Classification of chemical reactions

Types of chemical reactions based on chemical change- 10th CBSE- Physical science

Chemical reactions depict the chemical change from reactants to products. They are classified into 10 types based on the nature of reactants and reagents utilized.

This article explores these types of chemical reactions in detail, offering comprehensive examples. It is a crucial topic frequently featured in the 10th CBSE AISSE examinations, requiring students to grasp every aspect of a chemical reaction type, including the chemical transformation and reagents involved, along with suitable reaction conditions.

To aid in understanding, our blog post covers essential questions with detailed explanations, aiming to enhance proficiency in this topic and achieve high scores in chemistry.

What is meant by a chemical reaction?

As commonly understood, a chemical reaction entails a chemical change whereby reactants undergo transformation into products.

Reactants serve as the substances initiating the chemical reaction, with the bonds within their molecules being broken, leading to the formation of new chemical compounds.

Products, on the other hand, are the newly formed chemical substances resulting from the reaction of reactants, possessing distinct chemical and physical properties. They represent the newly bonded compounds arising from the chemical change in the reactants.

Categorization of Chemical Reactions:

Chemical reactions are classified based on the chemical changes, which are influenced by factors such as the physical states of reactants, energy involvement or release, catalysts, and the required temperature and pressure. 

Besides these factors, there are numerous other variables that can impact the chemical change or reaction rate. Various types of chemical reactions include:

1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Double displacement reaction
5. Precipitation reaction
6. Oxidation reaction
7. Reduction reaction
8. Redox reaction
9. Exothermic reaction
10. Endothermic reaction

Combination reaction:

When two or more substances (elements or compounds) unite to produce a single product, the reaction is termed a combination reaction.

Examples of combination reactions are;

(i) Burning of coal

Carbon + Oxygen -----------→ Carbon dioxide

C (s) + O₂ (g) ------------→ CO₂ (g)

(ii) Formation of sulphur dioxide

Sulphur + Oxygen --------------→ Sulphur dioxide

S (s) + O₂ (g) ----------→ SO₂ (g)

(iii) Formation of water

Hydrogen + Oxygen -----------→ Water

2H₂ (g) + O₂ (g) ------------→ 2H₂O (l)

(iv) Formation of hydrochloric acid

Hydrogen + Chlorine -----------→ Hydrochloric acid

H₂ (g) + Cl₂ (g) ------------→ 2HCl (g)

(v) Washing soda formation

Sodium carbonate + water ----------→ Washing soda

Na₂CO₃ (s) + 10 H₂O (l) -------------→ Na₂CO₃.10H₂O (s)

Important question & answers on combination reaction:

1. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

 

Answer:

The element ‘X’ is Copper (Cu)

The black coloured compound formed is Copper oxide.

Copper + Oxygen -----------→ Copper Oxide

2Cu (s) + O₂(g) -----------→ 2CuO (s)

At room temperature, copper appears as a brown-colored solid. Upon exposure to air, the copper metal undergoes a reaction with atmospheric oxygen, resulting in the formation of a black-colored copper oxide layer on its surface, giving it a black appearance.

Since copper oxide is produced through the combination of copper and oxygen, it serves as an instance of a chemical combination reaction.

Decomposition reaction:

Decomposition reactions involve the breakdown of a single substance into two or more substances.  

Examples of decomposition reactions are;

(i) Heating of Ferrous sulphate

Ferrous sulphate ------------→ Iron (III) oxide + Sulphur dioxide + Sulphur trioxide

                          Heat

2FeSO₄ (s) -----------------→ Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

(ii) Formation of Silver metal from AgCl

Silver chloride ----------------→ Silver + Chlorine

                     Sun light

2AgCl (s) ------------------→ 2Ag (s) + Cl₂ (g)

(iii) Decomposition of Zinc carbonate

Zinc carbonate -------------→ Zinc oxide + Carbon dioxide

ZnCO₃ (s) ---------------→ ZnO (s) + CO₂ (g)

Types of decomposition reactions:

Decomposition reactions necessitate energy, whether in the form of heat, light, or electricity, to break down the bonds in reactants.

Accordingly, decomposition reactions can be classified into three types.

1. Thermal decomposition
2. Electrolytic decomposition
3. Photolytic decomposition

Thermal decomposition:

When a decomposition reaction occurs through heating, it is referred to as thermal decomposition reaction. Such reactions are endothermic, involving the absorption of heat energy.

