Types of Chemical Reactions

In this article, we shall study the types of chemical reactions. Before understanding the types of chemical reactions, let us study the meaning of the terms “a physical change” and “a chemical change”

Physical Change:

A change in which only the physical properties of a substance are changed is called a physical change. In a physical change, there may be a change in shape, size, physical state, and appearance of the substance.

Characteristics of Physical Change:

• In a physical change the only physical properties of substance change.
• In a physical change, there is no change in the composition of the substance
• In a physical change, no new substance is formed.
• The change is temporary and can be easily reversed.
• There is no change in the mass of the substance as there is a change in its form and not in the bulk.
• There is no loss or gain of energy because the exact same amount of energy is released back during the reverse process.

Examples of Physical Changes:

• Heating of platinum wire.
• Melting of ice,
• Vapourization of water due to boiling.
• Magnetization of iron
• Sublimation of ammonium chloride or iodine.
• Heating of sulphur
• Dissolving salt or sugar in water.
• Glowing of an electric bulb.

Note: To decide whether the change is physical or chemical. If there is a change in the chemical composition of a substance or there is a gain or loss of energy in the change, the change is a chemical change otherwise it is a physical change.

Chemical Changes:

A change in which a substance is converted into another substance of different physical and chemical properties is called chemical change.

Characteristics of Chemical Change:

• In chemical change the physical and chemical properties of a substance take place
• In a chemical change, there is a change in the composition of the substance
• In a chemical change, one or more new substances are formed.
• The change is permanent and cannot be easily reversed.
• There may be a change in the mass of the substance (due to conversion of mass into energy) as there is a change in its chemical composition.
• There is loss or gain of energy because old bonds are broken and new ones are formed.

Examples of Chemical Changes:

• Rusting of iron: Iron is a grey lustrous hard meal. atmospheric oxygen and water react to form a hydrated iron oxide which is a dull, porous, brittle substance. Iron oxide has different properties than iron. Hence rusting of iron is a chemical change.
• Burning of wood or paper:
• Heating of sugar
• Formation of curd from milk
• Burning of magnesium in the air.
• Electrolysis of acidified water.
• Ripening of fruits.
• Clotting of blood
• Digestion of food.

Note: In a chemical change, there is a change in the chemical composition of a substance or there is a gain or loss of energy in the change.

Reactions involving Both a Physical and a Chemical Change:

Burning of Candle: When a candle is burning due to heat candle melts and resolidifies when it is cooled. It is a physical change. At the same time, wax undergoes combustion and forms carbon dioxide and water. It is a chemical change. Thus both types of changes are taking place.

Chemical Reaction:

In a chemical reaction one or more substances, called reactants undergo a chemical change to produce new substances called products of the reaction. Thus a chemical reaction is a process in which reactants undergo a change to produce products. In the reaction, 

C_(s) + O_2(g) → CO_2(g)

Carbon C_(s) and are oxygen O_2(g) reactants, while carbon dioxide CO_2(g) is a product.

Conditions Required for a Chemical Reaction to Take Place:

Physical Contact (Mixing) Between Reactants:

• For every chemical reaction to take place there should be physical contact between the reacting molecules.
• But some reactions just initiate by physical contact with the substance. For example, when sodium comes in contact with water or atmospheric water vapours, the reactions start instantly and sodium hydroxide is formed with the evolution of hydrogen. Hence sodium is kept under kerosene. (it is heavier than kerosene and do not react with kerosene).
• Fluorine and hydrogen when brought together, react with each other with explosion even in dark.

Solution:

• Some chemical reaction takes place only in solution form of the substances.
• Oxalic acid and sodium carbonate do not react with each other in crystalline (solid) form but when they are mixed with each other in water reaction between them takes place.

Heat:

Generally, chemical reactions are accompanied by absorption or release of energy. If heat is evolved in the reaction it is called exothermic reaction, while if heat is absorbed in the reaction, it is called endothermic reaction. Some energy is required to start the reaction. This energy can be provided by heating.

