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  • Class 10th

    Chemical Reaction and Equation Chemistry

    1

    Q#1 Define symbol. What information is conveyed by symbol?

    Ans. A symbol is a brief representation of the name of the element. It consists of either the first letter or the

    first letter along with another significant letter of the name of the element.

    A symbol conveys the following information:-

    1. The name of the element (qualitative significance)

    2. One atom of the element (quantitative significance)

    3. Atomic mass of the element (quantitative significance)

    The concept of symbol was first of all introduced by a Swedish Scientist Berzelius.

    Q#2 What is a chemical formula and write the significance of formula.

    Ans. The chemical formula of a compound is a brief representation of a substance in terms of symbols of

    various elements present in it. For example, formula H2O represents water.

    Significance of H2O:-

    1. H2O represents water. 2. H2O represents one mole of water. 3. H2O represents one molecule of water. 4. H2O shows that it is made of hydrogen and oxygen. 5. It shows it contains 2-atoms of hydrogen and one atom of oxygen. 6. It shows valency of oxygen is two and the valency of hydrogen is one. 7. It shows molecular mass of water as 18 a.m.u. 8. It indicates that 18 parts of water has 2 parts of hydrogen and 16 parts of oxygen by weight.

    Q#3 What is a chemical equation? Give the various steps involved in writing a chemical equation.

    Ans. The chemical equation is a statement that describes a chemical change in terms of symbols and

    formulae of the reactants and products.

    The substances which take part in a chemical reaction are called reactants and are written on the left

    hand side in a chemical equation while those substances which form during chemical reaction are called

    products and are always written on the right hand side in a chemical equation.

    Writing chemical equation:- The different steps of writing an equation are listed below:-

    1. The reactants are always written on the left side of an arrow. 2. The products are written on the right hand side of an arrow. 3. The formulae of reactants and products are connected by plus sign (+). 4. The sign of arrow shows the direction in which the reaction is predominant.

  • Class 10th

    Chemical Reaction and Equation Chemistry

    2

    Q#4 What are the essentials of chemical equation?

    Ans. Following are the essentials of chemical equations

    1. It must represent a true chemical change 2. It must be balanced. 3. It should be molecular.

    Q#5 What are the limitation of chemical equation?

    Ans. The chemical equation suffers from certain limitations. It does not convey information about the

    following facts:-

    1. The physical state of the reactants and products. 2. Concentration of the reactants. 3. Time used in the completion of a reaction. 4. Speed of the reaction. 5. Whether the reaction is exothermic or endothermic. 6. Whether the reaction is reversible or irreversible. 7. Essential condition for reaction like temperature, pressure or use of any catalyst. 8. Changes such as precipitation change in colour, evolution of heat, light, light or sound energy

    occurring during the chemical reaction.

    Q#6 Write pre-knowledge before writing a balanced chemical equation.

    Ans. Following knowledge is necessary before one starts writing a balanced chemical equation.

    1. Whether or not a chemical reaction takes place between two chemical substances. 2. One must know all the reactants and products formed during a chemical change. 3. One must know the correct symbols and formulae of all the reactants and products.

    Q#7 What is the significance of chemical equation.

    Ans. To understand the significance of chemical equation, we consider the equation

    2H2 +O2 2H2O

    The above equations conveys that

    1. Hydrogen gas combines with oxygen to form water. 2. Two moles of H react with one mole of oxygen to form two moles of water. 3. 2 molecules of hydrogen react with one molecule of oxygen to form two molecules of water. 4. 4 gms of hydrogen react with 32 gms of oxygen to form 36 gms of water.

  • Class 10th

    Chemical Reaction and Equation Chemistry

    3

    Q#8 What are the various ways in which a chemical equation can be made more informative? Give

    examples to illustrate your answer.

