Introduction

We have studied chemical change in previous classes about chemical change. Burning of paper, souring of milk, rusting of iron are some examples of chemical change. The nature and identity of initial substance changes during chemical change. Why does the identity of initial substance change ? This is because chemical reactions take place during chemical change. We shall discuss about chemical reaction and its various types in this chapter.

1.1 Chemical Reactions

Chemical Reaction : A process in which new substance(s) with new properties are formed is called chemical reaction.

Examples of chemical reaction are :

1. Burning of Mg Ribbon : When magnesium reacts with oxygen gas,it forms magnesium oxide. Here two substances magnesium and oxygen combine together and forming a new substance magnesium oxide, so this is an example of chemical reaction as new substance magnesium oxide forms in the process.
2. Reaction of Lead Nitrate With Potassium Iodide : When lead nitrate reacts with potassium iodide ,there forms lead iodide and potassium nitrate. In this process,two new substances lead iodide and potassium nitrate form. So this is an example of chemical reaction.
3. Reaction of Zinc with HCl : When zinc reacts with dilute hydrochloric acid , it forms zinc chloride and hydrogen gas. Clearly in the process, there forms zinc chloride and hydrogen gas which are the new substances different from initial substances zinc and hydrochloric acid. So this is an example of chemical reaction

1.2 Reactants and Products

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

Products : The new substances formed as a result of chemical reaction are called products.

Ex 1 : Magnesium + Oxygen → Magnesium Oxide ( Here Magnesium and Oxygen gas are taking part in chemical reaction so these are reactants and new substance formed after the reaction is Magnesium oxide. So Magnesium oxide is product.

Ex 2 : Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen Gas (Here zinc and hydrochloric acid are taking part in chemical reaction hence these are reactants. The new substances formed after the reaction are zinc chloride and hydrogen gas so these are products.

1.3 What happens during chemical reaction ?

Watch or perform the following three activities to conclude some point about what happens during a chemical reaction ?

1. Activity 1.1 : Burning of Mg Ribbon

Notes from the activity 1.1 :

• The form of Mg ribbon changes into powder form. It points that form or state may change during chemical reaction.
• Mg Ribbon burns with dazzling white flame . It implies that a flame may appear during a chemical reaction.
• It gives enormous amount of heat and light during reaction. It implies that heat my release in a reaction.
• We saw that MgO powder changed the colour of red litmus paper into blue. It means that MgO white powder is basic in nature. And generally metallic oxide is basic in nature.

2.Activity 1.2 : Reaction Of Lead Nitrate with Potassium Iodide

Notes from Activity 1.2 :

• We saw that when we reacted lead nitrate solution with potassium iodide ,the color of the content changed from white to yellow. It means that during a chemical reaction color may change.
• When we will leave the reaction ,An yellow substance lead iodide (if a substance forms which is insoluble in water ,it is known a precipitate) settle down in bottom of beaker which is precipitate. Thus precipitate may form in a chemical reaction.

3.Activity 1.3 : Reaction of Zinc with dilute Hydrochloric Acid

Notes from Activity 1.3 :

• We saw a gas released in this reaction. It means that during a chemical reaction a gas may evolve.
• We saw that when we brought a burning matchstick near the test tube opening ,it burnt with pop up sound which points out that this gas is hydrogen gas.
• If we touch the bottom of test tube containing zinc and HCl ,it feels hot. It points that during a chemical reaction temperature may change.

1.4 Characteristics of chemical reactions

On the basis of above three activities,we can easily see or observe the following features or characteristics when a chemical reaction take place.

Important Characteristics of chemical reactions

1. Change in state : When a candle burns(reacts with oxygen),It forms water and gases. Clearly, there is a change in state due to the chemical reaction.
2. Change in color : When Lead Nitrate reacts with Potassium iodide, there forms yellow color substance lead iodide.So we can say that color may change during chemical reaction. You can watch it in the activity 1.2.
3. Formation of Precipitate : When Lead nitrate reacts with Potassium iodide,there forms a precipitate of Lead iodide. (Watch activity 1.2 to be more certain).
4. Change in Temperature : When zinc reacts with dilute hydrchloric acid,the content of the test tube appear hot. This indicates that there may be change in temperature during a chemical reaction. Watch it in the activity 1.3.
5. Evolution Of Gases : When zinc reacts with dilute hydrochloric acid, hydrogen gas release in the reaction.

