CLASS X (CHEMICAL REACTIONS & EQUATIONS)

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INTRODUCTION

·         In Class IX we have learnt about physical changes and chemical changes. Chemical changes lead to chemical reactions between two or more substance.

·         In this chapter we will learn about chemical reactions and equations.

·         Any compound can be found in solid state (s), liquid state (l), gaseous state (g) or in aqueous solution (aq)

CHEMICAL REACTIONS

·         It is a process in which a substance(s) undergo chemical changes to produce new substances with new properties.

·         The substance which takes part in a chemical reaction is called reactant.

The substance which is formed after a chemical reaction is called a product. eg-

Na  + H₂O   -->   NaOH  + H₂↑

                                                       ^{(Sodium)    (Water)     (Sodium Hydroxide)  (Hydrogen Gas)}

CaCO₃   -->   CaO + CO₂↑

                                                    ^{(Calcium Carbonate)    (Calcium Oxide)   (Carbon Dioxide)}

These are some simple examples of chemical reaction. Where substance(s) before the arrow (which denotes chemical change) is reactant and the substances formed after the chemical change (denoted by arrow) is product.

·         Sometime chemical reaction occur with any breaking or forming of bonds, then change which occur is due to re-arrangement of existing bonds. eg-

·         These kind of compounds which are formed by rearrangement of existing bonds are called isomers. They have the same molecular formulae but different structure.

·         And these reactions are called isomerisation reaction.

Ø  CHARACTERISTICS OF CHEMICAL REACTIONS

·         When a chemical reaction occurs the following changes are observed:

1.    Evolution of gas: Some chemical reactions occur with evolution of one or more gases. eg-

        i.            Zinc reacts with hydrochloric acid to form zinc chloride and evolve hydrogen gas.

Zn + 2HCl   -->   ZnCl₂   + H₂↑

                                                    ^{(Zinc) (Hydrochloric Acid)   (Zinc Chloride) (Hydrogen Gas)}

      ii.            Lead nitrate is heated, it gets decomposed to give a solid residue of lead monoxide with the evolution of nitrogen dioxide and oxygen gasses.

2Pb(NO₃)₂   -->   2PbO   + 4NO₂↑ + O₂↑

                                               ^{(Lead Nitrate)            (Lead Monoxide) (Nitrogen Dioxide)(Oxygen Gas)}

2.    Formation of precipitate: Sometime when two solutions are mixed we see that the resulting product is a solid which gets settled at the bottom of the vessel, this solid substance is called precipitate. eg-

        i.            When an aqueous solution of sodium sulphate is mixed with a solution of barium chloride, a white ppt. of barium sulphate is formed in sodium chloride solution.

Na₂SO₄ + BaCl₂   -->   BaSO₄↓ + 2NaCl

                                       ^{(Sodium Sulphate)(Barium Chloride)     (Barium Sulphate)(Sodium Chloride)}

      ii.            When an aqueous solution of lead acetate and potassium iodide are mixed, an yellow precipitate of lead iodide is produced.

(CH₃COO)₂Pb + 2KI   -->   PbI₂↓   + 2CH₃COOK

                                       ^{(Lead Acetate)  (Potassium Iodide)             (Lead Iodide)   (Potassium Acetate)}

3.    Change in temperature: There are various chemical reaction which occur with increases or decrease in temperature. eg-

        i.            Exothermic reaction: This is a type of chemical reaction in which there is a release of heat or energy when the reaction occurs. Like when calcium oxide (quick lime) is treated with water it produces calcium hydroxide (lime water) and heat is released.

CaO + H₂O   -->   Ca(OH)₂ + Heat

                                            ^{(Calcium Oxide)(Water)              (Calcium Hydroxide)}              

      ii.            Endothermic reaction: This is a type of chemical reaction in which heat or energy is given to initiate the reaction. Like when ammonium chloride and barium hydroxide are mixed together the bottom of the vessel gets cold as heat is absorbed to form ammonium hydroxide and barium chloride.