Calcium carbonate --------------→ Calcium oxide + Carbon dioxide

                         Δ

CaCO₃ (s) ------------→ CaO (s) + CO₂ (g)

Copper nitrate -----------------→ Copper Oxide + Nitrogen dioxide + Oxygen

                               Heat

2Cu (NO₃)₂ (s) -------------→ 2CuO (s) + 4NO₂ (g) + O₂ (g)

Blue colour                             Black       Brown fumes

Electrolytic decomposition:

Electrolytic decomposition refers to the process of decomposition conducted by passing electricity through a substance.

Water --------------→ Hydrogen + Oxygen

                  Electric current

2H₂O (l) ------------------------→ 2H₂ (g) + O₂ (g)

This process is known as electrolysis of water

Photolytic decomposition:

Photolytic decomposition is the term used to describe decomposition that occurs in the presence of sunlight.

Silver bromide -----------------→ Silver + Bromine

                        Sunlight

2AgBr (s) ----------------------→  2Ag (s)    +   Br₂ (g)

Pale yellow                                white gray    dark reddish brown

Important question & answers on decomposition reaction:

1. Why are decomposition reactions called the opposite of combination reactions? Write equations for the reactions?

Answer:

A combination reaction occurs when two or more simpler chemical substances combine to form a single giant molecule, as seen in the formation of limestone.

Calcium oxide + Carbon dioxide -------------→ Calcium carbonate

CaO (s) + CO₂ (g) --------------→ CaCO₃ (s) 

Conversely, a decomposition reaction involves a single reactant molecule breaking down to yield two or more simpler products, as seen in the decomposition of limestone.

Calcium carbonate -----------------→ Calcium oxide + Carbon dioxide

CaCO₃ (s) -----------------→ CaO (s) + CO₂ (g)    

Upon closer examination of both examples, it becomes evident that the formation and decomposition of limestone are inversely related. Therefore, it can be concluded that a decomposition reaction is the opposite of a combination reaction.

2. Differentiate between combination and decomposition reactions

Answer:

Decomposition reactions occur when a single substance breaks down to yield two or more products. These reactions require energy absorption, such as light, electric current, or heat, as they involve bond breaking within the reactant molecule to form simpler product molecules.

Calcium carbonate -----------------→ Calcium oxide + Carbon dioxide

                         Heat

CaCO₃ (s) ----------------→ CaO (s) + CO₂ (g)    [Endothermic reaction]

Lime stone                       Quick lime

On the other hand, combination reactions involve the union of two simple chemical substances to form a single product molecule. This process entails the formation of new bonds between the reactant molecules, resulting in the creation of a single giant molecule. Combination reactions may involve either energy absorption or emission, making them either endothermic or exothermic.

Calcium oxide + water --------------→ Calcium hydroxide 

CaO (s) + H₂O (l) -------------→ Ca (OH)₂ (aq) + heat          [Exothermic reaction]

Nitrogen + Oxygen ----------------→ Nitric oxide

                   Heat

N₂ + O₂ -------------→ 2NO        [Endothermic reaction]

3. Define a decomposition reaction. How can we say that (i) electrolysis of water (ii) blackening of silver bromide when exposed to sun light are decomposition reactions? Mention the type of energy involved in each case

[2024, AISSE]

Answer:

In a decomposition reaction, a single chemical compound undergoes breakdown into two or more substances, and these reactions progress with the absorption of various forms of energy such as electric current, sunlight, and heat, depending on the nature of the reactants involved.

During electrolysis, the decomposition of a water molecule into hydrogen and oxygen gases occurs upon passing an electric current through it.

Water ---------------→ Hydrogen + Oxygen

                   Electric current

2H₂O (l) ------------------------→ 2H₂ (g) + O₂ (g)             

    

The use of electric current in breaking down the water molecule characterizes the process as electrolytic decomposition.

Similarly, when a pale yellow-colored silver bromide is exposed to sunlight, it transforms into gray-colored silver metal, accompanied by the release of bromine gas.

Silver bromide -----------------→  Silver + Bromine

                        Sunlight

2AgBr (s) ----------------------→ 2Ag (s) + Br₂ (g)

Pale yellow                                white gray    dark reddish brown

As sunlight is utilized in the decomposition of AgBr, the process is termed photolytic decomposition.

