid catalyst, the more molecules it can accommodate on its surface and the more wilt the rate of the reaction.

If a solid catalyst is crushed to powder, the surface area of the solid substances increases substantially and thus more reactants get the opportunity to react.

(The process by which the reactant molecules “sit” on the solid surface is called “adsorption”.) If the physical state of the catalyst is different from that of the reactants, then the catalyst is called a heterogeneous catalyst.

For example, in the production of ammonia from the reaction between nitrogen gas and hydrogen gas, a solid powder of iron is used as a heterogeneous catalyst. For a heterogeneously catalyzed reaction, adsorption always precedes the chemical reaction.

Organic catalysts: Enzymes

Chemical reactions are constantly occurring within our bodies too. Formation or breaking of chemical bonds rarely happens on their own within biological systems.

Enzymes are biological molecules that act as catalysts and help complex reactions occur. The enzymes are basically proteins, but the, ey may be associated with non-protein substances (known as coenzymes or prosthetic groups) that are essential for the action of the enzyme.

The term “enzyme” meaning “from yeast” was coined by German physiologist Wilhelm Kuhne in 1876. Eduard Buchner showed that fermentation, previously believed to depend on a mysterious “life force” contained only in living organisms, could be achieved by extracts from yeast (which is not “living”).

Enzymes are highly specific catalysts in the sense that a particular enzyme acts on only a particular reactant (generally called “substrate”) to complete a specific task.

Experiments have revealed the catalytic activity of an enzyme Some hands-on experiments can be done to observe the effect of an enzyme on a chemical reaction.

Urease Catalyses hydrolysis of urea \(\begin{aligned}
& \left(\mathrm{NH}_2 \mathrm{CONH}_2+\mathrm{H}_2 \mathrm{O} \rightarrow\right. \\
& \left.2 \mathrm{NH}_3+2 \mathrm{CO}_2\right)
\end{aligned}\)
Invertase Converts surcose to glucose and fructose
Pepsin (Found in our Stomach) Converts proteins into smaller amino acids
Trypsin (Found in our intestine) Smaller Amino acids
Catalase  decompose Hydrogen peroxide (H2O) to water and oxygen  \(2 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2\)

 

1. Let us mix equal volumes of hydrogen peroxide and water in a test tube and a small piece of fresh potato is added. The enzyme, catalase, is found in the fresh potato.

Almost immediately after the addition, bubbles of oxygen start evolving from the reaction mixture. We have already mentioned that catalase decomposes hydrogen peroxide into oxygen and water. The reaction can be repeated with fish or goat liver, which also contains the enzyme catalase.

Enzymes are indispensable for living things because all the biological reactions occurring within living things are catalyzed by enzymes.

Starting from digestion of food to synthesis is due to a relatively small region of the protein molecule present in the enzyme. This region is commonly referred to as an “active centre

2. Let us dissolve some urea in a small volume of water taken in a beaker and to it some amount of powdered ahar daal is added.

After 10 – 15 minutes, if we smell the reaction mixture, a faint pungent smell of ammonia is obtained. Since ammonia is a weak base, so if we add a few drops of phenolphthalein, the reaction mixture turns pink. Arhar daal and watermelon seeds contain the enzyme urease.

Urease catalyses the hydrolysis of urea. Microbes in urinals decompose urea present in urine and that is why we get the smell of ammonia in the urinal.

proteins, hormones, DNA etc., and enzymes are everywhere. Each enzyme performs only a particular action on a particular substrate. Enzymes are indispensable for living things because all the biological reactions occurring within living things are catalyzed by enzymes.

Starting from digestion of food to synthesis is due to a relatively small region of the protein molecule present in the enzyme.

This region is proteins, hormones, DNA etc., enzymes are everywhere. Each enzyme performs only a particular action on a particular substrate.

 

Exothermic And Endothermic Reaction

When two or more reactants are mixed, they react chemically. It is often found that either heat is evolved or heat is absorbed.

