Quick Views CH- 1 Chemical Equation and Reaction

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Class X 
    CH- 1 Chemical Equation and Reaction

Chemical reactions: Chemical equation, Balanced chemical equation, the implication of a balanced chemical equation, types of chemical reactions: Combination, decomposition, displacement, double displacement, precipitation, neutralization, oxidation and reduction

1. Chemical Equation is a representation of the equation of chemical reaction that represents the reactants, products, and their physical states symbolically.

2. Balanced Chemical equation- A chemical equation is balanced so that the numbers of atoms of each element involved in a chemical reaction are the same for both the reactant and the products. Equations must always be balanced.

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3. Changes can be taken place in a chemical reaction  –

Chemical change - is a change in which one or more substances are formed.

Example - Cooking of food, souring of milk, ripening of fruits and Rusting of iron.

i. Change in the state- By the electrolysis of water (liquid) converted into hydrogen gas and oxygen gas.       

ii.Change in colour – Light green Ferrous sulphate crystals (FeSO_4, 7H₂O) lose water when heated and the colour of the crystals changes to white.

iii.Evolution of a gas -  Formation of hydrogen gas by the action of dilute sulphuric acid on zinc.      

iv. Change in temperature - Calcium oxide (Quick lime) reacts vigorously with water to produce calcium hydroxide (slaked lime ) and releasing a large amount of heat.

4. Skeletal Equation – An unbalanced chemical equation is called the skeletal equation for a reaction.

The skeletal equation for the burning of magnesium in the air –

Mg + O₂ = MgO, this is an unbalanced chemical reaction because the masses or atoms are not the same on both sides of the equation.

5. Uses of calcium hydroxide- The solution of slaked lime Ca(OH)₂ is used for whitewashing the walls. Calcium hydroxide reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after some days of whitewashing and gives a shiny finish to the walls.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

6. Balanced equation - Balanced equation is the equation in which atoms of the various elements on the reactants and product sides are equal. According to the law of conservation of the mass, the number of atoms of elements in both the sides of the chemical equation should be equal.

7. Chemical reactions - Chemical reaction is a reaction in which any chemical change in the reactants takes place whether the reaction is combination, decomposition, displacement or rearrangement of elements which involves the transformation of a substance into one or more new substances.

8. Types of chemical reactions –

(i) Direct combination reaction (synthesis) – When two or more reactants are combined to form one compound is known as a combination reaction.                             Ex – 2Mg + O₂ = 2MgO

(ii) Decomposition – In this reaction single reactant decomposed into two or more products.

Ex – (i) 2Pb (NO₃)₂ → 2PbO + 4NO₂ + O₂                                                                         (Brown gas)

(ii) CaCO₃ Heat → CaO + CO₂

(iii) White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine in the presence of light. 

2AgCl(s) Sunlight → 2Ag(s) + Cl₂(g)

(iv) Displacement – A chemical change in which a more reactive element displaces a less reactive element from a compound.

Ex – CuSO₄ + Ca → CaSO₄ + Cu

2AgBr₂ →2Ag + Br₂ This reaction is used in black and white photography.

(v) Double decomposition or displacement reaction – In this reaction, two compounds react to form two other compounds by exchanging their radicals or ions and one of the products formed as insoluble form.

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

AgNO₃ + HCl → AgCl↓ +HNO₃ .

(vi) Precipitation reactions produce insoluble salts.

 BaCl₂ (aq)+ H₂SO₄ → BaSO₄ (s) + HCl

(vii) Exothermic reaction - Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Examples of exothermic reactions are –

(i) Burning of natural gas

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

(ii) Respiration is exothermic - Glucose (present in food) combines with oxygen (which is obtained from respiration) in the cells of our body and gives energy.

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

(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

(viii) Endothermic reaction - The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.

CaCO₃  +  heat   →   CaO   +   CO₂

(Calcium carbonate)    (Calcium oxide) (Carbon dioxide)

(ix) Neutralization reaction – The reaction between acid and base that forms a salt and water only is called a neutralization reaction.

H₂SO₄ + NaOH → Na₂SO₄ + H₂O

 (Acid)  (Base)       (Salt)   (Water)

(x) Oxidation – It is a reaction in which an atom or ion loses electrons by adding oxygen or by adding electronegative ions or by removing hydrogen or by removing electropositive elements.

(i) Ex – C+O₂ = CO₂ .           

(Carbon is oxidized to carbon dioxide by adding oxygen) 

(ii) Zn + 2HCl = ZnCl₂ + H_2.   

( Zn is oxidized to ZnCl₂ by adding electronegative ion chlorine)

(iii) H₂S + Cl₂ = 2HCl + S.        

(Hydrogen sulphide is oxidized to S by removing hydrogen)

(iv) 2KI + H₂O₂ = 2KOH + I₂   

(Potassium iodide oxidized to iodine by removing electropositive ion.)

(xi) Reduction reaction - It is a reaction in which an atom or ion gains electrons by removing oxygen or by removing electronegative ions or by adding hydrogen or by adding electropositive elements.

(i) CuO + H₂ → Cu + H₂O                       

(Copper Oxide reduced to copper by removing oxygen)

(ii) 2FeCl₃ + H₂S→2FeCl₂ +2HCl + S  

(Ferric chloride is reduced to ferrous chloride by removing electronegative ion Cl^-)

(iii) 2NH₃ + 3Cl₂ = N₂ + 6HCl                 

(Chlorine is reduce to hydrogen chloride by adding hydrogen)

(iv) HgCl₂ + Hg →  Hg₂Cl₂ →HgCl       

 (Mercuric chloride reduced to mercurous chloride by adding electro positive ion)

(v) CuCl₂ + Cu →  CuCl (Cu₂Cl₂) → CuCl        

(Cupric chloride reduction to cuprous chloride by adding electro positive ion)

(xii) Redox reaction – Reaction in which both oxidation and reduction takes place is called redox reaction.

Oxidation (Removal of hydrogen)

           ↑

          H₂S + Cl₂ →2HCl + S                                    

                                       ↓

       Reduction (Addition of hydrogen)

Oxidizing agent – A substance that helps to oxidize the other the substance is called an oxidizing agent.

Reducing agent – A substance that helps to reduce the other the substance is called a redesigning agent. CuO + H₂ → Cu + H₂O

In this reaction hydrogen is a reducing agent that reduces CuO to Cu.

And CuO is an oxidizing agent that oxidizes H₂ to H₂O.

9. Corrosion - When a metal is attacked by substances around it such as air, moisture, acids, etc, it is said to corrode and this process is called corrosion.

Examples of corrosion:

(i) Rusting –It is the corrosion of iron. When iron reacts with oxygen present in the air in the presence of water or moisture, hydrated iron (III) oxide is formed that known as rust.

4Fe + 3O₂ + 2xH₂O →2Fe₂O₃.xH₂O (Rust)

(ii) The black coating on silver – It is due to corrosion. When silver reacts to moisture and sulfur presents in the air, a black coating of silver sulfide (Ag₂S) is formed.

(iii) The green coating on copper – It is due to corrosion. When copper reacts with carbon dioxide and moisture, a coating of copper carbonate and copper hydroxide [CuCO₃ Cu(OH)₂] is formed.

10. Rancidity - When fats and oils present in food materials are oxidized by oxygen present in the air, they become rancid and their smell and taste change. This process is called rancidity. It is prevented by using antioxidants like reducing agents and nitrogen gas (nonreactive gas).

To avoid. rancidity or prevents from oxidized, the manufacturers of potato chips fill the plastic bags containing chips with nitrogen which is nonreactive gas.

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