4. Write down the names of elements used in black and white photography? Write a balanced chemical equations for them

Answer:

Silver halides like AgBr and AgCl find application in black and white photography due to their photosensitivity.

Either silver chloride or silver bromide serves as components of the photographic emulsion, which is suspended in gelatin to aid in the development of photographic images.

Due to their sensitivity to light, these silver halides undergo decomposition to produce gray-colored silver metal, along with the release of the corresponding halogen.

Silver Chloride --------------------→ Silver + Chlorine

                     Sun light

2AgCl (s) ------------------→ 2Ag (s) + Cl₂ (g)

White                                 Gray        Greenish yellow

Silver bromide ----------------→ Silver + Bromine

                        Sunlight

2AgBr (s) ----------------------→ 2Ag (s) + Br₂ (g)

Pale yellow                              white gray    dark reddish brown

Displacement reaction:

The chemical reaction where one element displaces another element from its solution. It involves a more reactive element replacing a less reactive element from its salt solution. 

Consequently, this technique is employed in the refining process of certain less reactive elements.

Examples of displacement reactions:

Zinc + Copper sulphate -----------------→ Zinc sulphate + Copper

Zn (s) + CuSO₄ (aq) ---------------→ ZnSO₄ (aq) + Cu (s)

Lead + Copper chloride ------------→ Lead chloride + Copper

Pb (s) + CuCl₂ (aq) -----------------→ PbCl₂ (aq) + Cu (s)

Due to their higher reactivity compared to copper, zinc and lead are capable of displacing copper from its compounds. 

However, copper, being less reactive than zinc, cannot displace zinc from its salt solution. Therefore, no displacement reaction occurs in this scenario.

Copper + Zinc sulphate ---------------→ No reaction

Cu (s) + ZnSO₄ (aq) ---------------→ No reaction

A similar principle applies to low-reactive metals like silver and gold, which are unable to react with acids and water by displacing hydrogen due to their lower reactivity compared to hydrogen.

Important question and answers on displacement reaction:

1. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

 

Answer:

Copper metal, owing to its higher reactivity compared to silver, finds application in the extraction and recovery process of silver from silver nitrate solutions.

Copper + Silver nitrate ----------------→ Copper nitrate + Silver

Cu (s) + AgNO₃ (aq) ---------------→ CuNO₃ (aq) + Ag (s)

Double displacement reaction:

A double displacement reaction occurs when two different atoms or groups of atoms exchange places with each other

Examples of double displacement reaction:

Sodium sulphate + Barium chloride ----------------→ Barium sulphate + Sodium chloride

Na₂SO₄ (aq) + BaCl₂ (aq) ---------------→ BaSO₄ (s) + 2 NaCl (aq)

Sodium chloride + Silver nitrate ----------------→ Silver chloride + Sodium nitrate

NaCl (aq) + AgNO₃ (aq) ---------------→ AgCl (s) + NaNO₃ (aq)

Sodium carbonate + Calcium chloride --------------→ Calcium carbonate + Sodium chloride

Na₂CO₃ (aq) + CaCl₂ (aq) --------------→ CaCO₃ (s) + 2NaCl (aq)

In all the above reactions, the mutual exchange of ions take place between the reactant molecules.

Important question and answers on double displacement reaction:

1. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

In displacement reactions, highly reactive metals displace less reactive metals in accordance with the reactivity series, resulting in a change in the position of a single atom or ion.

 

Iron + Copper sulphate ----------→ Iron sulphate + Copper

Fe (s) + CuSO₄ (aq) -----------------→ FeSO₄ (aq) + Cu (s)

Grey      Blue                                        Green            Brown   

 

On the other hand, in double displacement reactions, there is a mutual exchange in the positions of all the atoms or ions involved, which does not follow the reactivity series.

 

Sodium sulphate + Strontium chloride -------------------→ Strontium sulphate + Sodium chloride

Na₂SO₄ (aq) + SrCl₂ (aq) ---------------→ SrSO₄ (s) + 2 NaCl (aq)

                                                                     White color

2. Are all double displacement reactions precipitation reactions? Justify your answer

 

Answer:

The majority of double displacement reactions result in the formation of a precipitate, which is a water-insoluble compound. However, there are exceptions, such as neutralization reactions. When an acid reacts with a base, it produces salt and water.

 In this reaction, the anions mutually exchange positions between the two cations without forming a precipitate. Therefore, it can be concluded that not all double displacement reactions are precipitation reactions.