Depending upon whether heat is evolved or heat is absorbed in a chemical reaction, the chemical reactions are classified as exothermic reactions and endothermic reactions.

Exothermic reaction: Chemical reactions which proceed with the evolution of heat energy are called exothermic reactions.
An exothermic reaction can be, in general, represented as,

⇒ \(A+B \rightarrow C+\text { heat }(Q)\)

Endothermic reaction: Chemical reactions -which proceed with the absorption of heat energy are called endothermic reactions.
This type of reaction can be represented by the following general form,

⇒ \(A+B \rightarrow C-\text { heat }(Q)\)

Note that, to clarify exothermic or endothermic reactions, either +Q or -Q is written after the products to denote the corresponding evolution or absorption of heat (Q).

Examples Of Exothermic Reaction

1. Burning of coal: \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2+\text { heat (Q). }\)

For the formation of CO2 from carbon and oxygen, the bonds between oxygen atoms in the oxygen molecules should be broken.

For the breaking of this bond, some amount of energy is absorbed. At the same time, some amount of energy is released during the formation of new bonds between carbon and oxygen in CO2. If the energy released is more than the energy required, during the reaction, the difference in energy is released as heat energy.

2. Catalytic Oxidation Of Sulphur Dioxide To Sulphur Trioxide:

∴ \(2 \mathrm{SO}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{SO}_3+\text { heat }(\mathrm{Q})\)

3. Reaction Of Quicklime With Water:

∴ \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2 \text { + heat }(\mathrm{Q})\)

4. Formation of ammonia from nitrogen and hydrogen:

∴ \(\mathrm{N}_2+3 \mathrm{H}_2 \rightarrow 2 \mathrm{NH}_3+\text { heat }(\mathrm{Q})\)

5. Combustion of methane:

∴ \(\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\text { heat }(\mathrm{Q})\)

6. Combustion of acetylene :

∴ \(2 \mathrm{C}_2 \mathrm{H}_2+5 \mathrm{O}_2 \rightarrow 4 \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\text { heat (Q) }\)

(this reaction is used for welding)

Examples Of Endothermic Reaction:

1. Passage of nitrogen and oxygen through electricity are:

∴ \(\mathrm{N}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{NO}-\text { heat (Q). }\)

For the formation of nitric oxide (NO) from nitrogen and oxygen, some amount of energy is required to break the bonds between nitrogen and oxygen atoms present in the respective molecules.

Some amount of energy is released during the formation of bonds between nitrogen and oxygen. In this case, the energy absorbed is more than the energy released. Hence, the difference in energy is absorbed as heat energy during the reaction.

2. Passage Of Steam Over Heated Coke:

∴ \(\mathrm{C}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CO}+\mathrm{H}_2 \text { – heat (Q) }\)

3. Formation Of Carbon Disulphide When Sulphur Vapour Is Passed Over Red Hot Charcoal:

∴ \(\mathrm{C}+2 \mathrm{~S} \rightarrow \mathrm{CS}_2-\text { heat (Q) }\)

4. Hydrogen gas and iodine vapour when flown over the hot platinum wire, hydroiodic acid formed:

∴ \(\mathrm{H}_2+\mathrm{I}_2 \rightarrow 2 \mathrm{HI} \text { – heat (Q) }\)

5. Crystals of hydrated barium hydroxide and solid ammonium chloride, when mixed in a beaker, produces a solid paste of barium chloride:

∴ \(\mathrm{Ba}(\mathrm{OH})_2 \cdot 8 \mathrm{H}_2 \mathrm{O}+2 \mathrm{NH}_4 \mathrm{Cl} \rightarrow \cdot \mathrm{BaCl}_2 \cdot 2 \mathrm{H}_2 \mathrm{O} \text { neat }(\mathrm{Q})\)

6. Making quicklime from limestone:

∴ \(\mathrm{CaCO}_3 \rightarrow \mathrm{CaO}+\mathrm{CO}_2-\text { heat }(\mathrm{Q})\)

We should mention here that during physical changes also heat may be evolved or absorbed, that is physical changes can also be exothermic or endothermic.