2NH₄Cl + Ba(OH)₂    + Heat   -->   2NH₄OH + BaCl₂

                        ^{(Ammonium Chloride)(Barium Hydroxide)                              (Ammonium Hydroxide)(Barium Chloride)}

4.    Change in colour: In some chemical reaction there is a change of colour. eg-

        i.            When chlorine water (yellow) is added to a solution of potassium iodide (colourless), a brown solution is observed.

Cl₂ + 2KI   -->   2KCl + I₂

                                           ^{(Chlorine Water)(Potassium Iodide)(Potassium Chloride)(Iodine)}

      ii.            When iron (grey) is left in open it gets rusted (brown).

4Fe + 3O₂ + xH₂O   -->   2Fe₂O₃.xH₂O

                                                   ^{(Iron)(Oxygen Gas)(Water)                                          (Rust)}

5.    Change in physical state: In some chemical reaction we see that the physical state of the reactants change to form different products. Like burning of candle. Candle which is solid when burnt form carbon dioxide gas and water vapour.

v  One or more of the above changes certify that a chemical reaction is occurring.

CHEMICAL EQUATIONS

·         Chemical equations are symbolic representation of a chemical reaction. Where symbols and formulae are used to represent the change of reactant to product.

Ø  RULES FOR WRITING A CHEMICAL REACTION:

Ø  BALANCED AND UNBALANCED CHEMICAL EQUATION

1.      Balanced chemical equation: A balanced chemical equation is one which contains an equal number of atoms of each element on both sides of the equation.

1.      Unbalanced chemical equation: An unbalanced chemical equation is one which does not contain the equal number of atoms of each element on both side of the equation.

Ø  BALANCING CHEMICAL EQUATION

·         Chemical reactions follow law of conservation of mass, i.e., matter can neither be created nor be destroyed, they can be interconvert.

·         The following points should be kept in mind before balancing a chemical equation:

        i.            The correct formula or symbol for each substance involved in the reaction must be known.

      ii.            None of the subscripts of the formulae on the left hand side can be changed in balancing the equation.

·         The method used in our course of study for balancing a chemical equation is called hit and trial method.

·         In this method we balance the number of atoms on each side by changing the number of moles before the compounds.

·         Like for H₂ + O₂ à H₂O we write 2H₂ + O₂ à 2H₂O

Ø  Information conveyed by a chemical equation

Ø  Advantages of using a chemical equation

1.      Representation of a reaction becomes easy, it saves time and space for writing.

2.      To prepare a definite amount of product, the amount of reactant required can be calculated.

3.      The same symbols and formulae are used all over the world, hence there is no confusion in understanding a chemical equation.

Ø  Limitation of chemical equation

1.      It does not indicate the physical state of the substances.

2.      It does not indicate heat absorbed or released.

3.      It does not indicate conditions for the reaction like temperature, pressure, concentration, catalyst etc.

4.      It does not indicate if the reaction is fast or slow.

5.      It does not indicate if the reaction is explosive.

6.      It does not say anything about the actual amount of reactant consumed or the amount of product formed, it gives only the ratio.

Ø  How to overcome the limitations and make the chemical equation more informative

1.      The physical state can be expressed with symbols like ‘s’ for solid, ‘l’ for liquid, ‘g’ for gas and ‘aq’ for aqueous solution in subscript after writing the formula of the compound. eg-

Zn + H₂SO₄ à ZnSO₄ + H₂ can be written as Zn_(s) + H₂SO_4(aq) à ZnSO_4(aq) + H_2(g)

2.      Evolution of gas can be expressed with an upward arrow (↑) after writing the formula of the gas evolved. eg-

Zn_(s) + H₂SO_4(aq) à ZnSO_4(aq) + H_2(g)↑

3.      Formation of precipitate can be expressed with a downward arrow (↓) written after formula of the compound precipitating. eg-

NaCl_(aq) + AgNO_3(aq) à AgCl_(s)↓ + NaNO_3(aq)

