Chapter – 1 : (Chemical Reaction & Equation)

 

Change is Law of Nature. Scientist classify these changes as physical changes and chemical changes.

Physical Change

  • In Physical Change, the physical properties of the substance change but the chemical composition does not change.
  • In Physical Change, substance comes to its original state as soon as the cause of change is withdrawn.
  • Example : Ice

Chemical Change 

In a chemical change, at least one of the reacting substance changes into a new substances with a different composition. The new substances can not be changed back to the original substance even if the cause of change is withdrawn. Example : Curd

Difference Between Physical & Chemical Change

Physical change

Chemical change

The identity of the substance is maintained.

The identity of the original substance is completely lost.

The change is temporary, the substance returns to its original state as the cause of. change is withdrawn

The change is permanent.

No new substance is produced.

A new substance is always produced.

Heat change may or may not occur.

Heat change may occur.

Only the physical state or some of the physical properties of the substances are changed

The substance after the change can not come back to its original state even when the cause of change is withdrawn.

 

Chemical Reaction

The processes, in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, are known as Chemical Reaction. The nature and identity of products totally changes from the reactants. Observations which determines whether the chemical reaction has taken place or not.

  1. Chemical reaction must be associated with change in temperature i.e. Heat should be either evolved or absorbed.
  2. The reaction must occur between fixed quantities of the reactants.
  3. The chemical reaction should follow the law of conservation of mass.
  4. The products obtained must have properties different from those of the reactants.

Example

(i) When potassium nitrate is heated, it gives potassium nitrite and oxygen.

Rules for writing a word equation

  • The substances taking part in chemical reaction reactants are always written on the left hand side of arrow.
  • The substances formed after the chemical reaction, products are always written on the right hand side of arrow.
  • A plus sign (+) is put in between the reactants or between the products. If their number is two or more.
  • An arrow (→) is put between the reactants and products, the arrow shows the direction of the reaction in which the reaction proceeds. The arrow is read as “to yield” or “to form”.
  • In the word equation when symbols and chemical formula of the reactants and products are used then it is called as chemical equation.

Example : Na + H2O → NaOH + H2

    • i.e. A chemical equation is a statement that describes a chemical reaction in terms of symbols and formulae. In this equation the law of conservation of mass is not obeying. Such chemical equation is called skeletal equation or unbalanced equation.
    • A chemical equation expressed in symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of the products is called skeletal equation or unbalanced equation.
  • To make this equation meaning full, this equation is balanced then it is called balanced chemical equation

Charge Table

–1 Charge

Formula

–2 Charge

Formula

–3 Charge

Formula

Name of Ion

Name of ion

Name of ion

Bromide ion

Br

Oxide ion

O2–

Nitride ion

N3–

Chloride ion

Cl

Sulphide ion

S2–

Phosphide ion

P3–

Fluoride ion

F

 

 

Boride ion

B3–

Iodide ion

I

 

 

 

 

Hydrogen carbonate
or (bisulphate ion)

HCO3

Carbonate ion

CO32–

Phosphate ion

PO43–

Hydrogen sulphate or (bisulphate ion)

HSO4

Manganate ion

MnO42–

Arsenate ion

AsO43–

Hydroxide ion

OH

Thiosulphate ion

S2O32–

Arsenite ion

AsO33–

Nitrate ion

NO3

Silicate ion

SiO32–

 

 

Chlorate ion

ClO3

Sulphate ion

SO42–

Phosphite ion

PO33–

Nitrite ion

NO2

Sulphite ion

SO32–

Borate ion

BO33–

Permanganate ion

MnO4

Chromate ion

CrO42–

Ferricyanide ion

[Fe(CN)6]3–

Acetate ion

CH3COO

Dichromate ion

Cr2O72–

 

 

Cyanide ion

CN

Hydrogen
Phosphate ion

HPO42–

 

 

Hypophosphite ion

H2PO2

Oxalate ion

C2O42–

 

–4 Charge

Meta aluminate ion

AlO2

 

 

Carbide ion

C4–

 

+1 Charge

 

 

Ferrocyanide
ion

[Fe(CN)6]4–

Ammonium ion

NH4+

 

 

 

 

 

One which contains an equal number of atoms of each element on both sides of the equation.

Balancing Chemical Equations

The simple equations are balanced by “hit and trial method”. Which is done in following steps.
Step 1 : Count the number of atoms of various elements on both sides of the equation.

Example : Fe + H2O → Fe3O4 H2

S.No.

Element

No. of atoms in reactants (LHS)

No. of atoms in products (RHS)

1

Fe

1

3

2

H

2

2

3

O

1

4

Step 2 : Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.
According to this rule Fe3O4 has maximum number of atoms & oxygen has 4 atoms so it is selected.

