chemical reaction

Chemical Reaction and Chemical Equation

Chapter-3

(a) What is a chemical reaction? Give example of chemical reaction.

Ans.: Chemical reaction : The process which converts one or more substances into one or more different substances of different composition is generally called a chemical reaction.

Example : When magnesium burns in a jar full of oxygen the two elements combine together to form magnesium oxide whose properties are completely different from those of either magnesium or oxygen.

2Mg + O2 = 2MgO.

(b) Write down differences between physical and chemical changes.

Ans.: Difference between the physical and chemical changes is as follows:

Physical change Chemical change1.The physical changes are those changes which do not affect the main composition of the substances and only the physical condition or state of the substances are changed

2. Physical change is temporary

3. There is no change in mass of the substance due to physical change in it.

1.The chemical changes are those changes in which substances are changed into a new substances with different composition and properties

2. Chemical change is permanent

3. Because of the change in composition of the substance a

Answer to the Short Questions


4. During physical change thermal change may not occur.

substance of different mass is formed.4. During chemical change thermal change must always occur.

(c) What is chemical equation? What are the rules of writing equation?

Ans.: Chemical equation : A chemical equation is a shortened expression of a chemical reaction where the reactants and the products are represented by symbols and formulae along with few signs such as plus and equality between the reactants and the products.

Zn + H2SO4 ZnSO4 + H2

Zinc + sulphuric acid zinc sulphate + hydrogen

Rules of writing equations :

(1) In a chemical equation the formulae or symbols of the reacting substances or reactants are written on the left hand side of the equality sign (=), and the symbols or formulae of the products are written on the right hand side of it.

(2) If the reactants or products are more than one, a (+) sign is used in between their formulae or symbols.

(3) If the number of molecules involved is more than one, that number should be put before the symbols or formulae of the respective substance.

(4) An arrow sign is written from reactants towards products. Instead of arrow, a sign of equality (=) may also be written.

(5) The number of atoms of different elements present in the reactants before chemical reaction must be the same as the number of atoms of different elements in the products formed after the reaction. So to bring equality in the number of atoms of the elements in the chemical equation, the symbol and formula must be multiplied by a requisite number.

(d) Write down the description of the following chemical reaction with examples.

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General Science : Physics and Chemistry

(a) Combination (b) Decomposition (c) Displacement (d) Mutual exchange

Ans.: Combination: Combination is a process in which two or more elements or compounds directly combine with each other to form a compound. But when two or more elements react with each other to form compound the reaction is called synthesis.

Example :

(1) NH3 + HCl NH4ClAmmonia + Hydrogen chloride ammonium chloride

(2) H2 + O2 H2OHydrogen oxygen water

(3) C + O2 CO2

Carbon oxygen carbon dioxide

The above three reactions are combination. But in reaction (2) and (3) two elements combine together to form a compound; so these two reactions are synthesis.

Decomposition: Decomposition is a process where a compound is split up or decomposed into two or more simpler substances element or compound. It is the opposite reaction of combination. But when a compound is decomposed into two or more elements the reaction is called analysis.

Example :(i) CaCO3 CaO + CO2

Calcium carbonate calcium oxide + carbon dioxide

(ii) 2H2O 2H2 + O2

Water hydrogen oxygen

(iii) HCl H2 + Cl2

Hydrogen chloride hydrogen chlorine Above three reactions is decomposition. But in reaction (ii) and (iii) one compound is decomposed to form only elements; so these two reactions are analysis.

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Replacement or displacement: It is a process in which one atom of an element expels or replaces one or more atoms of another element present in a compound and takes its place.

Example :(i) Zn + H2SO4 ZnSO4 + H2


In this reaction zinc replaces hydrogen, occupies their position forming zinc sulphates and sets free H2 molecule.

(ii) Zn + CuSO4 ZnSO4 + CuZinc cupper sulphate zinc sulphate copper

In this reaction zinc displaces copper of copper sulphate forming zinc sulphate and setting free cupper molecules.

Double decomposition or mutual exchange: It is a process in which molecules of two different compounds reacts with each other and exchange their constituents or radicals to produce more than one new compound. It is called the double decomposition or mutual exchange reaction.