    Ans. The chemical equations can be made more informative in three ways:

    i) By indicating the physical state of the reactants and the products.

    ii) By indicating the heat changes taking place in the reaction.

    iii) By indicating the conditions under which the reactions take place.

    i) By indicating the physical state of the reactants and the products:-

    Solid state is indicated by the symbol (s)

    Liquid state is indicated by the symbol (l)

    Aqueous solution is indicated by the symbol (aq)

    Gaseous state is indicated by the symbol (g)

    The physical state of the reactants and products are shown by putting the symbols just after their

    formulae in the equation given below:

    CaCO3 (s) + 2HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)

    Calcium carbonate Hydrochloric acid Calcium chloride Water Carbon dioxide

    ii) By indicating the heat changes taking place in the reaction.

    There are two types of reactions on the basis of heat changes involved, exothermic reactions and

    endothermic reactions.

    An exothermic reaction is indicating by writing +Heat or +Heat energy or +Energy on the products side of an equation as shown below:

    C(s) + O2 (g) CO2 (g) + Heat

    Carbon Oxygen Carbon dioxide

    Similarly, an endothermic reaction is indicated by writing +Heat on the reactants side of an equation as shown in the equation given below

    CaCO3(s) + Heat CaO(s) + CO2

    Calcium carbonate Calcium oxide Carbon dioxide

    iii) By indicating the conditions under which the reactions take place.

    If heat is required for a reaction to take place, then sign delta () is put over the arrow of the equation. If a catalyst is required for a reaction to take place, then the symbol or formula of a catalyst is written

    above or below the arrow sign in the equation.

    2KClO3(s) 2KCl(s) + 3O2 (g)

    Potassium chlorate MnO

    2 Potassium chloride Oxygen

  • Class 10th

    Chemical Reaction and Equation Chemistry

    4

    Also the conditions of temperature and pressure at which the reaction takes place can also be indicated

    in an equation by writing their values above or below the arrow sign in the equation.

    CO (g) + 2H2 (g) 300 atm; 3000C CH3OH (l)

    Carbon dioxide Hydrogen ZnO + Cr2O3 Methanol

    Q#9 What is balanced and unbalanced chemical equation? Give some examples.

    Ans. Balanced equation:- A chemical equation in which no. of atoms of each element is same on the side of

    reactants and products.

    Example.

    2Mg + O2 2MgO

    Magnesium Oxygen Magnesium oxide

    2KNO3 2KNO2 + O2

    Potassium nitrate Potassium nitrite

    Unbalanced or skeleton equation:- A chemical equation in which number of atoms of various

    elements towards the side of reactants is not equal to number of atoms of various elements towards the

    side of products is called an unbalanced equation.

    Example

    KNO3 KNO2 + O2

    This equation is wrong as atoms can neither be created nor destroyed by chemical reactions.

    Q#10 What do you understand by frequency number in a chemical equation?

    Ans. The frequency of occurrence of various elements in an equation is called frequency number. E.g. In

    KNO3 KNO2 + O2

    frequency of K=2, frequency of Oxygen =3, Frequency of N=2

    Rules for balancing a chemical equation

    1. Write frequency numbers of all the elements in chemical equation. 2. Start balancing equation from that element which has least frequency no. 3. Other element should be balanced in the order of increasing frequency no. 4. If two or more elements have same frequency no. then balance metallic element first 5. If there are two or more metallic elements with same frequency number, balance the metal with

    highest at. no. first. Then balance the next metallic elements with low atomic nos.

    6. If there are two or more non-metallic elements, with same frequency number, balance the non-metallic element with highest atomic number first. Other non-metallic element should be balanced

    in the decreasing order of atomic numbers.

  • Class 10th

    Chemical Reaction and Equation Chemistry

    5

    Examples of finding frequency number:-

    Ca(HCO3)2 + Ca (OH)2 CaCO3 + H2O

    f.no. ca=3 f.no of H=3 , f.no of C=2, f.no. of O=4

    i. Element Ca H C O

    Frequency number 3 3 2 4

    Order of balancing each element 2nd

    3rd

    1st 4

    th

    Thus the equation is balanced as Ca (HCO3)2 + Ca (OH) 2 2CaCO3 + 2H2O

    ii Al2 (CO3)3 + HNO3 Al (NO3)3 +H2O +CO2

    Element Al C O H N

    Frequency number 2 2 5 2 2

    Order of balancing 1st 3

    rd 5

    th 4

    th 2

    nd

    Thus the equation is balanced as Al2 (CO3)3

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