For example, when sulphuric acid or caustic soda is slowly added to water with stirring, it is a physical change and heat is evolved. Similarly, when ammonium chloride or ammonium nitrate is dissolved in water, there occurs absorption of heat.

Exothermic reactions are sometimes very much beneficial to us. The most important example is the burning of fuel. When fossil fuel (containing the element carbon) is burnt in the air,

It produces so much heat that this heat energy can be effectively utilized to cook food, heat water to steam which then can be used to rotate turbine blades to generate electricity, etc.

Burning of acetylene gas (C2H2) produces so much heat that temperature may rise above 2000°C and at this temperature, even iron melts.

So this exothermic reaction can be utilized for welding. At this high temperature, light is emitted which contains a significant amount of ultraviolet (UV) radiation which is very harmful to our eyes.

So use of a proper spectacle or protective goggles which can shield our eyes from UV radiation is strongly recommended during welding.

Exothermic reactions are responsible for explosive reactions. Here so much heat is produced in the chemical reaction that it can immediately heat up a confined gas.

As the temperature rises, the hot gas expands very rapidly and tries to escape, thus creating shock waves, which are responsible for the extensive damage during an explosion.

Oxidation

Let us consider the following chemical reactions:

1. Burning of coal in air or oxygen

∴ \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2\)

2. Burning of sulphur in air or oxygen

∴ \(\mathrm{S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2\)

3. A strip of metallic magnesium is ignited in oxygen

∴ \(2 \mathrm{Mg}+\mathrm{O}_2 \rightarrow 2 \mathrm{MgO}\)

4. Reaction between hydrogen and oxygen

∴ \(2 \mathrm{H}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}\)

In all these reactions, we find that one thing is common and that is in all these reactions oxygen is combined with another element or compound. These reactions are called oxidation.

Other examples of oxidation reactions can also be mentioned. Many metals and non-metals combine with oxygen to form oxide compounds.

When exposed to moist air, pure iron is converted into reddish-brown iron oxides, which are commonly known as rust.

But oxidation does not mean only a combination of oxygen with other elements. Additionally, the oxidation reaction can also be described as follows:

Reduction

Let us now discuss a reduction, which is the opposite of oxidation.

1. So, we can say that the reduction is a chemical reaction involving the removal of oxygen from a substance.
For example, when hydrogen gas is passed over hot, black, cupric oxide, reddish-brown metallic copper is formed as residue.

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction removal of oxygen

2. Reduction can also be defined as a chemical reaction in which the addition of hydrogen occurs with an element or a compound.

But oxidation does not mean only a combination of oxygen with other elements. Additionally, the oxidation reaction can also be described as follows:

1. Oxidation reaction is a chemical reaction in which chlorine is added.

For example, Chlorine combines with iron to produce ferric chloride.

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Addition of chlorine

2. Oxidation reaction can also be described as a chemical reaction in which hydrogen is removed from a substance.

For example: When hydrogen sulphide (H, S) gas is passed through chlorine water, some amount of sulphur is precipitated and hydrogen chloride (HCI) is produced.

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction removal of Hydrogen

For example, hydrogen combines with nitrogen to form ammonia at 550°C temperature and 200 atm pressure in presence of iron powder as a catalyst.

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Addition of hydrogen

A chemical reaction involving oxidation and reduction is usually called a redox reaction. On the basis of our discussion so far we can also say that oxidation and reduction occur simultaneously.

In every redox reaction, one substance is oxidised and at the same time, another substance in the reaction gets reduced. Here, oxygen is removed from cupric oxide (reduction) and oxygen is added to hydrogen to form water (oxidation).

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction removal of oxygen

Here, hydrogen is removed from H2S to form sulphur (oxidation) and hydrogen is added to chlorine to form hydrogen chloride (reduction).

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Addition of Hydrogen 2

Here oxygen is removed from zinc oxide and metallic zinc is formed (reduction). Simultaneously oxygen is added to carbon and carbon monoxide is formed (oxidation).

