CBSE Class 10 Science Chemical Reactions And Chemical Equations Notes

CHEMICAL REACTIONS &  CHEMICAL EQUATIONS

1.1 INTRODUCTION:

Chemistry is defined as that branch of science which deals with the composition and properties of matter and the changes that matter undergone by various interactions. A chemical compound is formed as a result of a chemical change and in this process different type of energies such as heat, electrical energy, radiation etc. are either absorbed or evolved. The total mass of the substance remains the same throughout the chemical change.

1.2  CHEMICAL ACTION OR REACTION:

When a chemical change occurs, a chemical action is said to have taken place. A chemical change or chemical action is represented by a chemical equation. The matter undergoing change in known as reactant and new chemical component formed is known as product.

1.2 (a)  Characteristics of a Chemical Reaction:

When we heat sugar crystals they melt and on further heating they give steamy vapour, leaving behind brownish black mass. On cooling no sugar crystals appears. Thus change which takes place on heating sugar is a chemical change and the process which brings about this chemical change is called chemical reaction. 

1. In this reaction the substance which take part in bringing about chemical change are called reactants. 
2. The substance which are produced as a result of chemical change are called products. 
3. These reactions involve braking and making of chemical bonds. 
4. Product(s) of the reaction is/are new substances with new name(s) and chemical formula.
5. It is often difficult or impossible to reverse a chemical reaction. 
6. Properties of products formed during a chemical reaction are different from thos of the reactants. 
7. Apart from heat other forms of energies are light and electricity which are also used in carrying out chemical changes.

In all chemical reactions, the transformation from reactants to products is accompanied by various characteristics, which are-

(i) Evolution of gas: Some chemical reactions are characterized by evolution of a gas.

(ii) Change of colour: Certain chemical reactions are characterized by the change in colour of reacting substance.

(iii) Formation of precipitate : Some chemical reactions are characterized by the formation of precipitate (an insoluble substance), when the solutions of the soluble chemical compounds are mixed together.

(iv)Energy changes: all chemical reactions proceed either with the absorption or release of energy.One the basis of energy changes, there are two types of reactions:

(A) Endothermic reaction : A chemical reaction which is accompanied by the absorption of heat energy is called an endothermic reaction.

Light energy is essential for biochemical reaction, photosynthesis, by which green plants prepare their food from carbon dioxide & water.

(B) Exothermic reaction : A chemical reaction which is accompanied by the release of heat energy is called exothermic reaction.

When magnesium wire is heated from its tip in a bunsen flame, it catches fire and burns with a dazzling white flame with release of heat and light energy.

(v) Change of state: Some chemical reactions are characterised by a change in state i.e. solid, liquid or gas

1.3 CHEMICAL EQUATIONS : 

All chemical changes are accompanied by chemical reactions. These reactions can be described in sentence form, but the description would be quite long. Chemical equations have been framed to describe the chemical reactions.

 A chemical equation links together the substance which react (reactants) with the new substances that are formed (products).

Zinc + Hydrochloric acid → Zinc chloride + Hydrogen

  (Reactants)                                    (Products) 

A Chemical reaction can be summarised by chemical equation.

1.3 (a) Types of Chemical Equations :

(i) Word equations : A word equation links together the names of the reactants with those of the products. For example, the word equation, when magnesium ribbon burns in oxygen to form a white powder of magnesium oxide, may be written as follows-

Magnesium + Oxygen → Magnesium oxide

     (Reactants)                   (Product)

Similarly, the word equation for the chemical reaction between granulated zinc and hydrochloric acid may be written as -

 Zinc + Sulphuric acid→  Zinc sulphate + Hydrogen

In a word equation

1. The reactants are written on the left hand side with a plus sign (+) between them.
2. The products are written on the right hand side with a plus sign (+) between them.
3. An arrow (→) separates the reactants from the products.
4. The direction of the arrow head points towards the product.

Although word equations are quite useful, yet they don’t give the true picture of the chemical reactions.

(ii) Symbol equation : A brief representation of a chemical reaction in terms of symbols and formulae of the substance involved is known as a symbol equation.

In a symbol equation, the symbols and formulae of the elements and compounds are written instead of their word names.

For e.g. Burning of magnesium in oxygen to form magnesium oxide may be written as follows :   

Mg + O₂ →  MgO

Symbol equations are always written from the word equations.