1.5 Chemical Change Vs Chemical reaction

Chemical Change : A process in which new substance(s) are formed, is called chemical change.

Examples of chemical changes are :

1. Souring of milk(milk left at room temperature during summer) : This is an example of chemical change because if we taste sour milk ,It is sour and unpleasant and its identity and nature is different from fresh milk which indicates the formation of some new substances.
2. Rusting of iron : Iron oxide forms in the process which is completely different from iron. So this is an example of chemical change.
3. Cooking of food : Chemicals present in food form new substances when we supply heat to the food,so this is an example of chemical change.
4. Ripening of fruits : When fruits get ripened ,its identity,nature and taste is different from raw fruits which indicates that this is an example of chemical change
5. Digestion of food : Digestion means breaking of food into simpler substances. Food contains chemicals like fats,carbohydrates,protein etc. These chemicals are broken down into simpler substances by our various digestive organs such as liver,pancreas,intestine. When the fats,carbohydrates and protein are broken down by our digestive system ,the new substances formed have different identity and nature. Therefore,digestion of food is an example of chemical change.
6. Respiration in our body : In our daily life ,We tell or understand respiration as inhalation or exhalation but Scientifically respiration is a not inhalation or exhalation.It is the process in which glucose in our body reacts with oxygen and forms carbon dioxide ,water and energy. As this is a process in which new substances are being made,so this is an example of chemical change.

Chemical Reaction : A process in which new substance(s) are formed is called chemical reaction.

Question : Chemical change and chemical reaction have same definition, So you will ask that what is the difference between chemical change and chemical reaction because both have same definition?

Ans : Simply speaking both are same but in a chemical change, we don’t focus on the detail of new substance formed in the process ? We do not investigate which are the initial and final substances and what are their names ? We just check nature,identity and taste of the new substance formed in the process to identify a chemical change. .But in a chemical reaction ,We focus or investigate which are the initial substances and what are the final substances formed in the process. We also write the names of initial and final substances formed in a chemical reaction.In a chemical change like cooking of food( protein, fat and other chemicals in food react with oxygen with help of heat),there may be many reactions involved but in a chemical reaction there is only one reaction

Now we can say that whenever a chemical change happens,there occurs a chemical reaction and vice-versa.

2.1 Chemical Equations

Chemical Equation : The representation of a chemical reaction using symbols and formula of substances involved in chemical reaction is called chemical equation. Chemicals equations are short representation of a chemical reaction in more useful and scientific form.

Important notes in writing chemical equation

1. We write name of reactants with help of their symbols and formula in the left side of a equation.If there are more than one reactants, Use the symbol “+” between the reactants.
2. Write the name of products with help of their symbols and formula in the right side of a equation. If more than one products form during chemical reaction,Use the “+” symbol in between two products.
3. Put a symbol of arrow after all the reactants.

1.Writing Equation of burning of Mg ribbon :

Magnesium reacts with oxygen and forms magnesium oxide. We can write it in equation form as :

Magnesium + Oxygen → Magnesium Oxide (This equation is chemical equation written using name of the reactants and products involved in the reaction. If we write reactants and products involved in chemical reaction by their name using “+” and ” →” ,Then the equation is known as word equation.)

Writing this reaction using formula and symbols of reactants and products :

Symbol of Magnesium : Mg

Formula of Oxygen gas : O₂ ( Note that symbol of oxygen atom is O but oxygen gas is different from oxygen atom. Two oxygen atoms are bonded together in oxygen gas and It is written as O₂ . Writing oxygen gas as O is wrong representation,So formula of oxygen gas is O₂ not O.)

Formula of Magnesium Oxide : MgO (Because Valency of Mg is 2 and O is 2 ,You know from class 9th that Mg is multiplied with valency of O and O is multiplied that of Mg to get formula of Magnesium Oxide. We will get Mg₂O₂ = MgO)

Now we can write chemical equation using the formula and symbols of the reactants and products involved as ,

Mg + O₂ → MgO ( This is a chemical equation written with help of symbols and formula of chemicals involved.Informally ,we will call it as formula equation of a chemical equation)

2.Writing equation of reaction of zinc with hydrochloric acid :

When zinc reacts with hydrochloric acid , there forms zinc chloride and hydrogen gas. We can write it as

Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen Gas ( Word equation )

Formula equation

Symbol of Zinc : Zn

Formula of hydrochloric acid : HCl ( Because valency of both hydrogen and chlorine is 1)

Formula of zinc chloride : ZnCl₂ (Because Valence of Zn is 2 and Cl is 1,you know that from class 9th that Zn will be multiplied with valency of Cl and Cl with that of Zn to get formula of zinc chloride )

Formula of hydrogen gas : H₂ (Note that symbol of hydrogen atom is H but Hydrogen gas is different from hydrogen atom. Two hydrogen atoms are bonded or joined together in hydrogen gas and It is written as H₂ . Writing oxygen gas as H is wrong representation,So formula of oxygen gas is H₂ not H.

Now we can write the formula equation for the reaction of zinc with hydrochloric acid as,

Zn + HCl → ZnCL₂ + H₂ ( Formula equation)

2.2 Balanced and Unbalanced Chemical Equation

Balanced Chemical equation : A chemical equation in which number of atoms of different elements of in the reactants and products is same is called balanced chemical equation.

Unbalanced Chemical Equation : A chemical equation in which number of atoms of different elements of in the reactants and products is not same is called unbalanced chemical equation.

Consider Chemical equation for burning of Mg Ribbon :

Mg + O₂ → MgO

Let’s count the number of atoms of different elements in the reaction.

Number of Atoms	In Reactants	In Products
No of Mg atoms	1	1
No of O atoms	2	1

Clearly , No. of oxygen atoms in reactant side and product side is not same , So this is an unbalanced chemical equation.

Now think on the aspect of mass of reactants and products.

Mass of Reactants = mass of one Mg atoms + Mass of two oxygen atoms

Mass of product = Mass of one Mg atom + Mas of one Oxygen atom

Clearly ,Mass of reactants is greater than mass of product because It contains one additional mass of oxygen atom. So we can say that this equation violates the law of conservation of mass which is one of the fundamental laws of chemistry. Therefore, We need to write a balanced chemical equation such that mass of reactants should be equal to the mass of product. This is only possible if no. of atoms of different elements are same in the reactant as well as product side. So,We have to write balanced chemical equation in order to follow the law of conservation of mass.

To write balanced chemical equation, We multiply reactants and product with specific number so that number of different atoms in reactant side and product side becomes same. Let’s write the balanced chemical equation of this reaction.

2Mg + O₂ → 2MgO

Here we have multiplied Mg by 2 ( Note that It is written 2Mg not Mg₂ because meaning Mg₂ will be different. Meaning of Mg₂ is that two atoms of Mg are bonded or joined together)and MgO by 2. This makes the chemical equation balanced. Now you can check that mass of reactants and product is same in balanced chemical equation. A balanced chemical equation does not violates the law of conservation of mass ,so we should write chemical equation in balanced form.

2.3 How to Balance Chemical Equations ?

We balance a chemical equation by hit and trial Method in class 10th, There are some other methods which we will learn in advanced class. Let’s discuss hit and trial method

Hit And Trial Method

1. This is also known as trial and error method.This is based on simple guessing and we do not apply any specific rules.
2. In this method, We guess simply that what should be multiplied in the formula or symbols of different reactants such that total number of atoms of different elements become same in both reactants and products.
3. Let’s take example of Mg + O₂ → MgO , Clearly number of oxygen atoms in reactant side and product side is not same. Our simple guessing say that if we multiply Mg by 2 and MgO by so that it becomes 2Mg + O₂ →2MgO , Now We can say that multiplying 2 with Mg and MgO completes our job that number of atoms of different element in reactant side and product side becomes same.

Balancing equation of iron with water by hit and trial method :

Fe + H₂O → Fe₃O₄+H₂

1 . Clearly ,Number of Fe and O atoms are not same in reactant side and product side.

2 . If we multiply Fe by 3 in left side, then number of Fe atoms will be balanced. And Equation becomes 3Fe + H₂O → Fe₃O₄ + H₂

3 . Now we have to balance number of O atoms in both sides to get a balanced chemical equation. We are doing it by just guessing not applying any specific rule.