 

Hydrochloric acid + Sodium hydroxide -----------------→ Sodium chloride + water

HCl (aq) + NaOH (aq) -----------------→ NaCl (aq) + H₂O (l)

                                                           Water soluble salt

 

Precipitation reaction:

Precipitation reactions occur when water-insoluble compounds, known as precipitates, are formed.

In these reactions, two anions exchange positions among the cations, leading to the creation of water-insoluble salts that settle at the bottom of the reaction flask.

 

Examples of precipitation reaction:

 

Lead nitrate + Potassium iodide -----------------→ Potassium nitrate + Lead iodide

Pb (NO₃)₂ (aq) + 2KI (aq) ---------------------→ 2KNO₃ (aq) + PbI₂ (s)↓

                                                                                         Yellow colour precipitate

 

Potassium bromide + Barium iodide ----------------→ Barium bromide + Potassium iodide

2KBr (aq) + BaI₂ (aq) ------------------→ BaBr₂ (s) ↓ + 2KI (aq)

                                                   White colour precipitate

 

Important question and answers on precipitation reaction:

 

1. On adding a drop of barium chloride solution to an aqueous solution of sodium sulphate, white precipitate is obtained.

(a) Write a balanced chemical equation of the reaction involved

(b) What other name can be given to this precipitation reaction?

 

Answer:

Upon mixing an aqueous solution of sodium sulfate with barium chloride, a white precipitate of barium sulfate is obtained.

 

Barium chloride + Sodium sulphate ------------------→ Barium sulphate + Sodium chloride

BaCl₂ (aq) + Na₂SO₄ (aq) -----------------→ BaSO₄ (s) ↓ + 2 NaCl (aq)

                                                                   White precipitate

 

In this reaction, the sulfate ion and chloride ion exchange positions between sodium and barium ions, leading to the formation of the precipitate. Given that the reaction involves the exchange of positions between two atoms or ions, it is classified as a double displacement reaction.

2. Translate the following statement into chemical equation and then balance it 

Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. 

State the two types in which this reaction can be classified

Answer:

Barium chloride + Aluminium sulphate ----------------→ Barium sulphate + Aluminium chloride

3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) --------------------→ 3BaSO₄ (s) ↓ + 2AlCl₃ (aq)

                                                                             White precipitate    

         

Since the reaction results in the formation of a water-insoluble salt, it is termed a precipitation reaction. Additionally, because the reaction involves the exchange of ion positions among the cations, it is referred to as a double displacement reaction.

Oxidation reaction:

An oxidation reaction occurs when a substance gains oxygen or loses hydrogen. The substance that either provides oxygen or removes hydrogen in such a reaction is termed as an oxidizing agent.

 

Examples of Oxidation reaction:

 

Copper + Oxygen ------------→ Copper oxide

2Cu (s) + O₂ (g) -------------→ 2CuO (s)

 

Sodium + Oxygen ------------→ Sodium oxide

4Na (s) + O₂ (g) -------------→ 2Na₂O (s)

 

Aluminium + Oxygen ------------→ Aluminium oxide

4Al (s) + 3O₂ (g) -------------→ 2Al₂O₃ (s)

 

Carbon+ Oxygen ---------------→ Carbon dioxide

C (s) + O₂ (g) ----------------→ CO₂ (g)

 

Magnesium + Oxygen ------------→ Magnesium oxide

2Mg (s) + O₂ (g) -------------→ 2MgO (s)

 

Important question and answers on oxidation reaction:

 

1. Why combustion reactions are classified as oxidation reactions?

 

Answer:

All combustion reactions occur in the presence of atmospheric oxygen, leading to the addition of oxygen to a substance. Consequently, these reactions are classified as oxidation reactions.

 

Methane + Oxygen ---------------→ Carbon dioxide + water

CH₄ (g) + 2O₂ (g) -----------------→ CO₂ (g) + 2H₂O (g) 

Reduction reaction:

A reduction reaction occurs when a substance gains hydrogen or loses oxygen. 

The substance that either donates hydrogen or removes oxygen in a reduction reaction is referred to as a reducing agent. In simpler terms, the substance undergoing oxidation acts as the reducing agent.

