CBSE Class 10 Science Chemical Reactions And Chemical Equations Notes Set 03

1. CHEMICAL REACTION AND EQUATIONS

Physical and chemical changes

1. No new or different substance is formed. The composition of the substance, that undergoes the change, remains unchanged. / Results in the formation of at least one new substance. The constituent particles of the new substance are different from the constituent particles of the original substance.

2. It is temporary change and in most cases it can be reversed by the reversal of conditions. / It is permanent change and cannot be reversed by reversal of conditions.

3. No change occurs in the mass of the substances undergoing the change. / Mass of the individual substances that undergoes the change, always, either increases or decreases. However, the total mass of all the reactants is equal to the total mass of all the products.

Remember

• Usually both changes occur together.
• In burning of a wax candle, both physical and chemical changes take place.

Conditions for chemical change

The criterion for a chemical change is the production of one or more new substances. Therefore, certain conditions should be met in order to bring about a chemical change. The necessary conditions are:

• A minimum amount of energy needed to initiate a reaction, called the activation energy, should be supplied in the form of heat, light or electric current. In a chemical change, the reactants combine to form new products. For this process, it is necessary to break the old bonds of the reactants and forms fresh bonds in order to give new products. This requires a certain amount of energy.
• For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break old bonds and form new bonds.
• The speed with which the chemical reaction takes place is called the rate of the chemical reaction. This should be appreciable, to bring about the change.

The rate of a reaction depends on following factors

• Temperature : Certain chemical reactions do not take place at room temperature but occur readily at a higher temperature. Accordingly, heat is required to start the reaction e.g. fuels like coal and wood only start burning when heated to a certain temperature called its ignition temperature.
• Presence of Light : Some reactions take place only in light and do not take place in dark e.g. photosynthesis in green plants or reaction between \( H_2 \) and \( Cl_2 \) to form \( HCl \). (\( H_2 + Cl_2 \xrightarrow{h\nu} 2HCl \))
• Presence of a Catalyst : A catalyst is a substance that increases the rate of chemical reaction without itself undergoing any change. Example: Hydrogenation of oils to form fats takes place faster, in presence of nickel.
• Electricity : Certain reactions take place with the help of an electric current. Example: Decomposition of acidulated water to give hydrogen and oxygen gas.
  \( 2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2 \)
  \( PbBr_2 \xrightarrow{\text{electric current}} Pb + Br_2 \)
• Pressure : Some chemical reactions need very high pressure to proceed. For e.g., in the commercial manufacture of ammonia from nitrogen and hydrogen by Haber’s process, a pressure of over 200 atmosphere is required, in presence of a catalyst iron and a temperature of \( 450^{\circ}C \) to \( 500^{\circ}C \).

Chemical reactions

The process in which a substance or substances undergo a chemical change to produce new substances, with entire new properties are known as chemical reaction.

Reactant : The substance / substances, which takes part in a chemical reaction.

Products : The new substance / substances formed as a result of a chemical reaction are called products.

A chemical reaction should satisfy the following conditions:

• There must not be either a gain or loss of matter.
• There must be either an evolution or an absorption of heat i.e. an energy change.
• The products obtained as a result of chemical reaction must have properties different from those of the reactants.

Types of chemical reactions

1) Direct combination or synthesis reaction

In a synthesis reaction, two or more chemical species combine to form a more complex product.

\( A + B \to AB \)

The combination of iron and sulphur to form iron (II) sulfide is an example of a synthesis reaction.

\( 8 Fe + S_8 \to 8FeS \)

Decomposition or analysis reaction

In a decomposition reaction, a compound is broken into smaller chemical species.

\( AB \to A + B \)

The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction.

\( 2 H_2O \xrightarrow{\text{electric arc}} 2 H_2 + O_2 \)

Example

If decomposition takes place,

(i) In presence of heat, we call it Thermal decomposition/Thermolysis.

(a) Decomposition of lime stone on heating.

\( CaCO_3(s) \xrightarrow{\text{Heat}} CaO(S) + CO_2(g) \)
Limestone (Calcium carbonate) \( \xrightarrow{\text{Heat}} \) Quick lime (Calcium oxide) + Carbon dioxide

(b) Decomposition of zinc carbonate on heating,

\( ZnCO_3(s) \xrightarrow{\text{Heat}} ZnO(s) + CO_2(g) \)
Zinc carbonate \( \xrightarrow{\text{Heat}} \) Zinc oxide + Carbon dioxide

(ii) In presence of light, we call it Photo decomposition/Photolysis.