4 . To balance number of O atoms , We need to multiply H₂O by 4 , So our equation becomes 3Fe +4H₂O →Fe₃O₄ +H₂

5. Now check the number of H atoms both sides, There is 8H atoms in reactant and 4H atoms in product. So Now we need to balance H atoms in both sides

6. If we multiply H2 in product by 4 , number of H atoms will be balanced in both sides. Thus our balanced chemical equation is :

3Fe +4H₂O →Fe₃O₄ +4H₂

3. Types of Chemical Reactions

We will study following five important types of chemical reaction in this chapter.

1. Combination Reaction
2. Decomposition Reaction
3. Displacement Reaction
4. Double Displacement Reaction
5. Oxidation and Reduction Reaction

3.1 Combination Reaction

Combination Reaction : A reaction in which two or more reactants (element or compound) combine to form a single substance as product is called combination reaction.

Examples of Combination reaction :

(i) Reaction of Calcium Oxide (quick lime) With Water : Activity 1.4 of NCERT book

Let’s watch or perform the reaction of calcium oxide with water as activity 1.4 of NCERT book in order to observe and conclude some points.

Watch Activity 1.4

We saw in the activity that calcium oxide reacts with water and forms calcium hydroxide. It also releases heat. We can write the chemical equation as CaO(s) + H₂O (l)→ Ca(OH)₂ (aq) + Heat . Clearly from the equation calcium oxide(CaO) reacts with water(H₂O) to form a single product Calcium hydroxide (Ca(OH)₂). So this is an example of combination reaction.

Notes from Activity 1.4

1. This reaction gives us large amount of heat hence it is called exothermic reaction. The reaction which gives us heat in product side is exothermic reaction.
2. Slaked lime is basic in nature because it turns red litmus into blue color.

An Important Discussion

• Common name of Calcium Oxide is Lime or Quick Lime , Common name of calcium hydroxide is Slaked Lime and Common name of calcium carbonate is Chalk.
• When Calcium hydroxide or slaked lime reacts with CO₂ ,It forms Calcium Carbonate (CaCO₃ ). The reaction between slaked lime with CO₂ can be written as Ca(OH)₂ (aq)+ CO₂ (g) → CaCO₃(s) . Calcium Carbonate appears whitish and shiny.
• Solution of slaked lime is used to whitewash the walls in our room or building to make our walls shiny and whitish. Slaked lime reacts with CO₂ present in atmosphere and forms Calcium Carbonate ( CaCO₃) . Calcium Carbonate appears whitish and shiny so our walls become shiny and whitish

(ii) Burning of Mg ribbon : 2Mg(s) + O₂(g) → 2MgO (s)

We have seen this reaction in the activity 1.1, Magnesium (Mg) reacts with oxygen (O₂) to form a single substance magnesium oxide(MgO) . Clearly a single substance MgO forms in the reaction so this is an example of combination reaction.

(iii) Reaction of Sodium with Chlorine : 2Na(s) + Cl₂ (g) → 2NaCl(s)

(iv) Burning of Coal : C(s) + O₂(g) → CO₂ (g)

(v) Formation of water from : H₂ (g) + O₂ (g) → H₂O(l)

3.2 Decomposition Reaction

Decomposition Reaction : A reaction in which a single substance decomposes to give two or more substances is called decomposition reaction. Decomposition reactions are opposite to combination reactions.

A substance in decomposition reaction can be decomposed by heat,electrolysis (electrolysis means breaking of a reactant by electricity ) and sunlight. The decomposition by heat is known as thermal decomposition, the decomposition by electrolysis is called electrolytic decomposition and decomposition by sunlight is known as photo decomposition.

Examples of decomposition reaction :

(i). Decomposition of Ferrous Sulphate : Activity 1.5 of NCERT book

Let’s watch or perform the activity 1.5 of NCERT book in order to observe and conclude some points .

Watch Activity 1.5 of NCERT Book

We observed in the activity that ferrous sulphate on heating decomposes into ferric oxide,sulphur dioxide and sulphur trioxide. With the help of activity,we can write the chemical equation of the reaction as : FeSO₄ (s) ^heat→ Fe₂O₃ (s) + SO₂(g) + SO₃(g) . Clearly from the equation a single substance ferrous sulphate(FeSO₄) decomposes into ferric oxide(Fe₂O₃),sulphur dioxide(SO₂) and sulphur trioxide (SO₃),so this is an example of decomposition reaction as single substance breaks into two or more than two products. This decomposition will be called as thermal decomposition because it occurs with the help of heat.