1.3 (b) Unbalanced and Balanced Chemical Equations :

In an unbalanced equation, the number of atoms of different elements on both side of the equation are not equal. For example, in the equation given below, the number of Mg atoms on both sides of the equation is one (same), but the number of oxygen atoms are not equal, It is known as an unbalanced equations.

                                   Mg + O₂ → MgO 

An unbalanced equation is also called skeletal equation.

In a balanced equating, the number of different elements on both sides of the equation are always equal. The balanced equation for the burning of magnesium ribbon in oxygen is written as -

 2 Mg + O₂ → 2 MgO

(i) Importance of balanced chemical equation: The balancing of a chemical equation is essential or necessary to fulfill the requirement of “Law of conservation of mass”.

(ii) Balancing of chemical equations: Balancing of chemical equations may be defined as the process of making the number of different types of elements, on both side of the equations, equal.

The balancing of a chemical equation is done with the help of Hit and Trial method. In this method, the coefficients before the symbols or formulae of the reactants and products are adjusted in such a way that the total number of atoms of each element on both the side of the arrow head become equal. This balancing is also known as mass balancing because the atoms of elements on both side are equal and their masses will also be equal.

The major steps involved in balancing a chemical equation are as follow –

1.Write the chemical equations in the form a word equations. Keep the reactants on the left side and the products on the right side. Separate them by an arrow whose head(→)  points from the reactants towards the product.

2.Convert the word equation into the symbol equation by writing the symbols and formulae of all the reactants and product.

3.Make the atoms of different elements on both side of the equation equal by suitable method. This is known as balancing of equation.

4.Do not change the formulae of the substance while balancing the equation.

5.Make the equations more informative if possible.

Example :

Zinc reacts with dilute sulphuric acid to give zinc sulphate and hydrogen.

Solution : The word equation for the reaction is -

Zinc + Sulphuric acid→  Zinc sulphate + Hydrogen

The symbol equation for the same reactions is -

 Z n + H₂SO₄  ZnSO₄ + H₂

Let us count the number of atoms of all the elements in the reactants and products on both sides for the equations.

As the number of atoms of the elements involved in the reactants and products are equal, the equation is already balanced.

2.Iron reacts with water (steam) to form iron (II, III) oxide and liberates hydrogen gas.

Solution :- The word equation for the reactions is -

Iron + Water→  iron (II, III) oxide + Hydrogen

The symbol equation for the same reaction is-

Fe + H₂O →  Fe₃O₄ + H₂

The balancing of the equations is done is the following steps:

I : Let us count the number of atoms of all the elements in the reactants and products on both sides of the equation.

Thus, the number of H atoms are equal on both sides, At the same time, the number of Fe and O atoms are not equal.

II : On inspection, the number of O atoms in the reactant (H₂O) is 1 while in the product (Fe₃O₄), these are 4. To balance the atoms, put coefficient 4 before H₂O on the reactant side. The partially balance equation may be written as

 Fe + 4H₂O→  Fe₃O₄ + H₂

III : In order to equate H atoms, put coefficient 4 before H₂ on the product side, As a result, the H atoms on both side on of the equation become 8 and are thus balanced. The partially balanced equation may now be written as

Fe + 4H₂O → Fe₃O₄ + H₂

IV : In order to balance the Fe atoms, put coefficient 3 before Fe on the reactant side. The equation formed may be written as -

3Fe + 4H₂O→   Fe₃O₄ + 4H2

V : on final inspection, the number of atoms of all the elements on both sides of the equation are equal. Therefore, the equation is balanced.

1.3 (c) Writing State Symbols:

The chemical equations or symbol equations which we have enlisted don’t mention the physical states of the reactant and product species involved in the reaction. In order to make the equation more informative, the physical state are also mentioned with the help of certain specific symbols known as state symbols. These symbols are

1. (s) for solid state
2. (l)for liquid state
3. (g) for gaseous state
4. (aq) for aqueous solution i.e., solution prepared in water.

Sometimes a gas if evolved in a reaction is shown by the symbol  i.e., by an arrow pointing upwards. Similarly the precipitate, if formed during the reaction, is indicated by the symbol  i.e., by an arrow pointing downwards.

The abbreviation ‘ppt’ is also use to represent the precipitate, if formed. 

1.3 (d) Significance of State Symbols:

The state symbols are of most significance for those chemical reactions which are either accompanied by the evolution of heat (exothermic) or by the absorption of heat (endothermic). For example

Both these reactions are of exothermic nature because heat has been evolved in these. However, actual amounts of heat are different when water is in the liquid state i.e. H2O and when it is in the vapour state.