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction addition of oxygenf2

In this case, oxygen is added to carbon monoxide and carbon dioxide is formed (oxidation) and oxygen is removed from ferric oxide and metallic iron is formed (reduction).

Thus oxidation and reduction occur simultaneously. In every redox reaction, one substance is oxidised and at the same time, another substance in the reaction gets reduced.

Electronic Concept Of Oxidation And Reduction

So far we have discussed oxidation and reduction reactions in terms of the addition or removal of oxygen (or chlorine) and the removal or addition of hydrogen.

  1. To summarize, we can say that, oxidation is a chemical reaction, which involves,
  2. addition of oxygen or addition of chlorine
  3. elimination of hydrogen and reduction is a chemical reaction which involves,
  4. elimination of oxygen
  5. addition of hydrogen

But we will see later that not all chemical reactions involve only the addition/removal of oxygen or chlorine or hydrogen. So, for these reactions, we need an alternative definition.

Our objective is to define oxidation and reduction in such a way as to cover all the oxidation-reduction reactions.

Let us take an aqueous solution of copper sulphate in a beaker and a shining iron nail is immersed in it. The whole set-up is kept undisturbed for some time.

It is then found that the shining iron nail has been coated with a reddish-brown layer. This reddish-brown layer is of metallic copper.  If the aqueous solution is analyzed, it will confirm the presence of a ferrous ion (Fe2+). So, a chemical change has taken place.

The chemical reaction is as follows :

⇒ \(\mathrm{CuSO}_4+\mathrm{Fe} \rightarrow \mathrm{FeSO}_4+\mathrm{Cu}\)

In copper sulphate, copper exists as a cupric ion (Cu2+) whereas in ferrous sulphate, iron is present as the ferrous ion (Fe2+), Hence, the above reaction can be alternatively represented as follows:

∴ \(\mathrm{Cu}^{2+}+\mathrm{Fe} \rightarrow \mathrm{Cu}+\mathrm{Fe}^{2+}\)

Here, uncharged metallic iron (Fe) loses two electrons and becomes positively charged Fe2+and these two electrons are gained by positively charged Cu2+ and are converted into uncharged metallic copper (Cu).

∴ \(\begin{aligned}
& \mathrm{Fe}-2 \mathrm{e} \rightarrow \mathrm{Fe}^{2+} \\
& \mathrm{Cu}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Cu}
\end{aligned}\)

The above reaction is also an oxidation-reduction reaction but this reaction does not involve the addition/removal of oxygen or chlorine or hydrogen. This is explained in terms of the gain or loss of electron(s).

Oxidation is defined as a chemical reaction involving the loss of electrons from an atom or ion (de-electronation) and reduction is a chemical reaction involving the gain of electrons by an atom or ion (electronation).

The substance which accepts electron(s) is reduced. It is called the oxidising agent or oxidant. The substance that loses an electron(s) is itself oxidised. That is known as a reducing agent or reductant.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Agent or reductant

 

Another Example Can Be Discussed:

When some pieces of metallic zinc are dropped in an aqueous solution of copper sulphate, then after some time it is found that reddish-brown patches of metallic copper are deposited on the silver or grey-coloured pieces of zinc. Further analysis confirms the presence of Zn2+ ions in the aqueous solution.

The overall reaction is, \(\mathrm{CuSO}_4+\mathrm{Zn} \rightarrow \mathrm{ZnSO}_4+\mathrm{Cu}\)

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction gain of electrons and loss of electrons

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Catalytic reaction

When some pieces of metallic zinc are dropped into a dilute, aqueous solution of sulphuric acid, and taken in a test tube, a colourless, odourless gas evolves, which is hydrogen gas. Further analysis Here, metallic zinc loses two electrons and forms Zn2+.

In a dilute aqueous solution of sulphuric acid, hydrogen ion (H+) is present. H+ ion gains electrons and ultimately forms hydrogen gas ( H2).