Notes from Activity 1.5

1. ´We saw that when we heat ferrous sulphate crystal , it releases water in the surrounding of test tube. It points that ferrous sulphate crystal(FeSO₄.7H₂O) seems to be dry but contain water molecule in each molecule of ferrous sulphate. This is called water of crystallization. We call that ferrous sulphate contain 7 water of crystallization.
2. We saw that gases released in the process turned blue litmus paper to red, so the nature of these gases are acidic.
3. We saw in the bottom a blackish-brown substance forms.This is ferric oxide.
4. Vehicles, industrial plants and factories releases the SO₂ and SO₃ which causes pollution and acid rain.

(ii) Decomposition of Lead Nitrate : Activity 1.6 of NCERT book

Watch or perform the activity 1.6 of NCERT book in order to write some important points.

Watch activity 1.6 of NCERT book

We saw in the activity that lead nitrate on heating decomposes into lead oxide,nitrogen dioxide and oxygen gas.From the above activity we can write the chemical equation as : 2Pb(NO₃)₂ (s) ^heat→ 2PbO (s)+ 4NO₂ (g) + O₂ (g), Where Pb(NO₃)₂ = Lead Nitrate , PbO = Lead Oxide and O₂ = oxygen gas . Clearly from the equation a single substance lead nitrate decomposes into more than two substances ,So this is an example of decomposition reaction. It will be also called thermal decomposition because single substance is broken by supplying heat to the reactant.

Notes from activity 1.6

1. When we heat the test tube at bottom a brown fumes releases which is nitrogen dioxide (NO₂) and an yellow color solid forms in the test tube which is lead oxide(PbO)
2. This is an endothermic reaction as we supply heat in reactant side.
3. We saw that gas released in the process turned blue litmus to red colour ,It points out that NO₂ is acidic in nature.
4. The gases NO₂,SO₂, and SO₃  are released by vehicles ,industrial plants and factory which causes acid rain and pollution.

(iii) Decomposition of Water by electricity : Activity 1.7 of NCERT book

2H₂O(l) ^Electricity → 2H₂ (g)+ O₂ (g)

Watch or perform activity 1.7 of NCERT book in order to observe and conclude some points.

Watch Activity 1.7 of NCERT book

We saw that on passing electricity through water ,it decomposes into hydrogen and oxygen gas.Now we can write chemical equation as 2H₂O(l) → 2H₂(g) + O₂(g). Clearly from equation a single substance breaks down into two substance so this is an example of decomposition reaction. This is called electrolytic decomposition as it happens through electricity.

Notes From Activity 1.7

1. We saw that some gases released in both the test tube when we connected the electrode with battery. It points out that substances can be broken by electricity. Decomposition reaction which takes place with help of electricity is called electrolytic decomposition.
2. We saw that volume of gas in one test tube was double that of the other. Actually volume of hydrogen gas is double than that of oxygen. Why the volume of hydrogen gas is double ? Look at the balanced chemical equation 2H₂O(l) → 2H₂(g) + O₂(g) , Clearly no. of molecules of hydrogen gas is double that of oxygen gas that is the reason volume of hydrogen gas formed is double than the volume of oxygen gas in test tube. Note that volume of gases depends on number of molecule, We will study more about the relation between volume and molecules of gases in out 11th class or grade.

(iv) Decomposition of Silver Chloride by sunlight

When silver chloride is left or exposed in sunlight for some time, it starts turning into grey color from its white color. It points out toward some chemical reaction. When silver chloride is left in sunlight ,It is decomposed by sunlight to form silver and chloride. The reaction is 2AgCl(s) ^sunlight→ 2Ag(s) + Cl₂(g)

The reaction in which reactant is decomposed by the energy of sunlight is called photo decomposition. So this is an example of photo decomposition.

(v) Decomposition of silver bromide by sunlight

Silver bromide is decomposed similar to the decomposition of silver chloride if it is exposed in sunlight. The reaction can be written as 2AgBr(s) ^sunlight→ 2Ag(s) + Br₂(g)

3.2.1 Endothermic and Exothermic Reaction

We saw in decomposition of lead nitrate (activity 1.6) that we have to supply heat in order to proceed reaction, such a reaction is called endothermic reaction.

Endothermic Reaction : A reaction in which heat is absorbed by the reactants from surroundings is called endothermic reaction.In other words, We can say that a reaction in which we supply heat or energy to proceed the reaction is called endothermic reaction.

