CBSE Grade 10 Science Chapter 1 - Chemical Reactions and Equations

 CBSE Grade 10 Science 

Chapter 1 - Chemical Reactions and Equations

What is Physical Change ?

• Most of the substance around us undergoes various changes. Some of these changes are temporary with no new substance being formed. They are called Physical Changes. 

In Physical Change there is change in colour or state of the substance but no new substance is formed.

Example of Physical Changes: 

1)Water changes to ice on freezing. (There is only change in state from Liquid to Solid.)

2)Water changes to steam on boiling.(There is only change in state from Liquid to Vapour) 

What is Chemical Change ?

•  In certain other changes the new substance formed in which the reactant or the parent loses its identity to form new substance called Product. These changes are permanent changes as we won’t get the reactant back.They are called Chemical Changes. 

Example of Chemical Changes :

When copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and copper are formed.

Introduction to Chemical Reactions and Equation

What is a Chemical Reaction?

• Chemical Reaction is the process by which two or more substance react with each other to form a completely new substance with different properties.

• Only a rearrangement of atoms takes place in a chemical reaction.

• The substances which take part in a chemical reaction are called Reactants.

• The new substances produced as a result of a chemical reaction are called Products.

Example: The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.

2Mg(s) + O₂(g) → 2MgO(s)

 Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.

Reactant: Substances which take part in a chemical reaction are called reactants.
Example: Mg and O₂.

Product: New substance formed after a chemical reaction is called a product.
Example: MgO.

 Characteristics of Chemical Reactions

(i) Evolution of Gas: 

 The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑

(ii) Change in Colour:

The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.

The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterized by a change in colour from orange to green.

(iii) Change in State of the Substance: 

The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas 

Wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas

Thus, there are some chemical reactions which can show more than one characteristics.

(iv) Change in Temperature:

The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).

(v) Formation of Precipitate: 

The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.

BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) (ppt) + 2HCl(aq)

What is a Word Equation ?

• A Word Equation is a chemical reaction expressed in words rather than chemical formulas.

• It helps identify the reactants and products in a chemical reaction.

For example,

Sodium + Chlorine → Sodium chloride

The above equation means: “Sodium reacts with chlorine to form sodium chloride.”

Symbols of elements and their valencies

A symbol is the chemical code for an element. Each element has one or two-letter atomic symbol, which is the abbreviated form of its name.

Valency is the combining capacity of an element. It can be considered as the number of electrons lost, gain or shared by an atom when it combines with another atom to form a molecule.

 What is Chemical Equation ?

The symbolic representation of chemical reaction using symbols and formulae is known as Chemical Equation.

Reactants are written in left hand side whereas Products are written on the right hand side.

In this equation, A + B → C + D

A and B are reactants  

C and D are Products. 

The arrow shows the direction of the chemical reaction. Condition, if any, is written generally above the arrow.

Example : When hydrogen reacts with oxygen, it gives water. This reaction can be represented by the following chemical equation:

Hydrogen + Oxygen → Water

H₂ + O₂ → H₂O

In the first equation, words are used and in second, symbols of substances are used to write the chemical equation.

Note : 

By writing the physical states of substances, a chemical equation becomes more informative.

• Gaseous state is represented by symbol (g).
• Liquid state is represented by symbol (l).
• Solid state is written by symbol (s).
• Aqueous solution is written by symbol (aq).
• For gas produced in the reaction, it is represented by “(↑)”.
• For precipitate formed in the reaction, it is represented by “(↓)”.
• Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.

Thus, by writing the symbols of the physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

A chemical equation can be divided into two types:

1. Balanced Chemical Equation
2. Unbalanced Chemical Equation.

 (a) Balanced Chemical Equation: 

A balanced chemical equation has the number of atoms of each element equal on both sides.

Example: Zn + H₂SO₄ → ZnSO₄ + H₂ 

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a Balanced Chemical Equation.

According to the Law of Conservation of Mass,

 Mass can neither be created nor destroyed in a chemical reaction. 

To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

(b) Unbalanced Chemical Equation: 

If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in the product, then the chemical equation is called Unbalanced Chemical Equation.

Example: Fe + H₂O → Fe₃O₄ + H₂ 

In this example, the number of atoms of elements are not equal on two sides of the reaction. 

For example; on the left-hand side only one iron atom is present, while three iron atoms are present on the right-hand side. Therefore, it is an unbalanced chemical equation.

Steps for Balancing of a Chemical Reaction

Short-cut technique for balancing a chemical equation

Method 1 - Hit and Trial method:

 While balancing the equation, change the coefficients (the numbers in front of the compound or molecule) so that the number of atoms of each element is same on each side of the chemical equation.

Example: Consider the following Chemical Equation.

aCaCO₃ + bH₃PO₄ → cCa₃(PO₄)₂ + dH₂CO₃

Set up a series of simultaneous equations, one for each element.

