Notes-Class-10-CBSE-NCERT-Science-Chapter-1-Chemical Reactions and Equations

Topics to be learn : Chemical reactions: Chemical equation, Balanced chemical equation, Implications of a balanced chemical equation; Types of chemical reactions: Combination, Decomposition, Displacement, Double displacement, Precipitation, Neutralization, Oxidation and reduction; Corrosion, Rancidity.

Chemical Reaction : A process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds is called a chemical reaction.

Chemical equation :

A chemical equation is an expression for given chemical change in terms of symbols or formulae of the reactants and products.

Example :The reaction of zinc with dilute sulphuric acid to produce zinc sulphate and hydrogen is given by the following chemical equation:

Zn + H₂SO₄ —-> ZnSO₄ + H₂.

Changes in Chemical Reactions : When a chemical reaction occurs, one or more of the following changes take place

• Change in state
• Change in colour
• Evolution of a gas
• Evolution or absorption of heat (change in temperature).

Examples :

• Burning of magnesium ribbon in air : Burning of magnesium ribbon in air gives a powder of MgO. So there is a change of state with formation of new substance and heat is produced.
• Addition of lead nitrate solution to potassium iodide solution : Addition of colourless lead nitrate solution to potassium iodide solution gives yellow coloured precipitate (lead iodide). So there is a change of colour.
• Addition of dilute hydrochloric acid to zinc granules : Addition of dilute hydrochloric acid to zinc granules gives a gas (H₂) with effervescence and heat is evolved. There is change in temperature.

Balanced chemical equation :

In a chemical reaction, the number of atoms of the elements in the reactants is same as the number of atoms of those elements in the product, such an equation is called a balanced equation.

• Law of conservation of mass to be kept in mind while we balance a chemical equation.
• According to law of conservation of mass matter can neither be created nor destroyed.
• Thus during a chemical reaction the total mass of the reactants and products remains the same.
• Therefore, for a complete chemical equation, the number of atoms of various elements on both sides are made equal, i.e., the equation is balanced.

Example : AgNO₃ + NaCl —> AgCl + NaNO₃

• In the above reaction, the number of atoms of the elements in the reactants is same as the number of atoms of elements in the products.

Writing of skeletal equation :

Q. Write the skeletal equation for the following reactions:

(i) Hydrogen sulphide reacts with sulphur dioxide to form sulphur and water.

(ii) Methane on burning combines with oxygen to produce carbon dioxide and water. (CBSE 2012)

Steps in balancing a chemical reaction :

Consider the following equations as an example :

Example 1:

Sodium hydroxide + Sulphuric acid à Sodium sulphate + water.

Step 1 : Write the skeletal equation from the given word equation. Or the given chemical equation as it is.

NaOH + H₂SO₄ ——> Na₂SO₄+ H₂O ..............(1)

Stop 2 : Write the number of atoms of each element in the unbalanced equation on both sides of equations.

It is seen that the number of atoms of all the elements on the two sides are not the same. It means that the equation (1) is not balanced.

The number of oxygen and sulpher atoms on both sides of the equations are same, therefore equalize the number of sodium and hydrogen atoms.

Step 3 : To balance the number of sodium atoms

To equalise the number of sodium atoms, we use 2 as the factor of NaOH in the reactants. Now, the partly balanced equation becomes as follows :

2NaOH + H₂SO₄ ——> Na₂SO₄+ H₂O ….(2)

Check whether the equation (2) is balanced or not.

We find that the equation (2) is not balanced, as the number of oxygen (6 | 5) and hydrogen atoms (4 | 2) are unequal on the two sides.

First balance the hydrogen number as it requires a smaller factor.

Step 4 : Now, balance the number of hydrogen atoms :

To equalise the number of hydrogen atoms, we use 2 as the factor of H₂O in the products. The equation then becomes

2NaOH + H₂SO₄ ——> Na₂SO₄+ 2H₂O ….(3)

Now, count the atoms of each element on both sides of the equation. The number of atoms on both sides are equal. Hence the balanced equation is

2NaOH + H₂SO₄ ——> Na₂SO₄+ 2H₂O

In this way, a balanced equation is obtained from an unbalanced equation by applying proper factors to appropriate reactant/product so as to balance the number of each element in steps.

Thermochemical equation :. An equation in which information about heat change is included is called a thermochemical equation.

• Most chemical reactions are accompanied by either evolution or absorption of heat. These reactions are known as exothermic and endothermic reactions respectively
• It is very important in such a case to indicate the physical state of the various species involved.

Exothermic reactions: Chemical reactions in which energy is evolved (or given out) are known as exothermic reactions. For example,

C_(s) + O₂(g) ———> CO₂(g)     …. ΔH = −3935 kJ

Endothermic reactions: Reactions in which energy is absorbed are called endothermic reactions. For example,

C(graphite) + 2H₂(g) ———-> CH₄(g)   ….. ΔH = +74.25 kJ

Q. Classify the following as exothermic and endothermic reactions (CBSE 2014,1015)

• Photosynthesis
• Respiration
• Burning of natural gas
• Electrolysis of water
• Decomposition of calcium carbonate.
• Burning of magnesium ribbon in air.

Ionic equations : An equation which shows only the atoms and ions that actually take part in the reaction is called an ionic equation.

• Ions which remain as such and do not take part in the reaction are not indicated. .