1.3 (e) Specialties of Chemical Equation :

(i)  We get the information about the substance which are taking part and formed in the reaction.

(ii) We get the information about the number of molecules of elements or compounds which are either taking part or formed in the chemical reaction.

(iii) We also get the information of weight of reactant or products.

Total weight of reactants is equal to the total weight of products because matter is never destroyed. In the above example total weight of calcium carbonate (reactant) is 100 gram and of product is also 100 g (56 gram + 44 gram).

(iv) In a chemical equation if any reactant or product is in gaseous state, then its volume can also be determined. For example in the above reaction volume of carbon dioxide is 22.4 liters.

(vi) In a chemical equation with the help of product we can get information about the valency as well.

For example

In the above reaction one atom of Mg displaces two atoms of hydrogen, so valency of magnesium is two.

All chemical equations are written under N.T.P. Conditions (at 273 K and 1 atmosphere pressure) if conditions are not otherwise mentioned.

1.3 (f) Limitations of Chemical Equations :

(i) We do not get information about the physical state of reactants and products.

For example solid, liquid or gas.

 (ii) No information about the concentration of reactants and products is obtained.           

(iii) No information about the speed of reaction and sense of timing can be obtained. 

(iv) Information regarding the favorable conditions of the reactions such as pressure, temperature, catalyst etc. can’t be obtained during the reaction.           

(v) We do not get information whether heat is absorbed or evolved during the reaction.           

(vi) We do not get information whether the reaction of reversible or irreversible. 

(vii) We do not get information about the necessary precautions to be taken for the completion of reaction.

 The above limitations are rectified in the following manner – 

1.The physical sate of reactants and products are represented by writing them in bracket. 

2.The precipitate formed in the reaction is represented by(↓) symbol and gaseous substance by(↑) 

3.To express the concentration, dilute or conc. is written below the symbol.  

4.Favorable conditions required for the completion of reaction are written above and below the arrow.  

    
5.Reversible reaction is represented by symbol and irreversible reaction by (↑) 

6.The heat absorbed in the chemical reaction is written on the right side by putting negative (-) sign and heat evolved in the chemical reaction is written on the right side by putting positive (+) sign.

2.1 TYPES OF CHEMICAL REACTIONS:

2.1 (a) Addition Reactions :

It is a union of two or more than two substances to from a new substance. It may be brought about by the application of heat, light electricity or pressure.

For eg. H₂ + CI₂  2HCI

 In the above example H₂ and CI₂ two elements combine to from hydrogen chloride.

Addition reactions may be formed in the following conditions -

(i) When two or more elements combine to form a new compound.

Synthesis reaction: It is a type of addition reaction in which a new substance is formed by the union of its component elements.

For eg. N₂ + 3H₃  2NH₃ (Haber’s Process)

Ammonia is synthesised from its components, nitrogen and hydrogen, so it is a synthetic reaction.

All synthesis reaction are addition reactions but all addition reactions are not synthesis reactions.  

Other Example of synthesis reactions are -

(ii) When two or more compounds combine to from a new compound.   

For eg.

(iii) When and element and a compound combine to from a new compound.

For eg.

Only single substance is formed as a product in the addition reactions.

2.1 (b) Decomposition Reaction :

It is breaking up of a substance into simpler compounds and it may be brought about by the application of heat, light, electricity etc.

(i) A decomposition reaction brought by heat is known as thermal decomposition.

For eg.

(ii) Decomposition performed by electricity is known as electrolysis.

For eg.

(iii) A decomposition reaction brought by light is known as photo decomposition.

For eg.

(iv) Decomposition reaction in which a compound decomposes into its elements is known as analysis reaction.

For eg

All analysis reactions are decomposition reactions, but all decomposition reactions are not analysis reactions.

Decomposition reaction is just opposite of the addition reaction.

2.1 (c) Displacement Reactions :

It involves displacement of one of the constituents of a compound by another substance and may be regarded as a displacement reaction.

In general a more reactive element displaces a less reactive element from the soluble solution of its salt.

2.1 (d) Double Displacement :

It is mutual exchange of the radicals of two compounds taking part in the reaction and results in the formation of two new compounds.

For eg.

Acid base neutralisation reactions are double displacement reactions.