Example :

(i) AgNO3 + NaCl AgCl + NaNO3

Silver nitrate sodium chloride silver chloride sodium nitrate

In this reaction Ag and Na have mutually exchanged their positions and nitrate and chloride interchange their position.

(e) Write down the type of following reactions :(i) CaO + CO2 = CaCO3 (ii) Fe + CuSO4 = FeSO4 + Cu (iii) Na2SO4 + BaCl2 = BaSO4 + 2NaCl (iv) CaCO3 =CaO+ CO2

(v)Na2CO3 + CaCl2 = 2NaCl + CaCO3

(vi)H2SO4 + 2NaOH = Na2SO4 + 2H2OAns.: (i)CaO + CO2 = CaCO3 ( combination )(ii) Fe + CuSO4 = FeSO4 + Cu ( replacement )

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(iii) Na2SO4 + BaCl2 = BaSO4 + 2NaCl (double decomposition) (iv) CaCO3 = CaO + CO2 ( decomposition )(v)Na2CO3 + CaCl2 = 2NaCl + CaCO3 (double decomposition)(vi)H2SO4 + 2NaOH = Na2SO4 + 2H2O ( neutralization ).

(f) Complete the following reactions and balance them :N2 + H2 = MgO + HCl =CaCO3 + HCl =Na2CO3 + CaCl2 =FeCl3 + H2O =NH4OH + HCl =

Ans.: N2 + 3H2 = 2NH3 MgO + 2HCl = MgCl2 + H2OCaCO3 + 2HCl = CaCl2 + CO2 + H2ONa2CO3 + CaCl2 = 2NaCl + CaCO3

FeCl3 + 3H2O = Fe(OH)3 + 3HClNH4OH + HCl = NH4Cl + H2O

(a) What are physical and chemical changes? Name five physical and five chemical changes.

Ans.: Definition of physical change : Physical changes are those changes which do not affect the main composition of the substance and only the physical condition or state of the substance is changed.

Five physical changes are as follows:1. Magnetizing the iron piece into a magnet of iron 2. Sugar solution (dissolving the sugar into water), salt solution (dissolving the salt into water) and solution of blue vitriol etc.3. Converting the solid wax into molten wax, switching on the electric bulb.4. Converting ice into water and water into vapour or like wise converting vapour into water and water into ice.

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Answer to the Essay Type Questions


Heat Heat

Ice water steam Cool Cool

5. Converting the iron into red hot iron by heating or like wise converting the red hot iron into its original position.

Heat

Iron Red hot iron Cool

Definition of chemical change : The chemical changes are those changes in which a substance is changed into a new substance or substances with different composition and properties.

Examples of five chemical changes :(i) Rusting of iron,(ii) Conducting electricity through the water or hydrochloric acid solution.

2H2O 2H2 + O2

or, 2HCl H2 + Cl2

(ii) Combustion of magnesium and carbon by oxygen

2Mg + O2 2MgO or, C + O2 CO2

(iv) Combustion of kerosene

(v) Metal + acid salt + hydrogen Zn + H2SO4 ZnSO4 + H2.

(b) What is a chemical reaction? Write down the causes of chemical reaction?

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Ans.: Chemical reaction : The process which converts one or more substances into one or more different substances of different composition is generally called a chemical reaction. Completely new substances are produced in the chemical changes by the rearrangement of atoms and molecules. As a result the previous substances can not be obtained after a chemical reaction.

Cause of chemical reaction: There are two causes of a chemical reaction. These are (i) Chemical affinity and (ii) Appropriate environment.

(i) Chemical affinity: The strong attraction which exists between the reactants for reacting in a chemical reaction is called chemical affinity. Chemical affinity should be there between the reactants when a chemical reaction takes place.

