Class 10 Science Chapter 1

Some other examples of combination reactions are discussed below.

1. Combination of two elements

On heating, magnesium combines with oxygen present in the air to form magnesium oxide.

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

Hydrogen and oxygen combine to form water.

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

2. Combination of two compounds

Calcium oxide, also known as quick lime, when mixed with water reacts with it to form calcium hydroxide, also known as slaked lime. The chemical equation for the same is given as:

Hence, in combination reactions, two or more compounds combine to produce only one product. Generally, combination reactions are exothermic in nature i.e. energy is released when two or more compounds combine. This can be explained with the help of the given activity.

there are very few combination reactions which are endothermic in nature. One of the examples of such a reaction is combination of nitrogen and oxygen gas to form nitrogen dioxide gas:

N
2(g)+O2(g)→2NO(g)NitrogenOxygenNitricoxide

In this reaction, reactants absorb energy from the surroundings in order to form product.

What happens when coal is burned? 

On burning, coal combines with oxygen to produce carbon dioxide. It also gives a lot of heat energy. Hence, burning of coal is an exothermic reaction.

DO YOU KNOW? Lime water or slaked lime (Ca(OH)2) is used in white washing of walls. It combines with carbon dioxide present in the air to form a thin layer of calcium carbonate. The chemical formula of calcium carbonate is CaCO3. The chemical equation involved in the reaction can be represented as: Ca(OH2(aq)+CO2(g)→CaCO3(s)+H2O(l)

Question. Define a combination reaction. Give one example of a combination reaction which is also exothermic.

(2 marks) 2009 CBSE Delhi

Sol: In combination reactions, two or more substances combine to form a new compound. Only one product is obtained in such reactions. The reactants in such reactions can be elements as well as compounds. The general equation used to represent a combination reaction is:

A + Z AZ

For example, calcium oxide reacts vigorously with water to produce calcium hydroxide.

CaO(s)    +     H2O(l)→Ca(OH)2(aq)Calciumoxide Water        Calciumhydroxide

A large amount of heat is also evolved during this process, which increases the temperature of the system. Hence, the combination of calcium oxide and water is exothermic in nature.

What are decomposition reactions?

In these reactions, a compound breaks down or decomposes to form two or more substances. These reactions are exactly opposite to combination reactions. We know that there is only one product in combination reactions. Similarly, there is only one reactant in decomposition reactions. The general equation used to represent a decomposition reaction is:

Decomposition reactions require a source of energy in the form of heat, light, or electricity to decompose the compound involved. Hence, these reactions can be classified into three types, depending on the source of energy for the reaction.

a) Decomposition by heat or thermal decomposition

b) Decomposition by electricity or electrolysis

c) Decomposition by light or photolysis

Let us now study three different types of decomposition reactions.

a) Decomposition by heat or thermal energy

One of the most common examples of thermal decomposition reactions is the decomposition of calcium carbonate. Calcium carbonate when heated decomposes to form calcium oxide and carbon dioxide.

In this reaction, one compound i.e. calcium carbonate breaks down to form two compounds, namely calcium oxide and carbon dioxide. Hence, it is an example of decomposition reactions. Commercially, this reaction is very important as calcium oxide (obtained as a product in this reaction) is used in cement and glass industries.

Hands-on Activity

Take about 3 g of solid lead nitrate in a boiling tube. Note the colour of the compound. Heat it in the flame of the Bunsen burner. Observe the change taking place. You will observe that emission of brown fumes occurs. These fumes are of nitrogen dioxide. During this reaction, lead nitrate decomposes to form lead oxide, nitrogen dioxide, and oxygen gas. The following reaction takes place: 2Pb(NO3)2 (s)  →  2PbO (s)   +    4NO2 (g)    +   O2 (g) Lead nitrate        Lead oxide   Nitrogen dioxide   Oxygen

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Taj Mahal is made up of marble. Do you know that chemically, marble is nothing but calcium carbonate?

Question.

(a) What is the colour of ferrous sulphate crystals? How does this colour change after heating?

(b) Name the products formed on strongly heating ferrous sulphate crystals.

(2 marks) 2009 CBSE Delhi

Sol: a) The colour of ferrous sulphate crystals is green.