Examples of endothermic reaction:

(i) Decomposition of ferrous sulphate(Activity 1.5) : 2FeSO₄ (s) ^Heat→ Fe₂O₃ (s) + SO₂(g) + SO₃(g)

We supply or give heat to the reactant in order to reaction to happen happen that is heat is absorbed by ferrous sulphate from surroundings ,So this is an example of endothermic reaction.Watch activity 1.5 to be more clear.

(ii)Decomposition of Lead Nitrate : 2Pb(NO₃)₂ (s) ^heat→ 2PbO (s)+ 4NO₂ (g) + O₂ (g)

(iii) Decomposition Of Lime Stone : CaCO₃(s)  ^Heat→   CaO(s) + CO₂(g)

(iv) Decomposition Of Silver Chloride : 2AgCl(s)  ^Sunlight →2Ag(s)    +    Cl₂(g)

With the help of energy from sunlight ,This reaction proceed , so this is an example of endothermic reaction as it takes energy from sunlight.

Exothermic Reaction : We saw in the reaction of quick lime with water (activity 1.4 ), It gives us heat. This reaction release energy in the surroundings, Such a reaction is called exothermic reaction.

Exothermic Reaction : A reaction which releases heat in surroundings is called exothermic reaction. Heat or energy is given out along with products in an exothermic reaction.

Examples of exothermic Reaction :

(i) Reaction of Calcium Oxide with Water(activity 1.4) :CaO(s) + H₂O (l)→ Ca(OH)₂ (aq) + Heat

In this reaction ,Calcium oxide reacts with water and produce large amount of heat(watch activity 1.4) so this is an example of exothermic reaction.

(ii) Burning of natural gas :     CH₄(g) + 2O₂ (g) →     CO₂ (g) + 2H₂O (g) + Heat

(iii) Respiration in our body :C₆H₁₂O₆(aq) + 6O₂(aq)    →    6CO₂(aq) + 6H₂O(l) + energy

(iv) Burning Of Mg Ribbon : 2Mg + O2  →   2MgO + Heat

In this reaction, Note that we supply heat to Mg ribbon in order for reaction to proceed so we can say that this should be endothermic reaction,But remember that Mg ribbon also gives large amount of heat along with product. Energy given is greater than energy absorbed so this is an example of exothermic reaction.

3.3 Displacement Reaction

Displacement Reaction : A chemical reaction in which more reactive element displaces less reactive element from its compound is called displacement reaction. It is also known as single displacement reaction.Suppose we have metal A and Salt solution of Metal B and also consider that Metal A is more reactive than Metal B then It can be represented as

Metal A + Salt solution of Metal B  → Salt solution of Metal A + Metal B

Examples of displacement Reaction :

(i) Reaction of Iron with Copper Sulphate : Activity 1.9 of NCERT book

Let’s watch the activity 1.9 to see this reaction.

Activity 1.9 of NCERT book

We observed in the activity that iron displaces the copper from copper sulphate solution and forms iron(II) sulphate and copper. With the help of above activity , We can write the chemical equation as :Fe(s) +  CuSO₄(aq) → FeSO₄(aq) + Cu(s) . Clearly from the equation iron being more reactive displaces copper from copper sulphate. Hence this is an example of displacement reaction.

Important Notes From Activity 1.9

1. When we put iron nails in copper sulphate solution ,Its color turns out to be of light green color. The colour changes because iron being more reactive than copper displaces copper from copper sulphate and make iron sulphate which is of light green colour.
2. You can see copper is deposited on the iron nail in the reaction.

Reactivity Series : In order to decide which element will displace another element from its salt , We need reactivity series. Reactivity series is the list of element in increasing or decreasing order of reactivity of element.The following picture describes the reactivity series in decreasing order.

(ii)Reaction of Zinc with iron sulphate : Zn(s) + FeSO₄(aq)  →   ZnSO₄(aq) + Fe(s)

(iii)Reaction of copper with silver nitrate : Cu(s)  + AgNO3(aq)   →  Cu(NO3)2 + 2Ag(s)

3.4 Double Displacement Reaction

Double Displacement Reaction : A reaction in which atoms or group of atoms are exchanged to form new products is called double displacement reaction.