S. No.

Atoms of oxygen

In reactants

In products

1

Initial

1

4

2

To balance

1 × 4

4

 

So the partly balanced equation is

Fe + 4H2O → Fe3O4 + H2

Step 3 : Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.

S.No.

Atoms of Hydrogen

In reactants

In products

1

Initial

8

2

2

To balance

8

2 × 4

 

Now the equation becomes as

Fe + 4H2O → Fe3O4 + 4H2

Step 4 : Balance the Iron atoms similarly.

S.No.

Atoms of Iron

In reactants

In products

1

Initial

1

3

2

To balance

1 × 3

3

 

Now the equation is

3Fe + 4H2O → Fe3O4 + 4H2

Step 5 : Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.

S.No.

Element

No. of atoms in reactants (LHS)

No. of atoms in products (RHS)

1

Fe

3

3

2

H

8

8

3

O

4

4

 

Step 6 : To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq) is written. Now the equation becomes as

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

Symbol (g) with water is written to show that water is used in the form of steam in this equation.

If a gas is evolved in a reaction it can be shown by the symbol (­) after the formula i.e. arrow pointing upwards e.g.

2Na(s) + 2H2O(l) → 2 NaOH(aq) + H2(g) or H2(­↑)

The symbol (↓) or ppt is be written for precipitate.

NaCl + AgNO3 → NaNO3 + AgCl (↓) or ppt.

Reversible reaction is represented by  symbol and irreversible reaction by (→) symbol.
The heat evolved in chemical reaction is written on the right side by putting positive (+) sign and heat absorbed in the chemical reaction is written on the right hand side by putting negative (–) sign.

Some times the reaction conditions, such as temperature, pressure, catalyst etc. are written above or below the arrow in the equation e.g.

Exothermic reaction : The reaction in which heat is liberated (or given out) is called an exothermic reaction

Example : C(s) + O2(g) → CO2(g) + Heat (393 kJ/mol)

Endothermic reaction : The reaction in which heat is absorbed (or taken in ) is called an endothermic reaction

Example : C(s) + 2S(g) → CS2(g) – Heat (92 kJ/mol)

The reaction with + Heat term on the product side are called exothermic reaction, while those with –Heat term on the product side are called endothermic reactions.

During respiration, the digested food gets oxidised and the energy is released. That is why, it is considered as an exothermic reaction.

Balancing of a chemical equation is necessary because no matter (hence, no atom) is lost or gained during a chemical reaction.

Types of chemical reactions : Chemical reaction occurs as a result of breaking and making of bonds resulting in redistribution of atoms among various reacting species in different ways. Accordingly the reactions are classified in different types. They are

  1. Combination reaction or synthesis reaction
  2. Decomposition reaction/Analysis reaction
  3. Displacement reaction
  4. Double displacement reaction/Methasis reaction
  5. Oxidation and Reduction reaction.

Combination reaction 

The reaction in which two or more substances combine to form a single new substance are called combination or synthesis reaction.

C(s) + O2(g) → CO2(g)

Combination reactions are of three common types :

(i) Combination of two elements to form a Compound eg.

(a) H2(g) + Cl2(g) → 2HCl(g)

(b) 2H2(g) + O2(g) → 2H2O(l)

(ii) Combination of an element and a compound to form a new compound.

(a) 2SO2(g) + O2(g) → 2SO2(g)

(iii) Combination of two compounds to form a new compound :

(a) NH2(g) + HCl(g) → NH2Cl(g)

Decomposition reaction

The reaction in which a single compound breaks up into two or more simpler substances are known as decomposition reaction. The decomposition reaction generally takes place when energy in some forms such as heat, electricity or light is supplied to the reactants.

The decomposition of a compound with light is called photolysis.
Note : All the decomposition reaction requires energy i.e. these reactions are Endothermic reactions. These reactions are used in extractions of metals.
Decomposition reaction is called opposite of combination reaction. This can be supported by the following reactions:

Displacement Reactions

The chemical reaction in which one element takes the place of another element in a compound, are called displacement reactions e.g. 

Iron, zinc and lead are more reactive elements than so they displace copper from its compounds.

Double Displacement Reactions

The reactions in which two compounds react to form two different compounds by mutual exchange of ions, are called double displacement reactions. These reactions are also called as Metathesis reaction.

Two common types of double displacement reactions are

Precipitation reaction : Any reaction that produces a precipitate, (the insoluble substance formed), is called precipitation reaction e.g.

Two common types of double displacement reactions are

(B) Neutralization reaction : When an acid reacts with base to form salt and water by exchange of ions e.g.