Examples of reduction reaction:

Carbon + Hydrogen → Methane

C (s) + 2H₂ (g) → CH₄ (g)

 

Nitrogen + Hydrogen → Ammonia

N₂ (g) + 3H₂ (g) → 2NH₃ (g)

Important question and answers on reduction reaction:

1. Provide two examples of both oxidizing and reducing agents.

Answer:

An oxidizing agent is a substance that either provides oxygen or removes hydrogen in an oxidation reaction. Examples of oxidizing agents include alkaline potassium permanganate and acidified potassium dichromate solutions.

Conversely, a reducing agent is a substance that either provides hydrogen or removes oxygen in a reduction reaction. Examples of reducing agents are hydrogen in the presence of nickel, palladium, and platinum catalysts, as well as metals, particularly alkali metals like lithium, sodium, and potassium.

2. Identify the substance that is reduced in the following reaction

ZnO + CO → Zn + CO₂

Answer:

In the given reaction, zinc undergoes reduction as oxygen is removed from zinc oxide.

Redox reaction (Oxidation - reduction reaction):

An oxidation-reduction reaction, also known as a redox reaction, involves one reactant undergoing oxidation while the other undergoes reduction simultaneously.

Oxidation-reduction reactions involve the addition or removal of oxygen or hydrogen.

 

Examples of redox reaction:

 

Zinc oxide + Carbon → Zinc + Carbon monoxide

ZnO + C → Zn + CO

In the provided reaction, carbon is oxidized to carbon monoxide, and zinc oxide is reduced to zinc.

 

Manganese oxide + hydrochloric acid → Manganese chloride + Water + Chlorine

MnO₂ + 4 HCl → MnCl₂ + 2H₂O + Cl₂

In the given reaction, hydrochloric acid undergoes oxidation, losing a hydrogen atom to form chlorine, while manganese oxide is reduced to manganese chloride.

 

Copper oxide + Hydrogen → Copper + Water

CuO + H₂ → Cu + H₂O

Similarly, in the reaction, copper oxide is reduced to copper, and hydrogen is oxidized to water.

 

Important question and answers on redox reaction:

 

1. Can a displacement reaction be a redox reaction? Explain with the help of an example

 

Answer:

In redox reactions, oxidation and reduction occur simultaneously. Oxidation involves the loss of electrons, while reduction involves the gain of electrons. In redox reactions, electrons are exchanged between metals of different reactivities. 

This concept is evident in displacement reactions, where a highly reactive metal displaces a less reactive metal from its salt solution. As a result, the highly reactive metal loses electrons and undergoes oxidation, while the less reactive metal gains these electrons and undergoes reduction. 

This leads to an increase in the oxidation state of the highly reactive metal and a decrease in the oxidation state of the less reactive metal. Therefore, displacement reactions can be classified as redox reactions.

For example:

Copper sulphate+ Zinc -----------------→ Zinc sulphate + Copper 

CuSO₄ + Zn ----------------→ ZnSO₄ + Cu

In the reaction where zinc displaces copper from its salt solution, zinc undergoes oxidation, changing its oxidation state from 0 to +2. 

Meanwhile, copper is reduced, accepting the electrons lost by zinc and changing its oxidation state from +2 to 0. 

Since both oxidation and reduction occur simultaneously in this example, it is classified as a redox reaction.

2. Identify the substances that are oxidized and the substances that are reduced in the following reactions

(i) 4Na (s) + O₂ (g) --------------→ 2Na₂O (s)

(ii) CuO (s) + H₂ (g) -------------→ Cu (s) + H₂O (l) 

Answer: 

In the first question, sodium is undergoing oxidation as it gains oxygen to form sodium oxide.

In the second question, copper oxide is losing oxygen to form copper, while hydrogen is gaining oxygen to form water. Therefore, copper oxide is being reduced to copper, and hydrogen is being oxidized to water.

3. In what way are redox reactions employed to produce electrical energy in batteries?

Answer:

The generation of electricity in batteries exemplifies a redox reaction, where the flow of electrons occurs between the cathode and anode, enabling electrical conduction. 

At the anode, a substance loses electrons and undergoes oxidation, while at the cathode, a substance gains electrons and undergoes reduction. To maintain electron flow balance, charged ions also migrate through an electrolyte solution connected to both electrodes.

Batteries consist of separate anode and cathode compartments containing solutions where oxidation and reduction reactions take place. These compartments are linked by a salt bridge and a conductive wire.