3.1 OXIDATION AND REDUCTION :

3.1 (a) Oxidation :

Oxidation is a chemical reaction in which a substance gains oxygen or loses hydrogen. Since oxygen is an electronegative element and hydrogen is an electropositive element, so, oxidation is defined as a reaction in which a substance gains and electronegative radical or loses and electropositive radical.

(i) A reaction in which a substance gains oxygen is known as oxidation.

For eg.

(ii) Gain or addition of a electronegative radical

For eg.

(iii) Removal of a hydrogen atom.

For eg

(iv) Removal or loss of electropositive radical or element.

 For e.g.

3.1 ( b) Reduction :

It is a chemical reaction in which there is a gain of hydrogen or any electropositive radical or a loss of oxygen or electronegative radical.

 (i) Gain of hydrogen.

For eg.

(ii) Gain of any electropositive radical or element.

For eg.

(iii) Loss of oxygen atom.

For eg.

(iv) Loss of electronegative radical.

For eg.

3.2 REDOX REACTIONS :

Reduction is loss of electronegative element or radical. From all above example it is clear that oxidation and reduction occur side by side, i.e. there can be no oxidation without and equivalent reduction. In a reaction whenever one substance is oxidised the other is definitely reduced. The reverse is also true whenever one substance is reduced the other is oxidized. Such reactions in which oxidation and reduction take place simultaneously are known as redox reactions.

When hydrogen gas is passed through not cupric oxide, hydrogen is oxidised to water (H2O) while cupric oxide is reduced to metallic copper by loss of oxygen. Hydrogen gas helps in reduction of cupric oxide to metallic copper so it is known as reducing agent, where  as cupric oxide helps in oxidation of hydrogen so it is known as oxidizing agent. A substance, which brings about reduction, is called reducing agent. A substance, which brings about oxidation, is called an oxidizing agent.

3.2 (a) Electronic Interpretation of Oxidation:

The electronic theory attempts to interpret oxidation on the basis of electron transfer. According to octet rule, atom will try to complete its octet by losing gaining or sharing electrons. Sodium chloride is an electrovalent compound and consists of an ion pair (Na+) (CI-) even in the solid state. In its formation, the neutral sodium loses and electron and becomes positively charged sodium ion. Sodium is said to be oxidised and loss of electrons is termed as oxidation.

            2Na  2Na⁺ + 2e^-

            2Na⁺ + 2CI^- → 2NaCI

3.2 (b) Electronic Interpretation of Reduction :

Reduction which is also referred to as electronation is a process involving the gain of electrons and is the reverse of oxidation.

For example

Mg combines with oxygen and is oxidized to MgO. According to electronic theory magnesium atom loses two electrons from its outermost shell (M) and is oxidised to mG which oxygen atom gains these two electrons and gets reduced to oxide anion, hence oxidation involves loss of electrons and it is also referred as de- electronation. Reduction involves gain of electrons so it is referred to as electronation.

            2Mg+ O₂ →  2MgO

            Mg →   Mg⁺² + 2e-

            O + 2e^- →  O^2-

            Mg⁺² + O^2- →   MgO

3.3  EFFECT OF OXIDATION REACTIONS IN EVERYDAY LIFE :

We are all aware of the fact that oxygen is most essential for sustaining life. One can live without food or even water for a number of days but not without oxygen. It is involved in a variety of actions which have wide range of effects on our daily life. Most of them are quite useful while a few may be harmful in nature. Some of these effects are briefly discussed. Some examples are-

3.3 (a) Combustion Reactions:

A chemical reaction in which a substance burns or gets oxidised in the presence of air or oxygen in called combustion reaction. For example, kerosene, coal, charcoal, wood etc. burn in air and thus, undergo combustion. Methane (CH4) a major constituent of natural gas undergoes combustion in excess of oxygen upon heating.

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

            Methane

            Similarly, butane (C₄H₁₀) the main constituent of L.P.G. also undergoes combustion.

            C₄H₁₀ (g) + 13/2O₂(g) →  4CO₂(g) + 5H₂O(g)

            Butane

All combustion reactions are of exothermic nature and are accompanied by release of heat energy. The human body may be regarded as a furnace or machine in which various food stuffs that we eat undergo combustion or oxidation. The heat energy evolved keeps our body working. Carbohydrates such as glucose, fructose, starch etc. Are the major source of energy to the human body. They undergo combustion with the help of oxygen that we inhale to form carbon dioxide and water. For example.