(a) Photolysis of silver bromide (used in photography)

\( 2AgBr(s) \xrightarrow{\text{Sun light}} 2Ag(s) + Br_2(g) \)

(b) Photolytic decomposition of hydrogen peroxide. (Thats why it is stored in coloured bottles)

\( 2H_2O_2 \xrightarrow{\text{Light}} 2H_2O + O_2 \)
Hydrogen peroxide \( \xrightarrow{\text{Light}} \) Water + Oxygen

(iii) In presence of electricity, we call it Electro decomposition/Electrolysis.

(a) Electrolysis of molten sodium chloride.

\( 2NaCl \text{ (molten)} \xrightarrow{\text{Electric Current}} 2Na \text{ (Sodium metal)} + Cl_2 \text{ (Chlorine)} \)

(b) Electrolytic decomposition of molten alumina

\( 2Al_2O_3 \text{ (molten)} \xrightarrow{\text{Electric Current}} 4Al \text{ (Aluminium)} + 3O_2 \text{ (Oxygen)} \)

Single displacement or substitution reaction

A substitution or single displacement reaction is characterized by one element being displaced from a compound by another element.

\( A + BC \to AC + B \)

An example of a substitution reaction occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen:

\( Zn + 2 HCl \to ZnCl_2 + H_2 \)

Metathesis or double displacement reaction or double decomposition reaction.

In a double displacement or metathesis reaction, two compounds exchange bonds or ions in order to form different compounds.

\( AB + CD \to AD + CB \)

An example of a double displacement reaction occurs between sodium chloride and silver nitrate to form sodium nitrate and silver chloride.

\( NaCl(aq) + AgNO_3(aq) \to NaNO_3(aq) + AgCl(s) \)

Few examples

(i) \( ZnO + C \xrightarrow{\text{Heat}} Zn + CO \)
Oxidation: \( C \) is oxidised to \( CO \).
Reduction: \( ZnO \) is reduced to \( Zn \).

(ii) \( MnO_2 + 4HCl \xrightarrow{\text{Heat}} MnCl_2 + Cl_2 + H_2O \)
Oxidation: \( HCl \) is oxidised to \( Cl_2 \).
Reduction: \( MnO_2 \) is reduced to \( MnCl_2 \).

(iii) \( Fe_2O_3 + 2Al \xrightarrow{\text{Heat}} 2Fe + Al_2O_3 \)
Ferric oxide + Aluminium \( \to \) Iron + Aluminium oxide
Oxidation: \( Al \) is oxidised to \( Al_2O_3 \).
Reduction: \( Fe_2O_3 \) is reduced to \( Fe \).

(iv) \( H_2S + Cl_2 \to 2HCl + S \)
Hydrogen sulphide + Chlorine \( \to \) Hydrogen chloride + Sulphur
Oxidation: \( H_2S \) is oxidised to \( S \).
Reduction: \( Cl_2 \) is reduced to \( HCl \).

Corrosion

The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties is called corrosion. Or the process of slow eating up of the surface of certain metals when kept in open for a long time.

Rust

It is called rusting of iron. Iron corrode readily when exposed to moisture and gets covered with a brown flaky substance called rust. Rust is a hydrated iron (III) oxide \( Fe_2O_3 \cdot 2H_2O \).

Rusting of iron takes place under the following conditions.

• Presence of air (or oxygen)
• Presence of water (or moisture)

Some more examples of corrosion

• Copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and acquires a green coating of basic copper carbonate.
• Silver articles become black after sometime when exposed to air because it reacts with sulphur to form a coating of silver sulphide.
• Lead or stainless steel lose their lustre due to corrosion.

Rancidity

The oxidation of oils or fats in food, resulting into a bad taste and bad smell is called rancidity. It is caused due to prolonged exposure of food to air. Oxygen present in air oxidise fats / oil present in food and form volatile substances, which have bad odour.

Prevention of rancidity

Rancidity can be prevented by adding antioxidants to foods containing fats and oils. Antioxidants are reducing agents so when they are added to food it do not get oxidised easily and hence, do not turn rancid.

Common antioxidants are :

• BHA (Butylated Hydroxy Anisole)
• BHT (Butylated Hydroxy Toluene)

Vitamin-E and vitamin-C (ascorbic acid) are the two antioxidants occuring in natural fats.

• It can be retarded by keeping food in refrigerator.
• It can also be retarded by storing food in air tight containers.
• Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas.
• It can be retarded by storing foods away from light.