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction zinc table-2

When colourless stannous chloride (SnCI2) solution is added to yellow ferric chloride solution as, (FeCI3), a faintly greenish solution of ferrous chloride (FeCI2) and stannic chloride (SnCI4) are produced.

This is an oxidation reaction and Zn is the reducing agent. This is a reduction reaction and H+ is the oxidizing agent The reaction can be alternatively represented as the overall reaction is, \(\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{ZnSO}_4+\mathrm{H}_2\)

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction gain of electronss-2

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction the over all reactions is

 

Oxidant And Reductant

Some substances (elements, ions, compounds etc) bring about the oxidation of (or oxidise) some other substances in some chemical reactions.

Similarly, certain substances bring about the reduction of (or reduce) some other substances. Hence oxidant and reductant are defined as-

Oxidant: The substance which, in a chemical reaction, acts as an agent to oxidise another substance and, at the same time is itself reduced, is called an oxidant.

Reductant: The substance which, in a chemical reaction, acts as an agent to reduce another substance and, at the same time is self-oxidised, is called a reductant.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 1

 

1. When hydrogen gas is passed over hot cupric oxide, it is reduced to metallic copper and hydrogen is oxidized to water.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 2

 

2. When lead oxide is reacted with carbon, lead oxide is reduced to lead metal and carbon is oxidized to carbon monoxide.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 3

 

3. When methane is burnt in oxygen (i.e. when complete combustion of methane occurs), methane is oxidized to carbon dioxide and oxygen is reduced to water.

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 4

 

4. When a dilute solution of hydrochloric acid is reacted with black-coloured manganese dioxide, HCI is oxidized to chlorine gas (which is pungent smelling). Manganese dioxide is reduced to form manganous chloride.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 5

 

5. When chlorine gas is passed through a colourless, aqueous solution of hydrogen bromide, hydrogen bromide is oxidised to bromine (which is a red-coloured liquid). Chlorine is reduced to hydrogen chloride.

 

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant 6

 

Thus, the main role of an oxidant or oxidising agent is to add oxygen to the substance to be oxidised, or to abstract hydrogen from the substance to be oxidised.

In electronic terms, the main role of an oxidant is to abstract electrons from the substance undergoing oxidation and then to get itself reduced by taking up those electrons.

Similarly, the main role of a reductant or reducing agent is to add hydrogen to the substance to be reduced or to abstract oxygen from the substance to be reduced.

In electronic terms, the chief function of a reductant is to give up electrons to the substance undergoing reduction and thus, to get itself oxidised.

Some Common Oxidant And Reductants:

WBBSE Solutions For Class 8 Chapter-2 Element, compound and chemical reaction sec-3 Chemical reaction Oxidation and reductant table

Instances Of Redox Reaction In Everyday Life

1. Fossil fuels such as coal, petrol, diesel etc., are oxidized in the air, producing heat and light. In all these reactions, the carbon present in the fossil fuel combines with aerial oxygen and produces carbon dioxide. The hydrogen in the fuel combines with oxygen to form water.

C (in the fuel) + O2→ CO2
Hydrogen in the fuel + O2→HO2

Both processes are oxidation reactions and the process of oxidation is called combustion.

Rusting: Rusting of iron is a common form of corrosion in which the metal is eaten up gradually due to oxidation of the metal by the action of air, moisture or a chemical (such as acid) on its surface.

When metallic iron is exposed to moist air (i.e., in the presence of oxygen and water), it is converted to hydrated iron oxide (Fe203.n H20) [where n is the number of water molecules].

This is brittle and the mechanical strength of metallic iron is absent in this hydrated iron oxide. In other words, metal is “degraded”.

This is an example of an oxidation reaction, where metallic iron is oxidized to hydrated iron oxide.

∴ \(4 \mathrm{Fe}+3 \mathrm{O}_2+2 \mathrm{nH}_2 \mathrm{O} \rightarrow 2 \mathrm{Fe}_2 \mathrm{O}_3 \cdot \mathrm{nH}_2 \mathrm{O}\)

Iron oxygen water hydrated iron oxide or rust.