Example : When zinc metal and dilute sulphuric acid are reacted, then; zinc sulphate and hydrogen gas are formed. Now sulphuric acid exists as 2H⁺ ions and SO₄^2- ions. Similarly, zinc sulphate exists as Zn²⁺ ions and SO₄^2- ions. So, the above reaction can be written as:

In this equation, we have one sulphate ion, SO₄^2-, on both the sides, which remains as such and does not take part in the reaction. So, cancelling the sulphate ion from both sides, we get the following ionic equation for the above reaction:

Zn + 2H⁺  ———> Zn²⁺ + H²

Types Of Chemical Reactions :

Combination Reaction : The reaction of two or more substances (either elements or compounds) to produce a new substance is called a combination reaction.

Example : Magnesium (Mg) on burning in air combines with oxygen to give magnesium oxide.

2Mg(s) + O₂(g) ———> 2MgO(s)

Decomposition Reaction : The reaction in which a compound splits up into two or more simpler substances is called decomposition reaction.

Light Decomposition: Silver chloride when exposed to sunlight turns grey. This is due to decomposition of silver chloride into grey silver and chlorine in the presence of light.

This happens due to absorption of light energy. Such reactions are used in photography.

2AgCl(s) \(\overset{heat}{\rightarrow}\) 2Ag(s) + Cl₂(g)

white                                                 Grey

Silver bromide also behaves in the same way.

2AgBr(s) \(\overset{heat}{\rightarrow}\) 2Ag(s) + Br2( g)

Brown                        Grey

Dissociation :

When a substance breaks up into + ve and — ve ions in water, it is called dissociation.

For example, acetic acid in Water dissociates into CH₃COO^- and H⁺ ions.

CH₃COOH + H2O ⇔ CH₃COO^- + H₃O⁺

Hydrochloric acid gas in water dissociates to H⁺ and Cl^-  ions.

HCl(g) + H₂O(l) ——> H⁺(aq) (H₃⁺O) + Cl^-(aq)

Double decomposition or displacement :

The reaction in which two reacting molecules exchange their partner ions in aqueous solution is double decomposition.

Example : The precipitation of AgCl by adding AgNO₃ to NaCl solution.

AgNO₃(aq) + NaCl(aq) ———> AgCl(s) + NaNO₃(aq)

Types of Combination reactions :

There are three types of combination reactions. These are discussed as follows:

(i) Combination between two elements: In these reactions, two elements combine under suitable conditions to form a compound. A few examples of this type of combination reactions are:

(a) Carbon element burns in oxygen to form carbon dioxide

C + O₂ \(\overset{combustion}{\rightarrow}\) CO₂

 (b) Iron and sulphur elements when heated form iron sulphide

Displacement OR substitution reaction :

The reaction in which an atom or a group of atoms in the molecule is displaced by another atom or a group of atoms is called displacement or substitution reaction.

Fe + S\(\overset{heat}{\rightarrow}\) FeS

(ii) Combination between an element and a compound: In these reactions, one of the combining substances is an element whereas the other is a compound. A few examples of this type of combination reactions are:

(a) Carbon monoxide combines with oxygen to form carbon dioxide.

2CO + O₂ ———> 2CO₂

(b) Sulphur dioxide combines with oxygen upon heating to form sulphur trioxide.

2SO₂   +  O₂ \(\overset{heat}{\rightarrow}\) 2SO₃

Oxidation and Reduction :

Oxidation: It is a process

• In which oxygen or any electronegative element is added up or
• Hydrogen or any electropositive element is removed

Reduction: It is a process in which

• hydrogen or an electropositive element is added up.
• Oxygen or electronegative element is removed.

Redox reaction : A chemical reaction in which one substance is oxidised and the other is reduced is called a redox reaction.

• All oxldatlon-reduction reactions are redox reactions.
• In a chemical reaction, a substance gets oxidised only when another substance is present, which gets reduced.

CuO + H₂  \(\underrightarrow{Heat}\) Cu + H₂O

Here CuO is losing oxygen, is being reduced. The hydrogen is gaining oxygen, is being oxidised

• It is true that oxidation cannot take place without reduction and vice versa. As electrons do not exist free, electrons lost during oxidation must be gained during reduction. Such reactions are called redox reactions.

Q. Select the oxidising agent and the reducing agent from the following reactions:

(i) H₂S + I₂ ——> 2HI + S

(ii) CuO + H2 ——> Cu + H₂0

(iii) Zn + CuSO₄ ——> ZnSO₄ + Cu.

Q. State some reactions of oxidation that you observe in your everyday life.

Q. Can a displacement reaction be a redox reaction? Explain with the help of an example. (CBSE 2015)

Have you observed the effects of oxidation Reactions in everyday life? :

Corrosion : Due to the effect of moisture and acids, metals get corroded. This effect is called corrosion.

Effects of corrosion :

Corrosion causes damage to metal articles lik car bodies, bridges, iron railings, Ships and other substances of daily use.

Corrosion and rusting :

Corrosion is seen in all metals, when exposed to air. A layer of metal compound is formed on the surface due to presence of moisture, acids or other gases present in air.

Rusting : Rusting is the process in which iron metal reacts with air and moisture to form brownish powder called rust.

• Shiny iron articles on exposure to moisture get coated with a brownish powder which peels off easily. In this way enormous amount of iron is damaged and lost.
• Rust is a complex compound of iron and it takes several months to form when iron is exposed to air and moisture, therefor it is called slow reaction. This is a combination reaction.

Rancidity : Fats and oils in food kept for long time get oxidised and become rancid and taste of food changes and causes infection on eating. This is called rancidity.