4.      In case of exothermic reactions heat can be written on the product side of the equation and in case of endothermic reaction heat can be written on the reactant side of the equation. Heating can be denoted by ‘Δ’ and cooling by ‘∇’. eg-

C_(s) + O_2(g) à CO_2(g) + Heat

N_2(g) + O_2(g) + Heat à 2NO_(g)

5.    Specific conditions like temperature, pressure and catalyst can be indicated above or below  the arrow line. eg-

TYPES OF CHEMICAL REACTIONS

1.    COMBINATION REACTIONS

·         The reaction in which two or more reactants combine to form a single product is known as combination reaction. They are of three type. [“हम साथ सात हैं ”]

A.      Combination of two or more elements. eg-

2H₂ +  O₂        -->            2H₂O

2Mg + O₂           -->         2MgO

2Na + Cl₂            -->       2NaCl

B.      Combination of two or more compounds. eg-

NH₃ + HCl        -->          NH₄Cl

CaO +  CO₂       -->           CaCO₃  + Heat

C.      Combination of an element with a compound. eg-

2CO + O₂           -->         2CO₂

2SO₂ + O₂        -->           2SO₃

2.    DECOMPOSITION REACTION

·         The reaction in which a single reactant breaks down into two or more simpler products is known as decomposition reaction. They are of three type. [“एक से बने अनेक”]

A.      Thermal decomposition (decomposition by applying heat). eg-

CaCO₃  + Δ        -->          CaO + CO₂

2Pb(NO₃)₂ + Δ          -->            2PbO + 4NO₂ + O₂

2NaHCO₃ + Δ           -->              Na₂CO₃ + CO₂            + H₂O

B.      Electrolytic decomposition (decomposition by applying electricity). eg-

2H₂O      +      electricity                -->               2H₂ +  O₂

2NaCl                +       electricity             -->                  2Na + Cl

2Al₂O₃     +     electricity               -->                4Al + 3O₂

C.      Photolytic decomposition (decomposition by applying sunlight). eg-

2AgCl + hν       -->          2Ag + Cl₂

2AgBr + hν       -->         2Ag + Br₂

3.    SUBSTITUTION OR DISPLACEMENT REACTION

·         The reaction in which an atom or a group of atom is replaced by another atom or group is called a substitution or displacement reaction. They are of 3 types. [“इसकी टोपी उसके सिर”]

A.      Displacement of less reactive metal by more reactive metal. eg-

Fe + CuSO₄             -->               FeSO₄  + Cu

Cu + 2AgNO₃             -->            Cu(NO₃)₂ + 2Ag

Zn + CuSO₄             -->               ZnSO₄  + Cu

B.      Displacement of less active non-metal from its compound. eg-

2NaBr + l₂         -->        2NaCl  + Br₂

2KI +  Cl₂         -->          2KCl +            I₂

C.      Displacement of hydrogen from acids by active (reactive) metals. eg-

Zn + H₂SO₄                -->            ZnSO₄  + H₂↑

4.    DOUBLE DISPLACEMENT REACTION

·         The reaction in which there is an exchange of an atom or a group of atom between two compounds is called double displacement reaction. eg-

NaCl +            AgNO₃               -->                     AgCl↓  + NaNO₃

BaCl₂   + Na₂SO₄          -->          BaSO₄↓ + 2NaCl

FeS +  H₂SO₄             -->             FeSO₄  + H₂S↑

CuSO₄ + H₂S             -->             CuS↓    + H₂SO₄

5.    PRECIPITATION REACTION

·         The reaction in which a solid compound formed in a reaction settles down in the solution is called precipitation reaction. example given above.

