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CBSE NOTES CLASS 10 SCIENCE CHAPTER 1

Chemical Reactions And Equations

Chemical Change/Reaction

Chemical Equations

Reactants

Products

Symbolic Form of Chemical Equation

Balanced Chemical Equation

Balancing Chemical Equations

Writing Symbols of Physical States

Exothermic Reactions

Decomposition Reaction

Thermal Decomposition

Endothermic Reaction

Electrolytic Decomposition of Water

Displacement Reaction

Double Displacement Reaction

Oxidation and Reduction

Corrosion

Rancidity

CBSE NOTES CLASS 10 SCIENCE CHAPTER 1

Chemical Reactions And Equations

Chemical Change/Reaction

A chemical change or reaction involves formation of one or more new substance.

We say that a chemical reaction has taken place if one of the following has taken place.

Examples of chemical reactions

[Magnesium ribbon must be cleaned before burning in air so that the layer of magnesium oxide (which is formed due to reaction of magnesium with air) can be removed in order to get the desired chemical reaction.]

Chemical Equations

The description of a chemical reaction can be done in a sentence form which is quite long.

It can also be written as word-equation.

Magnesium+ Oxygen Reactants Magnesium OxideProducts 

Reactants

The substances that undergo chemical change in the reaction, for example, magnesium and oxygen, are called reactants.

Products

The new substances formed during the reaction, for example magnesium oxide, are called the products.

A word-equation shows change of reactants to products by placing an arrow between them.

Symbolic Form of Chemical Equation

A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae.

The reactants are written on the left-hand side (LHS) separated by plus sign (+) from each other.

Products are written on the right-hand side (RHS) separated by plus sign (+) from each other.

The arrowhead points towards the products, and shows the direction of the reaction.

Balanced Chemical Equation

The chemical equation needs to be balanced so that it follows the law of conservation of mass.

The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.

This means that the number of atoms of each element remains the same, before and after a chemical reaction.

For example the word-equation

May be represented by chemical equation,

Balancing Chemical Equations

Let us try to balance the following chemical equation –

Step I: Draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

Step II: List the number of atoms of different elements present in the unbalanced equation

Element

Number of atoms in reactants (LHS)

Number of atoms

in products (RHS)

Fe

1

3

H

2

2

O

1

4

Step III: Start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum number of atoms. Hence we select Fe3O4 and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS.

To balance the oxygen atoms – we need to multiply H2O by 4. We cannot alter the formulae of the compounds or elements involved in the reactions (within the boxes)

Step IV: Fe and H atoms are still not balanced. Let us balance hydrogen atoms. There are 8 atoms of H on the LHS (in H2O) and 2 on the RHS (in H2). To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.

Step V: There are now 1 Fe atom on LHS and 3 on the RHS. Multiply Fe on the left by 3 to balance this.

Step VI: Finally, to check the correctness of the balanced equation, the balanced equation is,

This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.

Step VII: Writing Symbols of Physical States - To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water. Usually physical states are not included in a chemical equation unless it is necessary to specify them.

The reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example –

Precautions to be taken while doing expermients

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(i) Do not hold the test tube by hand. Use tongs for the purpose.

(ii) Do not point the mouth of the boiling tube at yourself or at your neighbours.

(iii) Do not smell the gas directly. Waft the gas gently towards the nose.

(iv) Do not touch the chemicals by hand. Use spoon or spatula.

(v) Do not heat the volatile substances, ike alcohol, directly over burner. Use water bath instead.

(vi) Do not add water to acid or base. Add acid or to base slowly to the water, with constant stirring for diluting to desired level. This is because the dissolution of acid or base in water is highly exhothermic. The heat evolved may cause explosion or may break the glass tube, which in turn may cause bodily harm.

Types of Chemical Reactions

Combination Reaction

A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

For example calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

A solution of slaked lime is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. The chemical formula for marble is also CaCO3.

More examples of combination reactions

(i) Burning of coal

(ii) Formation of water from H2 (g) and O2 (g)

Exothermic Reactions: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Decomposition Reaction

The reaction, in which one compound decomposes or breaks into two or more different products, is called decomposition reaction.

Displacement Reaction

Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. (Remember the reactivity series)

Double Displacement Reaction

Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

A substance, which is insoluble in water, is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

Oxidation and Reduction

If a substance gains oxygen or losses hydrogen during a reaction, it is said to be oxidized.

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

One reactant gets oxidised while the other gets reduced simultaneously during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.

Few Other Examples:

Corrosion

It is the process in which metals are slowly eaten up by the action of air, moisture or chemicals.

Methods of protecting the metals from corrosion

Rancidity

When the substances containing oils and fats are exposed to air they get oxidized and become rancid due to which their smell, taste and colour change. This process is known as rancidity. For example when when butter is kept open for a long time, its smell and taste gets changed.

Preventing Rancidity