NaOH(aq)     +     HCl(aq)      →    NaCl(aq) +          H2O(l)

    (Base)                   (Acid)                  (Salt)                (Water)

Oxidation

(i) The addition of oxygen to an element or compound.

additon of oxygen

(ii) Removal of hydrogen from a compound is known as oxidation.

removal of hydrogen
removal of hydrogen

Reduction

(i) The addition of hydrogen to an element or compound

H2 + O2 → 2H2O

(ii) Removal of oxygen from a compound.

removal-of-oxygen
removal-of-oxygen

Oxidising agent

The substance which gives oxygen or removes hydrogen for oxidation is called oxidising agent and the substance which gains oxygen during reaction is said to be oxidised.

Reducing agent

The substance which gives hydrogen or removes oxygen for reduction is called reducing agent. The substance which gains, hydrogen during reaction is said to be reduced.

Those reactions in which oxidation and reduction (both) occurs simultaneously are called redox reactions.

In the name Redox the term ‘red’ stands for reduction and ‘ox’ stands for oxidation.

EXAMPLE :

redox reaction example
redox reaction example

SO2 is reduced to sulphur, so it is oxidising agent.

H2S is oxidised to sulphur, so it is reducing agent.

It should be noted that substance which undergoes oxidation acts as reducing agent where as the substance undergoes reduction act as oxidising agent.

There is an another concept of oxidation and reduction in terms of metals and nonmetals. This is as follows

The addition of nonmetallic element (or removal of metallic element) is called oxidation.

The addiition of metallic element (or removal of nonmetallic element) is called reduction.

Electronic concept for oxidation and reduction

Oxidation : The loss of an electron by atoms or ions is called oxidation.

Atom  →  Cation + electrons

A  →  An+ + ne

Atom ‘A’ loses n electrons to become a positively changed ion An+. It is called cation.

Reduction : The gain of an electron by an atom or ion is called reduction.

B + ne  →  Bn–

The atom B gains n’ electrons to become negatively charged ion Bn–, it is called anion.

Oxidation and reduction reactions occurs simultaneously and are called as Redox Reactions. Only oxidation or only reduction is called half reaction. i.e.

A   →   An+ + ne – Oxidation

B   +   ne → Bn– – Reduction

A   +   B → A+B  →   AB Redox

e.g. Na + Cl → NaCl

In this process sodium loses one electron and oxidised to Na+, chlorine gains this electron and is reduced to Cl.

Na  →  Na+ + e (loss of an electron is oxidation )

Cl  +  e  →  Cl (Gain of an electron is reduction)

gain and loose of electrons
gain and loose of electrons

Effects of oxidation reactions in everyday life

 Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metals is studied as corrosion and that on food is studied as rancidity. Thus there are two common effects of oxidation reactions are as

(i) Corrosion of metals

(ii) Rancidity of food

Corrosion of metals : Corrosion is the process of deterioration of metals as a result of its reaction with air, moisture and acids. (Present in environment) surrounding it.

The corrosion causes damage to buildings, bridges, ships and many other articles especially made of iron.

Rust : Iron corrode readily when exposed to moisture and gets covered with a brown feaky substance called rust. It is called rusting of iron, Rust is a hydrated Iron (III) oxide Fe2O3 · 2H2O. Rusting of iron takes place under the following conditions

(a) Presence of air (or oxygen)

(b) Presence of water (or moisture)

It has been observed that

(a) Presence of impurities in the metal speed up the rusting process. Pure iron does not rust.

(b) Presence of electrolytes in water also speeds up the process of rusting

(c) The position of the metal in the electrochemical series determines the extent of corrosion. More the reactivity of the metal, there will be more possibility of the metal getting corroded. Other examples of corrosion are

(i) 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.

(ii) Silver articles become black after sometime when exposed to air because it reacts with sulphur to form a coating of silver sulphide.

(iii) Lead or stainless steel lose their lusture due to corrosion.

Unreactive metals such as gold, platinum, palladium, titanium etc. do not corrode.

Rancidity 

Fresh foods containing fats and oils smell and taste pleasant but when it remains exposed in air for a long time it’s smell and taste changes to unpleasant. It is said that the food has become rancid.

OR

It is due to the oxidation of fats and oils, butter, ghee, boiled rice etc, after prolonged exposure to air i.e. The condition produced by the aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.

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. The two common anti oxidants are
(a) BHA (Butylated Hydroxy Anisole)

(b) BHT (Butylated Hydroxy Toluene)

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

(ii) Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas.

(iii) It can be retarted by keeping food in refrigerator.

(iv) It can also be retarded by storing food in air tight containers.

(v) It can be retarded by storing foods away from light.

Important 

Difference Between Physical & Chemical Change

  1. In physical change the identity of the substance is maintained but in chemical changes the identity of original substance is completely lost.
  2. In Physical change the change is temporary, the substance returns to its original state as the cause of change is withdrawn yet chemical change is permanent.
  3. When physical changes happen no new substance is produced but in chemical change a new substance is always produced.

 

Leave a Comment