(ii) Appropriate environment: The causes which affect chemical reactions are temperature, pressure, electric current, contact of acid, presence of light etc. The following are some of the examples of chemical reactions and chemical changes:

2Mg + O2 ∆ 2MgOMagnesium oxygen heat magnesium oxide

2H2O Electric current 2H2 + O2

Acidified water hydrogen oxygen

Zn + H2SO4 ZnSO4 + H2

Zinc sulphuric acid zinc sulphate hydrogen

CaCO3 + HCl CaCl2 + CO2 + H2OCalcium hydrochloric calcium carbon watercarbonate acid chloride dioxide

If hydrogen and oxygen gas is mixed together in a flask and kept them years together water is not likely to be formed. But if electric current is passed through the mixture, water will be formed immediately.

(c) Describe the different types of chemical reaction?

Ans.: Different types of chemical reactions :

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(1)Combination: Combination is a process in which two or more elements or compounds directly combine with each other to form a compound. But when two or more elements react with each other to form compound the reaction is called synthesis. Example :

(1)NH3 + HCl NH4ClAmmonia + Hydrogen chloride ammonium chloride

(2) 2H2 + O2 2H2OHydrogen oxygen water

(3) C + O2 CO2

Carbon oxygen carbon dioxide

The above three reactions are combination. But in reaction (2) and (3) two elements combine together to form a compound; so these two reactions are synthesis.

(2) Decomposition: Decomposition is a process where a compound is split up or decomposed into two or more simpler substances element or compound. It is the opposite reaction of combination. But when a compound is decomposed into two or more elements the reaction is called analysis. Example :

(i) CaCO3 CaO + CO2

Calcium carbonate calcium oxide + carbon dioxide

(ii) 2H2O 2H2 + O2

Water hydrogen oxygen

(iii) HCl H2 + Cl2

Hydrogen chloride hydrogen chlorine

Above three reactions are decomposition. But in reaction (ii)and (iii) one compound is decomposed to form only elements; so these two reactions are analysis.(3) Replacement or displacement: It is a process in which one atom of an element expels or replaces one or more atoms of another element present in a compound and takes its place.

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Example :

(i) Zn+ H2SO4 ZnSO4 + H2


In this reaction zinc replaces hydrogen, occupies their position forming zinc sulphate and sets free H2 molecule.

(ii) Zn + CuSO4 ZnSO4 + CuZinc + cupper sulphate zinc sulphate + copper

In this reaction zinc displaces copper of copper sulphate forming zinc sulphate and setting free cupper molecules.

(4) Double decomposition or mutual exchange: It is a process in which molecules of two different compounds reacts with each other and exchange their constituents or radicals to produce more than one new compounds. It is called the double decomposition or mutual exchange reaction.

Example : (i) AgNO3 + NaCl AgCl + NaNO3

Silver nitrate sodium chloride silver chloride sodium nitrate

In this reaction Ag and Na have mutually exchanged their positions and nitrate and chloride interchange their position.

(5) Neutralization : A chemical reaction, where an acid and a base combine together to form salt and water, is called neutralization reaction.

Example : Hydrochloric acid and sodium chloride react together to form magnesium chloride salt and water. So it is neutralization.

2HCl + MgO MgCl2 + H2OHydrochloric magnesium magnesium water Acid oxide chloride(6) Combustion : When a substance is burnt in presence of oxygen or air to produce a different compound the process is called burning or combustion.

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Example : Carbon + oxygen carbon dioxide

C + O2 CO2

Here, carbon is burnt in the presence of air or oxygen and produces carbon dioxide which is completely different substance than the reactants.

(d)What is meant by chemical affinity?

Ans.: Chemical affinity : Chemical affinity can be defined as the affinity for which atoms of one or more than one elements or compounds are rearranged and formed different elements or compounds. This affinity depends on the structure of the atoms of reacting substances. Chemical affinity is influenced by some factors. These are temperature, pressure, electric current etc.

Example : If hydrogen is burnt with oxygen of air, it reacts together to form water. The affinity of hydrogen and oxygen to combine together is called chemical affinity. The compound is formed by this chemical affinity.

Reaction : 2H2 + O2 = 2H2O

(e) Describe the use of valency in writing formula of element and compound.