In lead storage batteries, the anode comprises lead, while the cathode consists of lead oxide-filled lead grids. Dilute sulphuric acid serves as the electrolytic solution.

The overall reaction can be summarized as follows:

Lead + Lead oxide + dilute sulphuric acid ------------------→ Lead sulphate + Water

Pb(s) + PbO₂(s) + H₂SO₄(aq) --------------------→ 2PbSO₄(s) + 2H₂O(l) 

At the anode: 

Solid lead oxidizes in the presence of sulphuric acid to produce lead sulphate and release two electrons.

 Pb(s) + SO₂⁻⁴ (aq) ------------→ PbSO₄(s) + 2e⁻ 

At the cathode: 

Solid lead oxide undergoes reduction in the presence of sulphuric acid, forming lead sulphate and water.

 PbO₂(s) + SO₂⁻⁴(aq) + 4e⁻+ 2H⁺ ---------------→PbSO₄(s) + 2H₂O(l)

Exothermic reaction:

These chemical reactions involve the release of heat upon the formation of products. Examples of exothermic reactions include the burning of candle or sugar, rusting of iron, and the digestion of food in our bodies.

Additionally, the decomposition of vegetable matter into compost is an exothermic process, characterized by the liberation of heat. Initially, bacteria break down large vegetable matter into compost, requiring some energy for bond breaking. However, the overall decomposition reaction results in the release of a significant amount of heat energy.

Examples of exothermic reaction:

Burning of natural gas

Methane + Oxygen ------------→ Carbon dioxide + water + energy

CH₄ (g) + 2O₂ (g) ------------→ CO₂ (g) + 2H₂O (g) + 891 KJ/mol

Combustion of ethane

Ethane + Oxygen ----------------→ Carbon dioxide + water + energy

2C₂H₆ (g) + 7O₂ (g) ------------→ 4CO₂ (g) + 6H₂O (g) + 1560 KJ/mol

Important question and answer on exothermic reaction:

1. Why is respiration considered an exothermic reaction? Explain

Answer: 

It is common knowledge that energy is essential for our survival, obtained from the food we consume. Upon digestion, carbohydrates are broken down into glucose, which then reacts with oxygen within our body's cells to generate energy. 

This process is referred to as respiration, and it is characterized by the release of heat energy, making it an exothermic reaction.

Glucose + Oxygen -----------→ Carbon dioxide + water + energy

C₆H₁₂O₆ (aq) + 6O₂ (aq) ------------------→ 6CO₂ (aq) + 6H₂O (l) + energy

Endothermic reaction:

Endothermic reactions are those that require heat energy for product formation. This energy is utilized to break the bonds between reactant molecules. Therefore, all thermal decomposition reactions are endothermic.

Endothermic reactions absorb heat from the surroundings, resulting in a decrease in temperature and producing a cooling effect.

For example, when barium hydroxide reacts with ammonium chloride, it absorbs heat. Barium hydroxide is a strong base, fully ionizing into Ba²⁺ and OH^- ions in water. In contrast, ammonium chloride is a weakly acidic salt that ionizes into NH₄⁺ and Cl^- ions in water, absorbing heat energy in the process.

Barium hydroxide + Ammonium chloride ------------→ Barium chloride + Ammonium hydroxide

Ba(OH)₂ + 2NH₄Cl -------------→ BaCl₂ + 2NH₄OH

This overall double displacement reaction is an acid-base reaction that can cause a temperature drop of -20 degrees Celsius. As a result, the bottom of the test tube containing the reaction feels cold to the touch.

Examples of endothermic reaction:

Lead nitrate + heat ---------------→ Lead oxide + Nitrogen dioxide + Oxygen

2Pb(NO₃)₂ (s) + heat -------------------→ 2PbO (s)  + 4NO₂ (g) + O₂ (g)           

 

Glucose + water + heat --------------→ Glucose solution

C₆H₁₂O₆ (s) + H₂O + heat ----------------→ C₆H₁₂O₆ (aq)

 

Zinc carbonate + heat --------------→ Zinc oxide + Carbon dioxide

ZnCO₃ + heat -----------------→ ZnO + CO₂

 

Nitrogen + Oxygen + heat ----------------→ Nitric oxide

N₂ + O₂ + heat ------------------→ 2NO

Important question and answer on endothermic reaction:

1. The type of reactions in which (i) calcium oxide is formed and (ii) calcium hydroxide is formed are opposite reactions to each other. Justify this statement with the help of chemical equations

[2024, AISSE] 

Answer:

Calcium carbonate undergoes thermal decomposition upon heating to produce calcium oxide and carbon dioxide. This reaction is endothermic, involving the absorption of heat energy.