6.    NEUTRALIZATION REACTION

·      The reaction in which an acid reacts with a base to form a salt and water is known as neutralization reaction. eg-

HCl + NaOH                 -->          NaCl +            H₂O

H₂SO₄  + 2KOH              -->                   K₂SO₄  + 2H₂O

7.    REDOX (REDUCTION-OXIDATION) REACTION

·         Oxidation: A process in which Oxygen atom(s) are added to the substance or Hydrogen atom(s) are removed from the substance or there is a loss of electron(s) from the substance, is known as oxidation. eg-

2Ca +  O₂         -->           2CaO

2HCl       -->         Cl₂ + H₂

·         Reduction: A process in which Hydrogen atom(s) are added to the substance or Oxygen atom(s) are removed from the substance or there is a gain of electron in the substance, is known as reduction. eg-

2H₂ + O₂         -->            2H₂O

CuO + H₂           -->         Cu + H₂O

·         Redox: A reaction in which the process of oxidation and reduction takes place simultaneously, is known as redox reaction. eg-

·         Oxidising Agent: The substance which reduces itself to oxidise other substances in a redox reaction, is known as Oxidising agent. eg- CuO acts as oxidising agent as it oxidises H₂ to H₂O in the above reaction, and Cl₂ acts as oxidising agent as it oxidises H₂S to S in the above reaction.

·         Reducting Agent: The substance which oxidises itself to reduce other substances in a redox reaction, is known as Reducing agent. eg- H₂ acts as reducing agent as it reduces CuO to Cu in the above reaction, and H₂S acts as reducing agent as it reduces Cl₂ to HCl in the above reaction.

8.    COMBUSTION REACTION OR BURNING

·         A reaction in which a substance reacts with oxygen to produce heat and light is known as combustion. It is sort of a fast oxidation reaction.  eg-

C + O₂             -->            CO₂ +  Heat +            Light

CH₄ + 2O₂       -->           CO₂ + 2H₂O + Heat   + Light

2Mg + O₂         -->           2MgO  + Heat  + Light

C₆H₁₂O₆ + 6O₂          -->             6CO₂ + 6H₂O  + Energy        [Respiration]

·         Conditions required for combustion:

        i.            The substance needs to be combustible, i.e., it should be able to burn.

      ii.            Oxygen should be present for combustion.

    iii.            A certain temperature must be provided for a substance to be able to burn, this is called ignition temperature.

FLAMES

Ø  What is flame?

·        A flame is a zone in which chemical combustion between gases takes place accompanied by evolution of heat and light

·        A flame consists essentially of a mass of intensely ignited gaseous matter.

·         A flame is the phenomenon produced at the surface where two gases meet and undergo combination with the evolution of heat and light.

        i.            Luminous flame: Flame which produce ample amount of heat and light.

      ii.            Non-luminous flame: Flame which produce ample amount of heat but very little light.

·         There are many cases where a solid burns, produces heat and light but there is no flame, like burning coal.

·         A burning solid does not produce flame unless a vapour is produced which burns.

·         A substance burn with blue flame if enough oxygen s present.

·         A substance burns with yellow flame if there is a lack of oxygen.

·         A substance burns with sooty flame if there is incomplete combustion. In that case there will be more smoke than flame.

Ø  The Candle Flame

·         Wax is the matter in candles which melts and goes to the wick and burns. It consists of carbon and hydrogen (Hydrocarbons) which actually burns and produces vapour which burns to produce flame. Flame of a candle is yellow due to unburnt carbon particles leading to incomplete combustion.  It gives out light as it is luminous flame as the solid carbon particles present in the flame produces light.

Ø  RANCIDITY

·         When fat or oily food substances are kept in open for some amount of time, the fat or oil present in it get oxidised, leading to a foul smell and taste, this phenomenon is called rancidity. [“बासी”]

·         Rancidity can be prevented by various methods:

1.      Refrigeration

2.      Adding antioxidants

3.      Keeping in airtight containers

4.      Keeping in inert atmosphere (flushing with N₂ gas)

5.      Adding preservatives

There is also one topic corrosion, but we will be covering it in Metals & Non-metals in detail.

Let us solve some questions:

Answers to some of these questions:

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Atish BainHOD (Department of Chemistry)St. Thomas High School (10+2), Dhanbad

Do check out the video on Chemical Reactions & Equations