C₅H₁₂O₆(s) + 6O₂(g)→  6CO₂(g) + 6H₂O  + energy

All combustion reactions are not accompanied by flame. Combustion is basically oxidation accompanied by release of energy.

3.3 (b) Respiration :

Respiration is the most important biochemical reaction which releases energy in the cells. When we breathe in air, oxygen enters our lungs and passes into thousands of smalls air sacs (alveoli). These air sacs occupy a large area of membranes and oxygen diffuses from the membranes into blood. It binds itself to hemoglobin present in red blood cells and is carried to millions of cells in the body. Respiration occurs in these cells and is accompanied by the combustion of glucose producing carbon dioxide and water. Since the reaction is of exothermic nature, the energy released during respiration carry out many cell reactions and also keeps our hart and muscles working. It also provides the desired warmth to the body. Both carbon dioxide and water pas back into the blood and we ultimately breathe them out. Respiration takes place in the cells of all living beings.

Fish takes up oxygen dissolved in water through their gills while plants take up air through small pores (stomata) present in their leaves.

3.3 (c) Harmful Effects of Combustion:

We have discussed the utility of combustion in releasing energy which our body needs to keep warm and working; however, combustion has harmful effects also. The environmental pollution is basically due to combustion. Poisonous gases like carbon monoxide (CO), sulphur dioxide (SO₂) sulphur trioxide (SO₃) and oxide of nitrogen (NO_X) etc. are being released into the atmosphere as a result of variety of combustion reaction which are taking place. They pollute the atmosphere and make our lives miserable. In addition to these, other harmful effects of combustion are corrosion and rancidity. These are briefly discussed.

(i) Corrosion: Corrosion may be defined as the process of slow eating up of the surfaces of certain metals when kept in open for a long time.

Quite often, when we open the bonnet of a car after a long time, we find a deposit around the terminals of the battery. This is an example of corrosion. Black coating on the surface of silver and green layer on the surface of copper are the examples of corrosion. In case of iron, corrosion is called rusting. Rust is a chemical substance brown in colour and is formed by the chemical action of moist air (containing O₂ and H₂O) on iron. It is basically an oxidation reaction and the formula of rust is Fe₂O3, xH₂O. It is very slow in nature and once started keeps on.

Both corrosion and rusting are very harmful and case damage to the building, Railway tracks, cars and other objects/ materials where metals are used. We quite often hear that an old building has collapsed on its own causing loss of both lives and property. This is on account of the rusting of iron which is used in making the structure particularly the roof.

(ii) Rancidity: Oxidation has damaging effects on food and eatables. When the fats and oils present in butter and margarine are oxidised, they become rancid. As a result, their smell and taste change. They become quite unpleasant. This is known a rancidity. It can be checked in a number of away.

(A) Manufacturer sometimes add certain food additives to the food materials. These are known as antioxidant and check their oxidation.

(B) Keeping food in air tight containers prevents its oxidation.

(C) Refrigeration of food also slows down rancidity because the temperature inside refrigerator is very low and direct contact with air or oxygen is avoided.

(D) Chips manufacturers generally flush their bags with nitrogen before packing so that they may not be oxidised.

GIST OF THE LESSON

1) Chemical reaction— Chemical changes or chemical reactions are the changes in which one or more new substances are formed.

2 )Chemical Equations – Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation.

3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation.

4) The chemical reactions can be classified into different types such as—

a) Combination reaction – The reactions in which two or more substances combine to form a new substance are called combination reaction. For example,

2Mg(s) + O₂ (g) →2 MgO (s)

b) Decomposition reaction - The reaction in which a single compound breaks up into two or ore simpler substances are called decomposition reactions. For example,

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

The decomposition of a substance by passing electric current through it is known as electrolysis.

The decomposition of a substance on heating is known as thermal decomposition.

The decomposition of a substance by absorbing light energy is called photochemical decomposition.

c) Displacement reactions -The chemical reactions in which a more reactive element displaces a less reactive element from a compound are known as displacement reactions. For example,

i) Zn (s) + CuSO₄ (aq)→ ZnSO₄ (aq) + Cu (s).

ii) Cu (s) + 2AgNO₃ (aq) → Cu (NO₃)₂ (aq) +2Ag (s).

d) Double Displacement Reactions - The chemical reactions in which compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions.

These reactions take place in solution two common types of this reaction are precipitation reactions and neutralization reactions

i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed whereby

Some salts precipitate due to mutual exchange of ions between the two salts. For example

AgNO₃ (aq) + NaCI (aq) -----> AgCl(s) + NaNO₃.

ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt and water by exchange of ions.

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

e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction.

Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.

Reduction: Reaction that shows the loss of oxygen or gain of hydrogen.

Both oxidation and reduction take place simultaneously and hence called redox reaction.

ZnO + C →Zn + CO

ZnO reduce to Zn ---- reduction

C oxidize to CO ------oxidation

f) Exothermic reaction and endothermic reaction: On the basis of energy changes during chemical reaction, they can be classified as

i) Exothermic reaction: A chemical reaction in which heat energy is produced.

C + O2→ CO₂ (g) + heat

ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed.

CaCO₃ + Heat →CaO + CO₂

5 Corrosion – The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in the atmosphere is called corrosion. Rusting – Iron when reacts with oxygen and moisture forms red substance called rust.

6 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 can be prevented by using various methods such as by adding antioxidants to the food materials, Storing food in air tight container and by flushing out air with nitrogen. 

Q.1 What is a ‘Reaction’?

Ans.Reaction is a term used for depicting a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.

Q.2 What is a ‘Chemical Reaction’?

Ans.A chemical change is always accompanied by a chemical reaction. a chemical change or reaction can be confirmed by any or all of the following observations

=> change in state

=> change in colour

=> change in temperature

=> evolution of gas.

Q.3 What is a ‘Chemical Equation’?

Ans.A Chemical Equation represents a chemical reaction. A complete chemical equation represents the reactants, products, and their physical states symbolically. A chemical equation also indicates a number of atoms of each element involved in a reaction. A chemical equation can be written as balanced or unbalanced. An unbalanced chemical equation is also known as Skeletal Chemical Equation for a reaction. An equation having an equal number of atoms of each element on both the sides is called a balanced chemical equation.

Types of Chemical Reaction

Chemical Reactions are primarily of five types as listed below:

1. Combination reaction

2. Decomposition reaction

3. Displacement reaction

4. Double displacement reactions

5. Oxidation and Reduction reactions

What are ‘Combination Reactions’?

When two or more substances (elements or compounds) combine to form a singleproduct, the reactions are called ‘Combination Reactions’. Generally, CombinationReactions are exothermic in nature. The following are some examples of CombinationReactions:

CaO (s) + H₂O (l) -----> Ca(OH)₂ (aq)

C (s) + O₂ (g) ------> CO₂ (g)

What are ‘Decomposition Reactions’?

Decomposition reactions are opposite to combination reactions. In a Decomposition reaction, a single substance decomposes to give two or more substances.

What are ‘Displacement Reactions’?

When an element displaces another element from its compound, such reaction is called a displacement reaction e.g. -

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

What are ‘Double Displacement Reactions’?

A double displacement reaction is a bimolecular process in which parts of twocompounds are exchanged to give two new compounds. The general equation used torepresent double displacement reactions can be written as -

AB + CD -----> AD + BC

Examples of double displacement reactions are -

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

“Double Displacement Reactions ” have two commoj features -

1. Two compounds exchange their ions resulting in the formation of two new compounds.

2. One of the new compounds formed would be separated from the mixture in some way, commonly as a solid or gas. Double displacement reactions can be further classified as Precipitation, Gas formation, and Acid-base neutralization reactions.

Sample Hot Questions

Q.1: Which of the following is a Combination and which is a Displacement Reaction?

(a) Cl₂ + 2KI -----> 2KCl + I₂

(b) 2K + Cl₂ ------> 2KCl

Ans:

(a) Displacement reaction

(b) Combination reaction

Q.2: What is a catalytic decomposition?

Ans: Those reactions in which a compound splits up into two or more simpler substances in the presence of catalyst are called catalytic reactions. For example,digestion of food in human body.

Q.3: What is the term used for the symbolic representation of a reaction.

Ans: Chemical equation.

Q.4: Write two uses of decomposition reactions.

Ans: Two uses of decomposition reactions-

(i) These reactions are used to extract several metals from their oxides and salts. When the oxide or salt is electrolyzed, metal is obtained as per the following equation:

2Al₂O₃ ------> 4Al + 3O₂

(ii) Digestion of food in our body is also an example of decomposition reaction. Thestarch (carbohydrates) present in the body is decomposed to give simple substance(glucose) and the proteins are broken down into amino acids.

Q.5: Explain the terms ‘oxidizing agent’and reducing agent’.