Ca: a=3c

C: a=d

O: 3a+4b=8c+3d

H: 3b=2d

P: b=2c

Let’s set c=1

Then a=3 and

d = a = 3

b = 2c = 2

So a=3; b=2; c=1; d=3

The balanced equation is

3CaCO₃ + 2H₃PO₄ → Ca₃ (PO₄)₂ + 3H₂CO₃

Method 2 - Tabular Comparision method:

 Example : Consider the following Chemical Equation.

Fe + H₂O → Fe₃O₄ + H₂

To balance the given or any chemical equation, follow these steps:

Step 1:Write the number of atoms of elements present in reactants and in products in a table as shown here.

Name of atom	No. of atoms in the reactant	No. of atoms in the product
Iron	1	3
Hydrogen	2	2
Oxygen	1	4

Step 2 : Balance the atom which is maximum in number on either side of a chemical equation.

• In this equation, the number of oxygen atom is the maximum on the RHS.

To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so that, the number of oxygen atoms becomes equal on both sides.

Fe + 4 × H₂O → Fe₃O₄ + H₂

• Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

Fe + 4 × H₂O → Fe₃O₄ + 4 × H₂ 

• After that, the number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

3 × Fe + 4 × H₂O → Fe₃O₄ + 4 × H₂ 

• Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.

Name of atom	No. of atoms in the reactant	No. of atoms in the product
Iron	3	3
Hydrogen	8	8
Oxygen	4	4

After balancing, the above equation can be written as follows: 

3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Types of Chemical Reaction

Taking into consideration different factors, chemical reactions are grouped into multiple categories.

● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation

 (i) Combination Reaction: 

Reactions in which two or more reactants combine to form one product are called Combination Reactions.

A general combination reaction can be represented by the chemical equation given here:

A + B → AB

Examples of Combination Reaction :

1) When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

Mg(s) + O₂(g) → 2MgO(s)

Magnesium + Oxygen → Magnesium Oxide

2) When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.

C (s) + O₂(g) → CO₂(g) 

Carbon + Oxygen → Carbon dioxide

3) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

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

 

A solution of slaked lime produced by the reaction is used for white washing walls.

4) Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO₃.

Ca(OH)₂ (aq) + CO₂ (g)   →   CaCO₃ (s) + H₂O (l)
(slaked lime)                   (calcium carbonate)

5) Other examples are

Burning of coal: C (s) + O₂ (g) → CO₂ (aq)
Formation of water: H₂ (g) + O₂ (g) →  H₂O (aq)
Burning of natural gas (Methane): CH₄ (g) + O₂ (g) →  CO₂ (g) + H₂O (l)

(ii) Exothermic reaction: 

Those reactions in which energy is released in the form of heat are called Exothermic Reactions.

Examples of Exothermic Reaction :

1) All combustion reactions e.g.

CH₄+ 2O₂ —> CO₂ + 2H₂O + Heat

2) Thermite reactions e.g.

2A₁ + Fe₂O₃ —> 2Fe + Al₂O₃ + Heat

3) Combinations are generally exothermic in nature. The decomposition of organic matters into compost is an example of exothermic reaction.

4) The rice, potatoes and bread we eat contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is Respiration is an exothermic reaction.

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

(iii) Endothermic reaction: 

Those reactions in which energy is absorbed are called Endothermic Reactions.

 Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. 

These reactions lower the temperature of their surrounding area, thereby creating a cooling effect.

Examples of Endothermic Reaction

1) When ammonium chloride (NH4Cl) is dissolved in water, an endothermic reaction takes place. The salt dissociates into ammonium (NH4+) and chloride (Cl–) ions. The chemical equation can be written as follows:

NH4Cl (s) + H2O (l) ⟶ NH4Cl (aq) – Heat

• Ammonium nitrate (NH4NO3), an important component in instant cold packs, dissociates into the ammonium cation (NH4+) and the nitrate anion (NO3–) when dissolved in water. These ions go on to form ammonium hydroxide (NH4OH) and nitric acid (HNO3) respectively (by reacting with the OH– and H+ ions in water). This reaction is endothermic in nature since it cools the surroundings by absorbing heat from it.

• The formation of nitric oxide from the reaction between nitrogen and oxygen is endothermic since it involves the absorption of approximately 180.5 kilojoules of heat for every mole of N2 and O2.

2) The melting of ice to form water.

3) Evaporation of liquid water, forming water vapour.

4) Sublimation of solid CO₂

5) The baking of bread.

(iv) Decomposition Reaction: 

Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction.

A general decomposition reaction can be represented as follows :

AB → A + B

Examples of Decomposition Reactions are :

1) When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.

CaCO₃(s)  −→− →       CaO(s) + CO₂(g)

Calcium carbonate → Calcium oxide + Carbon dioxide

2) When ferric hydroxide is heated, it decomposes into ferric oxide and water

2Fe(OH)₃(s)        →→ Fe₂O₃(s) + 3H₂O(l)

Types of Decomposition Reactions:

(a) Thermal Decomposition

• Decomposition reactions which require heat is known as Thermal Decomposition or Thermolysis.
  