Ans.: Suppose an element A combines with another element B to form a compound. Suppose also the valency of A is x and the valency of B is y . Now at the time of forming compound of A with B the number of valency of A is to be written just below the right side of B, for example (Bx); like wise the number of valency of B is to be written just below the right side of A like (Ay).

Example :1. Valency of sodium = 1 and valency of chlorine = 1.So the formula of sodium chloride (formed by the combination sodium and chlorine) is Na1Cl1or NaCl.

2. Valency of hydrogen = 1 and valency of oxygen = 2So the formiula of water is H2O.3. Valency of nitrogen = 3 and valency of hydrogen = 1So the formula of ammonia (formed by the combination of nitrogen and hydrogen) is N1H3 or NH3.

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Some examples of the formula of molecules are given below according to the valency of the elements.

Up valency down

symbol

Valency written on the

opposite corner(diagonal

valency)

Formula of

molecules formed

Equality of

valency

Name of the

compound

1 1 H Cl

1 1H Cl HCl 1=1

Hydrogen chloride

2 2 Mg O

2 2Mg O

Mg2 O2 orMgO 2=2

Magnesium oxide

1 2 H O

1 2H O

H2 O1 orH2O 2=2

Water

3 1 N H

3 1N H

N1 H3 orNH3

3=3 Ammonia

(f) Classify the following changes into physical and chemical changes (i) Combustion of wax (ii) Rusting (iii) preparation of powder of milk (iv) Formation of film on lime water (v) Combustion of kerosene lamp (vi) Melting of Iron (vii) Magnetization of Iron (viii) Preparation of sulphate solution (ix) Mixing water with calcium oxide (x) Preparation of sugar solution (xi) burning of magnesium ribbon.

Ans.: Physical change Chemical change

(i) Combustion of wax (ii) Rusting (iii) Preparation of powder milk (iv)Formation of film on lime water(vi) Melting of Iron (v)Combustion of kerosene lamp(vii) Magnetization of Iron (viii) Preparation of sulphate solution(x) Preparation of sugar solution (xi) Burning of magnesium ribbon

(ix) Mixing water with calcium oxide

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1. What are the differences between combination and synthesis reaction?

Ans.: The differences between combination and synthesis reaction are as follows:Combination reaction Synthesis reaction

(1) The reaction in which a compound is produced from its constituent substances through direct combination is called combination reaction. For example –2H2 + O2 = 2H2O3H2 + N2 = 2NH3

CaO + CO2= CaCO3

(1)The reaction in which a compound is produced from its constituent elements through direct combination is called synthesis. For example –2H2 + O2 = 2H2O2H2 + N2 = 2NH3

(2) All combination reactions are not synthesis reaction.

(2) All synthesis reactions are combination reaction.

2. Write down the names of reactions and the causes of for the names :(i) NaCl + AgNO3 = NaNO3 + AgCl(ii) HCl + NaOH = NaCl + H2O(iii) 2KClO3 + heat = 2KCl + 3O2

(iv) Zn + H2SO4 = ZnSO4 + H2

(v) AlCl3 + 3H2O = Al(OH) 3 + 3HCl(vi) CaO + CO2 = CaCO3

(vi) CaO + CO2 = CaCO3

(vii) NH4CNO + heat = H2N – CO – NH2

Ans.: (i) NaCl + AgNO3 = NaNO3 + AgCl : It is a double decomposition reaction; because here reactants are decomposed and exchanged the Cl and NO3 radical with each other and form two different compounds NaNO3 and AgCl.(ii) HCl + NaOH = NaCl + H2O : It is a neutralization reaction, because in this case, acid (HCl) and base (NaOH) reacts together to form salt (NaCl) and water.

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Answer to the Extra Questions


(iii) 2KClO3 + heat = 2KCl + 3O2;It is a decomposition reaction, because here, reactant is decomposed to produce KCl and O2 by the application of heat.

(iv) Zn + H2SO4 = ZnSO4 + H2;It is a replacement reaction. Because here zinc replaces hydrogen from sulphuric and produces hydrogen gas.

(v) AlCl3 + 3H2O = Al(OH) 3 + 3HCl ;It is a hydrolysis reaction. Because water reacts with alluminium chloride and produces Al(OH) 3 and HCl.