Ans:

Oxidizing agent

The substance, which oxidizes elements or compounds, is known as oxidizing agent e.g. -

4Fe + 3O₂ ------> 2Fe₂O₃

Hence, oxygen is an oxidizing agent. In this equation it oxidizes iron (FeII) to ferric oxide (Fe₂O₃).

Reducing agent

The substance, which oxidizes elements or compounds, is known as oxidizing agent e.g. -

SnCl₂ + 2FeCl₃ -----> SnCl₄ + 2FeCl₂

Here stannous chloride is a reducing agent. It reduces ferric chloride to ferrous chloride.

(e) Redox reaction : If a substance gains oxygen or looses hydrogen during a reaction it is oxidized. On the other hand if a substance loses oxygen or gains hydrogen during a reaction, it is reduced. Both oxidation and reduction take place simultaneously and hence these are called redox reaction. Oxidation

   ZnO           +         C       →     Zn + CO

Reduction            Oxidation

    Cl₂          +       H2S      →     2HCl + S

Reduction

(f) Exothermic reaction and endothermic reaction : On the basis of energy changes during chemical reaction, they can be classified as

(i) Exothermic reaction- A chemical reaction in which heat energy is produced is called exothermic reaction .For e.g. (i) C (s) + O₂ CO₂ (g) + Heat

(ii) Endothermic reaction- A chemical reaction in which heat energy is absorbed is called endothermic reaction. CaCO₃ + Heat CaO + CO₂

5. Effects of some oxidation reaction in everyday life:

(a) Corrosion – The process of slow conversion of metals into their undesirable compounds due to their reaction with moisture and other gases present in the atmosphere is called corrosion of metals. Some common examples of corrosion are rusting of iron, formation of green layer on the surface of copper, tarnishing of silver etc. Formation of brown layer on the surface of iron is called rusting of iron. Formula of rust is Fe2O3.xH2o. Rusting of iron is a serious problem.

(b) Rancidity- The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is known as rancidity. Rancidity is caused due to oxidation of fat and oil present in food materials. Rancidity can be prevented by using various methods such as by adding antioxidants to the food materials, storing food in air tight container and by flushing out air with some inert gases like nitrogen.

Top definitions:

1. Chemical reaction: A chemical reaction involves a chemical change in which substances react to form new substances with entirely new properties. Substances that react or take part in the reaction are known as reactants and the substances formed are known as products.

2. Physical change: If a change involves change in colour or state but no new substance is formed, then it is a physical change.

3. Chemical change: If a change involves formation of new substances, it is a chemical change.

4. Chemical equation: The symbolic representation of a chemical reaction is called a chemical equation.

5. Exothermic and endothermic reactions: If heat is evolved during a reaction, then such a reaction is known as exothermic reaction. If heat is absorbed from the surroundings, then such a reaction is known as endothermic reaction

6. Combination reaction: Combination reaction is a reaction in which 2 or more substances combine to give a single product.

7. Decomposition reaction: In a decomposition reaction, a single reactant decomposes to give 2 or more products. Decomposition reactions require energy in the form of heat, light or electricity

8. Displacement reaction: A reaction in which a more active element displaces less active element from its salt solution.

9. Reactivity series: The Reactivity series is a list of metals arranged in the order of decreasing reactivity. The most reactive metal is placed at the top and the least reactive metal is placed at the bottom.

10.Double displacement reaction: A chemical reaction in which there is an exchange of ions between the reactants to give new substances is called a double displacement reaction. 

11.Precipitation reaction: An insoluble solid known as precipitate is formed during a double displacement reaction. Such reactions are also known as precipitation reactions.

12.Redox reaction: A reaction, in which oxidation and reduction takes place simultaneously is known as redox reaction.

13.Oxidation: Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.

14.Reduction: Reduction is a chemical process in which a substance gains hydrogen or loses oxygen. 

Key learnings:

1. A chemical reaction involves a chemical change in which substances react to form new substances with entirely new properties. Substances that react or take part in the reaction are known as reactants and the substances formed are known as products.

2. During a chemical reaction, there is a breaking of bonds between atoms of the reacting molecules to give product

3. A chemical reaction can be observed with the help of any of the following observations:

a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state

4. Physical change: If a change involves change in colour or state but no new substance is formed, then it is a physical chang

5. Chemical change: If a change involves formation of new substances, it is a chemical change.

6. Exothermic and endothermic reactions: If heat is evolved during a reaction, then such a reaction is known as Exothermic reacti If heat is absorbed from the surroundings, then such a reaction is known as endothermic reaction.