Examples of double displacement reactions :

(i) Reaction of Sodium Sulphate with Barium Chloride : Activity 1.10

Watch or perform the activity 1.10 as described in the video. Watching the activity ,we can write the reaction as

We saw in the activity that sodium sulphate reacts with barium chloride and forms barium sulphate and sodium chloride. With the help of above activity ,We can write the chemical equation as : Na₂SO₄(aq) + BaCl₂(aq)  →  BaSO₄(s)  +   2NaCl(aq). This is an example of double displacement reaction because ions of the two ionic compounds exchange to form new substances.

Important Notes From Activity 1.10

1. We saw in the activity that a white colored solid precipitate forms which is barium sulphate. As a precipitate forms in this reaction , this is also an example of precipitation reaction.
2. In this reaction, ions of both the reactants interchange to form new products . Sodium ions in Sodium Sulphate by interchanging joins with chloride ions of Sodium Chloride and Sulphate ions join with barium ions to make new products. A reaction in which ions are interchanged to make new products is called double displacement reaction. This is an example of double displacement reaction.

(ii) Reaction of Lead Nitrate with Potassium Iodide : Activity 1.2 of NCERT Book

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

(iii) Reaction of Sodium Chloride with Silver Nitrate

NaCl   +   AgNO₃   → NaNO₃  + AgCl

3.5 Oxidation and Reduction

When we will go in higher class , we will find more precise definition of oxidation and reduction. In class 10th, we will define oxidation and reduction in terms of gain or loss of oxygen and hydrogen.

Definition Of Oxidation :

1. Gain of oxygen in a substance is called oxidation.
2. Loss of hydrogen from a substance is also called oxidation.

Definition of Reduction :

1. Gain of hydrogen in a substance is called reduction.
2. Loss of oxygen from a substance is called reduction.

Note : If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidized. And If a substance loses oxygen or gains hydrogen during a reaction, it is said to be reduced.

Examples of Oxidation and Reduction :

(i)Reaction of copper with Oxygen : Activity 1.11 Of Ncert Book

Watch or perform the activity 1.10 of NCERT book

We saw in the activity that copper reacts with oxygen and forms copper(II) oxide.With the help of the activity we can write the reaction as : 2Cu + O₂    ^Heat→  2CuO .Clearly from the equation,there is addition of oxygen in copper so this is an oxidation reaction. Clearly,copper gains oxygen so it is being oxidized.

(ii) Reaction of nitrogen gas with hydrogen gas : N₂   +  3H₂  → 2NH₃

Clearly ,there is gain of hydrogen in the nitrogen gas so this is a reduction. As nitrogen gas gains hydrogen hence it is being reduced.

3.5.1 Redox Reaction

Redox Reaction : A reaction in which one reactant gets oxidized and other reactant gets reduced is called redox reaction.In other words,a reaction in which oxidation and reduction take place simultaneously is called redox reaction.

Examples :

(i) Reaction Of Copper Oxide with Hydrogen gas

In this reaction , Copper Oxide is losing oxygen so this is a reduction and copper oxide is being reduced .And the hydrogen is gaining oxygen so this is an oxidation and hydrogen is being oxidized here. In this reaction , Oxidation and Reduction takes place simultaneously so this is an example of redox reaction.

(ii) Reaction Of Zinc Oxide with Carbon

ZnO + C  → Zn +CO

3.5.2 Effects of oxidation and reduction in everyday life

Effect of Oxidation :

1. Iron and iron like material aluminium ,copper and silver are left exposed to air ,then these materials get oxidized. As a result of oxidation ,the black or green coating forms on the outer layer of these metal. Rusting of iron is an example of oxidation of iron.
2. When iron like materials are attacked by air, water vapours and acids in surrounding then it is said to be corroded. This process is called corrosion.

Prevention from Oxidation :

1. Electroplating is modern way to prevent corrosion in iron like material. A layer of chromium is deposited on the outer layer of iron like material to prevent corrosion because chromium does not corrode and gives a shiny cover to these materials.
2. Most of the materials like car building parts, bath taps ,bicycle handles etc are electroplated by chromium.
3. You can also paint these kind of materials to prevent corrosion.

Rancidity :

1. When fats /oils are oxidised, they become rancid and their smell and taste change. Antioxidants are added in substances which contain fats or oil.
2. Oil/fats containing foods are flushed with nitrogen to prevent rancidity.

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