The number of water molecules (n) in the rust varies, it is not fixed. Rusting involves unwanted oxidation of iron which occurs in nature on its own. It is a continuous process.

This single class of reaction is responsible for the destruction of various materials, instruments and infrastructures made of iron.

Everything made of iron and which is exposed to moist air is vulnerable to corrosion. Every year, crores of rupees are required for round-the-clock maintenance of costly instruments and infrastructures.

Corrosion control can be achieved by recognizing and understanding the corrosion mechanism, by using corrosion-resistant materials and designs and by using protective systems and devices and treatments.

For example, it is found that the more the exposed area of metallic iron is too moist air, the more the extent of rusting. Hence, if the exposed surface of iron is coated with some paints (such as coal tar or some synthetic paints) which prevent direct contact between iron and moist air, the rate of corrosion can be significantly reduced.

Probably this is the cheapest way to prevent corrosion Another more sophisticated way to prevent rusting is to coat an iron surface with another metal.

For example, metallic zinc can be electroplated on the iron surface to prevent rusting. The process of coating the surface of any substance with metallic zinc is known as galvanization.

We have mentioned that during rusting metallic iron is converted to hydrated iron oxide. Alternatively, we can say that Fe is converted to Fe2+ ion.

But we also know that oxidation and reduction occur simultaneously. So, if Fe-2e → Fe2+ is the oxidation reaction, which one is the reduction reaction? In fact, in an acidic solution, there are two types of reduction reactions, as follows:

∴ \(\begin{aligned}
& 2 \mathrm{H}^{+} \text {(aqueous) }+2 \mathrm{e} \rightarrow \mathrm{H}_2 \text { (gas) } \\
& 4 \mathrm{H}^{+} \text {(aqueous) }+\mathrm{O}_2 \text { (gas) }+4 \mathrm{e} \rightarrow 2 \mathrm{H}_2 \mathrm{O} \\
& \text { (liquid) }
\end{aligned}\)

A simple experiment can be performed to show that water and oxygen, both are necessary for the rusting of iron.

Experiment: Let us take three beakers. In the first beaker, some iron nails are placed and it is left open in the air for a few days.

In the second beaker, some normal water is taken. A few iron nails are immersed in it, and then some molten wax is poured into the beaker in such a way, so as to create a layer of wax on the surface of the water, which prevents the passage of air through it into the water.

In the third beaker, instead of normal water some amount of boiled water is taken and the iron nails are immersed in it. In this case, also, the surface of the water is covered with a layer of wax,

so that water does not come in direct contact with the air outside. The second and third beakers are also left undisturbed for a few days.

Observation: After some days, it will be found that the nails in the first beaker are rusted. Some rusting takes place in the nails kept in the second beaker. But no rusting takes place in the nails kept in the third beaker.

Inference: This is because the nails are in direct contact with moisture and oxygen present in the air in the first beaker.

So rusting occurs. In the second beaker, nails are in direct contact with water and oxygen (which remains dissolved in water under ordinary temperature and pressure).

So here also rusting of the nails occurs. But in the third beaker, boiled water is taken. When water is properly boiled, the dissolved oxygen is driven out.

So, in absence of any oxygen rusting does not occur, although the nails are in direct contact with water.
This conclusively proves that for rusting to occur, the presence of both water and oxygen is necessary.

3. We have always noticed that if coconut oil or mustard oil is exposed to air for a long time, the smell of the substance becomes very disagreeable.

This oil is called “rancid oil”. This occurs due to the oxidation of some compounds present in this oil. The moisture of the air also reacts with various components of this oil.

These reactions are responsible for the disagreeable smell. If some fresh fruit, for example, an apple, is cut and then left open in the air for a long time, brown patches develop on the exposed surface of the fruit.

This is also due to the oxidation of some substances present in the fruit by aerial oxygen. Brown patches are due to the formation of oxidized products.

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