Example: 

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

(b) Electrolytic Decomposition

• Decomposition reactions which require electricity is known as Electrolytic Decomposition or Electrolysis. 
  

• In these reactions, compounds decompose into simpler compounds because of passing of electricity.

Example: 

 

When electricity is passed in water, it decomposes into hydrogen and oxygen.

2H₂O(l)  →rightarrow 2H₂(g) + O₂(g)

(c) Photolysis or Photo Decomposition Reaction: 

• Decomposition reactions which require light are known as Photolytic Decomposition or Photolysis.

• In these reactions , compound decomposes because of sunlight.

Example: 

 

When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.

2AgCl(s) (white)  →ight−→−−−−− 2Ag(s) (grey) + Cl₂(g)

Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.

(v) Displacement Reaction: 

Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound or solution.

Displacement reactions are also known as Substitution Reaction or Single Displacement/ replacement reactions  

A general displacement reaction can be represented by using a chemical equation as follows :

A + BC → AC + B

Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taking place.

Examples of Displacement Reactions :

1) When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

2) When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

 note : Both metals and non-metals take part in displacement reactions. 

3) Reaction of iron nails with copper sulphate solution.

 

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

4) Reaction of lead with copper chloride solution.

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

(vi) Double Displacement Reaction:

 An exchange of ions between the reactants takes place to give new products.

A double displacement reaction, also known as a double replacement reaction or metathesis, is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

General Form of double displacement reactions can be given by the equation 

AB + CD → AC + BD

Examples of Double displacement reactions:

1) When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

 

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

2) When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.

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

Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.

(vii) Precipitation Reaction

An insoluble compound called precipitate forms when two solutions containing soluble salts are combined.

Examples of Precipitation Reactions:

1) When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

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

(viii) Neutralization Reaction

The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called Neutralization Reaction.

 Examples of Neutralization Reactions:

1) When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.

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

(ix)Redox Reaction:

Oxidation: 

• Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as Oxidation.
• Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be Oxidized.
  

Reduction: 

•  Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called Reduction.
• The compound or element which goes under reduction in called to be Reduced.
  

Oxidation and Reduction take place together.

Oxidizing agent:

• The substance which gives oxygen for oxidation is called an Oxidizing agent.
• The substance which removes hydrogen is also called an Oxidizing agent.

Reducing agent:

• The substance which gives hydrogen for reduction is called a Reducing agent.
• The substance which removes oxygen is also called a Reducing agent.

The reaction in which oxidation and reduction both take place simultaneously is called Redox reaction.

 Examples of redox Reactions

1) When copper oxide is heated with hydrogen, then copper metal and hydrogen are formed.

CuO + H₂ → Cu + H₂O

(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from copper oxide. Removal of oxygen from a substance is called Reduction, so copper oxide is being reduced to copper.

(ii) In this reaction, H₂ is changing to H₂O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called Oxidation, so hydrogen is being oxidised to water.

• The substance which gets oxidised is the reducing agent.
• The substance which gets reduced is the oxidizing agent.

What is Corrosion ?

• Corrosion is a Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.

• Metals are prone to corrosion. It is a slow conversion of metals into some undesirable compounds. 
• This occur may be due to reaction with oxygen, gases, acids etc.

(1) When irons reacts with atmospheric oxygen and moisture, a red layer is formed on the surface of the iron, this process is known as Rusting.

4Fe(s) + 3O₂(from air) + xH₂O(moisture) → 2Fe₂O₃.xH₂O(rust)

Rusting: Iron when reacts with oxygen and moisture forms red substance which is called Rust.

The rusting of iron is a redox reaction.

Corrosion (rusting) weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships etc. and cuts short their life.

Methods to Prevent Rusting

• By painting.
• By greasing and oiling.
• By galvanisation.

 (2) Corrosion of copper

Copper objects lose their lustre and shine after some time because the surface of these objects acquires a green coating of basic copper carbonate, CuCO₃.Cu(OH)₂ when exposed to air.

Cu(s) + H₂O(moisture) + CO₂(from air) → CuCO₃.Cu(OH)₂(green)

(3) Corrosion of silver

The surface of silver metal gets tarnished (becomes dull) on exposure to air, due to the formation of a coating of black silver sulphide(Ag₂S) on its surface by the action of H₂S gas present in the air.

Ag(s) + H₂S (from air) → Ag₂S(black) + H₂(g)

What is Rancidity ?

The taste and odour of food materials containing fat and oil changes when they are left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil present in food material.

It gives foul smell and bad taste to food. 

 

Rancid food causes stomach infection on consumption.

Methods to Prevent Rancidity

• Store cooking oils from direct sunlight.
  

• Food should be placed at low temperature or under Refrigeration

• By adding antioxidants or preservatives , food can be protected from rancidity.

• Packing material should replace the air with nitrogen.
  

• Minimize the use of salts in fried foods.

• Store the food in air-tight containers