On heating, ferrous sulphate crystals (FeSO₄.7H₂O) lose their water of crystallisation and due to this, the colour of the compound changes to white/colourless.

(b) On strong heating, ferrous sulphate crystals give ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃) as products.

In displacement reactions, a more reactive metal replaces a less reactive metal from the latter’s salt.

Reactions in which a more reactive element replaces a less reactive element from the salt solution of the less reactive element are called displacement reactions. 

you know that displacement reactions are of two types? They are:

1. Single Displacement Reactions
2. Double Displacement Reactions

Single Displacement Reactions can be better understood with the help of the following figure.

In the above figure, you have three blocks. It will be observed that while red and blue blocks are fixed in, green block is aloof. Now, if a blue block is detached from the red and fixed with the green, it will mean that the green block displaces the red block.

Thus, in a single displacement reaction, an uncombined single element replaces the other element present in a compound.

 Double Displacement Reaction is of three types.

• Precipitation reaction

In precipitation reaction, soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of the solution as a solid. This insoluble compound is called a precipitate.

Example:

If an aqueous solution of sodium sulphate is mixed with barium chloride, it will be observed that a white insoluble substance is formed. The white insoluble substance is called a precipitate. Here, barium chloride reacts with sodium sulphate to produce barium sulphate (white insoluble precipitate) and sodium chloride. Thus, this is an example of a double displacement reaction. The chemical equation involved in the reaction is

You must have observed that when butter is kept in the open for a long time, it becomes rancid. Also, its smell and taste undergo a change.

Do you know why?

This is because butter undergoes oxidation i.e. it reacts with oxygen and gets oxidized. This process is called rancidity.

What is Oxidation?

Oxidation is defined as a process that involves a gain of oxygen or a loss of hydrogen. When a substance gains oxygen or loses hydrogen during a reaction, it is oxidized.

Let us perform an activity to understand more about these reactions.

Activity:

Take around 1g copper powder (reddish brown in colour) in a china dish and heat it over a burner (as shown in the given figure).

What do you observe?

It will be observed that after some time, the surface of the powder is covered by the layer of a black substance. When copper powder is heated, it combines with oxygen to form copper oxide.

Actually, in the process, copper powder gains oxygen. Thus, it gets oxidized to form copper oxide on heating. This process is called oxidation.

Now, if hydrogen gas is passed over heated copper (II) oxide, then the black coating on the surface turns brown. This is because a reverse reaction takes place and copper is re-obtained.

Here, copper (II) oxide loses oxygen and gets reduced to copper. This process is called reduction.

What is Reduction?

Reduction is defined as a process that involves a gain of hydrogen or a loss of oxygen. When a substance loses oxygen or gains hydrogen during a reaction, it is reduced.

Oxidation and reduction always take place simultaneously. Therefore, reactions involving oxidation and reduction are known as Redox (‘Red’ for reduction and ‘ox’ for oxidation) reactions. In a redox reaction, one substance is oxidized, while the other is reduced.

The substances that are reduced (provide oxygen or remove hydrogen) in course of the reaction are called oxidizing agents. These substances oxidise other chemicals in the reaction and are reduced in the process. On the other hand, the substances that are oxidized (remove oxygen or provide hydrogen) are called reducing agents.

For example:

In the above reaction, CO₂ gets reduced to CO and here, CO₂ is the oxidizing agent. On the other hand, hydrogen gets oxidised to form water and here, H₂ is the reducing agent.

We come across many examples of redox reactions in our daily life. For example, in the process of corrosion, metal combines with oxygen and gets corroded. Again, food gets spoiled, when it is oxidised and the process is called rancidity. We will discuss them separately.

Corrosion:

It may be defined as a process where materials, usually metals, are deteriorated because of a chemical reaction with air, moisture, chemicals, etc. For example, corrosion causes damage to car bodies, bridges, iron railings, ships, and all objects made of metals (especially those made from iron). Iron, in the presence of moisture, reacts with oxygen to form iron (III) oxide. This reaction is represented as:

This hydrated iron (III) oxide is rust. If not controlled, rusting can corrode the entire iron present in an object. As rust is softer than iron, the strength of the object decreases when rusting takes place. Every year, a large amount of money is spent on the maintenance of structures made of iron such as bridges, rails, ships etc.

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