7. Chemical equation: The symbolic representation of a chemical reaction is called a chemical equation.

8. Features of a chemical equation:

a. The reactants are written on the left hand side with a plus sign between them.

b. The products are written on the right hand side with a plus sign between them.

c. An arrow separates the reactants from the products. The arrow head points towards the products and indicates the direction of the reaction.

9. Skeletal chemical equation: A chemical equation which simply represents the symbols and formulae of reactants and products taking part in the reaction is known as skeletal chemical equation for a reaction. For example: For the burning of Magnesium in the air, Mg + O2 → MgO is the skeletal equation.

10. Balanced chemical equation: A balanced equation is a chemical equation in which number of atoms of each element is equal on both sides of the equation i.e. number of atoms of an element on reactant side = number of atoms of that element on the product side.

11. As per the law of conservation of mass, the total mass of the elements present in the products of a chemical reaction is equal to the total mass of the elements present in the reactants.

12. The process of equating the number of atoms on both the sides of a chemical equation is known as balancing of a chemical equation.

a. The first step in balancing a chemical equation is to write the number of atoms of each element present on the left hand side and right hand side.

b. We should always start balancing with the compound that contains maximum number of atom It can be reactant or a product. Then in that compound select the element which has the maximum number of atoms.

c. While balancing a chemical equation, the molecular formulae of the reactants and products should not change. The molecular formulae are simply multiplied by suitable coefficients.

d. To make a chemical equation more informative, the reaction conditions such as temperature, pressure or catalyst are written on the arrow separating the reactants and products.

e. The evolution of gas is indicated by an upward arrow.

f. The formation of precipitate is indicated by a downward arrow.

g. Heat evolved during the reaction is written as + Heat on the product side.

h. Heat absorbed during the reaction is written as + Heat on the reactant side.

13.Types of chemical reactions:

a. Combination reaction
b. Decomposition reaction
c. Displacement reaction
d. Redox reaction

14.Combination reaction is a reaction in which 2 or more substances combine to give a single product. Combination reaction can be between 2 elements, between an element and a compound or between 2 compounds.

15.Decomposition reaction: In a decomposition reaction, a single reactant decomposes to give 2 or more products. Decomposition reactions require energy in the form of heat, light or electricity

16.Types of decomposition reactions:

a. Decomposition reactions which require heat are known as thermolytic decomposition reactions
b. Decomposition reactions which require light are known as photolytic decomposition reactions
c. Decomposition reactions which require electricity are known as electrolytic decomposition reactions

17.Displacement reaction: A reaction in which a more active element displaces less active element from its salt solution.

18.The reactivity series is a list of metals arranged in the order of decreasing reactivity. The most reactive metal is placed at the top and the least reactive metal is placed at the bottom.

19.Double displacement reaction: A chemical reaction in which there is an exchange of ions between the reactants to give new substances is called double displacement reaction.

20.Precipitation reaction: An insoluble solid known as precipitate is formed during a double displacement reaction. Such reactions are also known as precipitation reactions.

21.Redox reaction: A reaction in which oxidation and reduction take place simultaneously in a reaction, is known as a redox reaction.

22.Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.

23.Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.

24.If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised.

25.If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced.

26.A substance that loses oxygen or gains hydrogen is known as an oxidising agent.

27.A substance that loses hydrogen or gains oxygen is known as a reducing agent.

28.An oxidising agent gets reduced whereas a reducing agent gets oxidised.

29. In terms of electronic concept, Oxidation is defined as a loss of electrons while reduction is defined as a gain of electr

30. Corrosion is the slow eating up of metals by the action of air and moisture on their surface Corrosion in case of Iron is known as Rusting.

31.Chemically, rust is hydrated ferric oxide (Fe₂O₃.xH₂O)

32.Advantages of corrosion: Though corrosion is undesirable, it can be advantageous in case of aluminium which on exposure to air, gets coated with a protective layer of aluminium oxide. This protects the metal underneath from further corrosion and damage.

33.Rancidity: When oils and fats or foods containing oils and fats are exposed to air, they get oxidised due to which the food becomes stale and gives a bad taste or smell. This is called Rancidity.

34. Rancidity can be prevented by:

a) Adding antioxidants i.e. the substances which prevent oxidation
b) Refrigeration
c) Storing the food in air-tight containers