 

The equation representing this process is:

Calcium carbonate + heat → Calcium oxide + Carbon dioxide

CaCO₃ (s) + heat → CaO (s) + CO₂ (g)

 

Furthermore, calcium oxide reacts vigorously with water to yield slaked lime (calcium hydroxide), releasing a substantial amount of heat. This reaction, a combination reaction, is exothermic.

The equation for this reaction is:

Calcium oxide + Water → Calcium hydroxide

CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat

 

As exothermic and endothermic reactions are opposites, the formation of calcium oxide and calcium hydroxide represents opposite reactions.

Corrosion:

When a metal reacts with substances in its environment, such as moisture or acids, it undergoes a process known as corrosion. Corrosion is detrimental to metallic objects, particularly those made of iron.

Iron objects, initially shiny when new, develop a reddish-brown powder over time due to corrosion, a phenomenon commonly referred to as rusting.

 

4Fe (s) + 3O₂ (g) + 2x H₂O (aq) ----------------------→ 2 Fe₂O₃. xH₂O (aq)

                                                                                               Rust

One advantage of rusting, particularly in the case of iron, is its role in facilitating the transportation of oxygen in the body. The corrosion of iron aids in maintaining the red coloration of blood cells.

There are two primary methods to prevent rusting: galvanization, which involves applying a protective zinc coating to iron materials, and painting.

Additional examples of corrosion include the black coating observed on silver and the greenish coating found on copper.

Chemical equation for the formation of silver sulphide is;

Silver + Oxygen ---------------------→ Silver Oxide

4Ag + O₂ ------------------→ 2Ag₂O

Silver oxide + Hydrogen Sulphide ------------------------→ Silver sulphide + water

Ag₂O + H₂S --------------------→ Ag₂S + H₂O

                                                     Black

Chemical equation for tarnishing of copper is;

Copper + Oxygen + Water + Carbon dioxide -----------------------→ Copper carbonate and copper hydroxide

2Cu + O₂ + H₂O + CO₂ -------------------------→ CuCO₃ . Cu(OH)₂

                                                                                       Green 

Important questions and answers on corrosion:

1. Why do we apply paint on iron articles?

Answer:

Iron objects are susceptible to corrosion when exposed to atmospheric moisture and oxygen. Rusting of iron results in the formation of a reddish-brown powder known as iron oxide.

To prevent rusting, a protective layer of paint is applied to iron articles. This layer acts as a barrier, shielding the iron from exposure to atmospheric oxygen and moisture, thereby inhibiting oxidation. In this way, painting serves as an effective method to prevent the rusting of iron.

2. Anjana has been collecting silver coins and copper coins. One day she observed a black coating on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coating? Write the chemical name of black and green coatings?

Answer:

Corrosion, a chemical process, is accountable for the tarnishing of silver and copper. When these metals come into contact with atmospheric moisture, oxygen, or acids, they undergo oxidation, leading to corrosion. 

The black tarnish on silver coins results from the formation of silver sulfide when exposed to oxygen and hydrogen sulfide. 

Similarly, the greenish coating on copper coins indicates the formation of copper carbonate and copper hydroxide upon exposure to moisture, oxygen, and carbon dioxide.

Rancidity:

When fats and oils undergo oxidation, they undergo a process called rancidity, altering their smell and taste. To prevent this, substances known as antioxidants are often added to food products containing fats and oils. 

Storing food in airtight containers can also slow down the oxidation process. Therefore, chip manufacturers typically flush bags of chips with gases like nitrogen to inhibit oxidation and maintain freshness.

Important question and answer on rancidity:

1. What happens when food materials containing fats and oils are left for a long time. List two observable changes and suggest three ways by which this phenomenon can be prevented?

Answer:

When food items containing fats and oils are stored for extended periods, they may become rancid, altering their smell and taste.

To prevent this occurrence, three methods are commonly employed:

1. Incorporating antioxidants into food products containing fats and oils.
2. Storing such food items in airtight containers to minimize contact with atmospheric oxygen.
3. Using nitrogen during the storage of food items to hinder their oxidation process.