(vi) CaO + CO2 = CaCO3

It is a combination reaction. Because the compound CaCO3 is produced from the direct combination of its constituent compounds CaO and CO2.

(vii) NH4CNO + heat = H2N – CO – NH2 It is an isomerization reaction, because here reactant and product both are isomers to each other.

3. Write down the main characteristics of chemical reaction.

Ans.: Characteristics of chemical reaction:

1. The properties of the products formed in a chemical reaction are different from those of the reactants.2. There is always absorption or emission of heat.3. The total mass of the reactants and the total mass of the products are equal.

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Answer to the Objective Questions


1.Multiple Choice Questions :Tick (√) Correct Answer.

1. What is the valency of Nitrogen in N2O3 compound?(a) 2 √ (b) 3(c) 4 (d) 6

2. What is the total number of atoms in one molecule of H2SO4?(a) 2 (b) 5

√ (c) 7 (d) 83. What is the valency of Iron in Fe2O3?

(a) 2 √ (b) 3(c) 4 (d) 6

4. Which one is the correct formula of Ferrous Sulphate?(a) Fe2 (SO4) 3 (b) Fe2 (SO4)2

√ (c) FeSO4 (d) Fe2SO4

5. What is the difference between SO2 and SO4?(a) SO2 is an element and SO4 is a compound?(b) SO2 and SO4 both are compound

√ (c) SO2 is a compound and SO4 is radical(d) SO2 and SO4 both are element.

6. Zn + H2SO4 = ZnSO4 + H2; what is the type of this reaction?(a) Mutual exchange (b) Combination(c) Analysis √ (d) Displacement

7. What is the difference between 2O and O2?√ (a) 2O means two atoms of Oxygen and O2 means is a molecule of

Oxygen.(b) 2O means two molecule Oxygen and O2 means two molecules of Oxygen.(c) 2O is an element and O2 is compound.(d) There is no difference between 2O and O2.

2. Fill in the blanks :

(a) H2 + ………… = 2HCl(b) …..Ca + O2 = …CaO(c) MgO + 2HCl = ………+ H2O

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(d) Zn + H2SO4 = ……….+ H2

(e) CaO + …….. = ………..+ CaCl2

(f) The substance taking part in the reaction is ………….and the substance obtained after the reaction is called ……………

(g) In chemical reaction ………..molecules of reactant takes place.(h) The reaction between carbon and oxygen is called …………….(i) The reaction between hydrogen and oxygen forming water is

called………….(j) The number of atoms in reactants and products in chemical reaction

remain ……………

Ans.: (a) Cl2; (b) 2, 2; (c) MgCl2; (d) ZnSO4

(e) 2HCl, H2O; (f) reactant, product; (g) rearrangement of(h) Combustion; (i) synthesis reaction; (j) always equal or unchangeable

3. Match left side sentence with that on right side.

Left Right a. In the equation of chemical reaction the substance in the right side is called product and

a. mutual exchange.

b. The number of atoms in reactants and products

b. One compound molecule is changed into different molecules of different substances.

c. In decomposition reactions c. combinationd. Chemical combination between two reactants is called

d. substance on left hand side is called reactant

e. when there is no change in the fundamental structure or composition of the substances the change is called

e. physical change

f. AgNO3 + NaCl = AgCl + NaNO3

f. H2 + Cl2 = 2HCl

g. In synthesis a compound is formed by direct union of two element; for example

g. remain always equal

h. chemical change.

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Ans.: a. In the equation of chemical reaction the substance in the right side is called product and substance on left hand side is called reactant.b. The number of atoms in reactants and products remain always equal.c. In decomposition reactions one compound molecule is changed into different molecules of different substances.d. Chemical combination between two reactants is called combination.e. When there is no change in the fundamental structure or composition of the substances the change is called physical change. f. AgNO3 + NaCl = AgCl + NaNO3 mutual exchange.g. In synthesis a compound is formed by direct union of two elements; for example H2 + Cl2 = 2HCl.

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chemical reaction

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