Examples:

CaCO_3(s)       CaO_(s)  + CO_2(g)      ΔH = 178 kJ

KNO_3(s)       KNO_2(s)  + O_2(g)      ΔH = 244.7 kJ

Some reactions are accompanied by the release of heat energy.

CH_4(g)  + 2O_2(g)    →   CO_2(g)  + 2H₂O_(g)      ΔH = – 890.4 kJ

Light:

Certain reactions takes place in presence of light. Such reactions are called photochemical reactions.

H_2(g)  + Cl_2(g)      HCl_(g)

CO_2(g)  + 6H₂O_(l)     C₆H₁₂O_6(s)     Photosynthesis reaction

Some reactions are accompanied by the release of light energy

CH_4(g)  + 2O_2(g)    →   CO_2(g)  + 2H₂O_{(g)  + Light} 

_2Mg(s)  + O_2(g)    →   2MgO_(s)  + Light

Pressure:

Some reactions require higher  pressure to take place.

N_2(g)  + 3H_2(g)    →   2NH_3(g)     Haber’s process at pressure = 200 atm

Catalyst:

Some chemical reactions are extremely slow. To increase their speed or two facilitate the reaction catalysts are used. A catalyst is a substance which alters the rate of chemical reaction. The composition of the catalyst does not change in the reaction. The catalysts which increase the rate of reaction are called positive catalysts, while those decrease the rate of reaction are called negative catalysts.

N_2(g)  + 3H_2(g)    →   2NH_3(g)

Haber’s process catalyst used is iron. (a positive catalyst). Iron increases the rate of reaction

2 H₂O_2(i)   →   2H₂O_(l) + O_2(g)

the catalyst used is phosphoric acid. (a negative catalyst). Phosphoric acid retards the decomposition of hydrogen peroxide.

There are certain substances which increase the rate of a chemical reaction by increasing the efficiency of the catalyst, such substances are called promoters. In Haber’s process, molybdenum is used as a promoter which increases the efficiency of iron which is used as the catalyst.

Energy Change in Chemical Reactions:

Every substance has a fixed amount of stored energy, which is in the form of potential energy. This energy is called the chemical energy of the substance.

In a chemical reaction, reactants are changed into products. The change in energy in chemical reaction refers to the difference between the total chemical energy of reactant and total chemical energy of products. When bond is broken energy absorbed and when bonds are formed energy is released. Energy may be released in the form of heat, light, sound or electricity.

The types of chemical reactions on the basis of the absorption or release of energy, are as follows:

Exothermic Reactions:

The chemical reactions in which heat is evolved are called exothermic reactions.

• Example:
• Nitrogen combines with hydrogen to give ammonia with evolution of 100 kJ of heat

N_2(g)      +        3 H_2(g)       →           NH_3(g)    + 100 kJ

• Respiration is an exothermic process.

Characteristic of Exothermic Reaction:

• In an exothermic reaction, heat is evolved.
• For an exothermic reaction, the change in enthalpy is negative.
• In an exothermic reaction, the enthalpy of reactants is more compared to that of products.
• Products are more stable than the reactants.

Endothermic Reactions:

The chemical reactions in which heat is absorbed are called endothermic reactions.
Examples :

• Nitrogen combines with oxygen to give nitric oxide with absorption of 100 kJ of heat

N_2(g)  +    O_2(g)         →   2NO_(g)   – 180 kJ

• Hydrogen reacts with iodine to give hydrogen iodide with the absorption of 51.88 kJ.

H_2(g)      +           I_2(g)         →          2HI_(g)   – 51.88 kJ

Some endothermic reaction takes place with the absorption of light energy are called photochemical reactions.

H_2(g)  + Cl_2(g)      HCl_(g)

Some endothermic reaction takes place with the absorption of electrical energy are called electrochemical reactions. e.g. Electrolysis of water

2H₂O_(l)  →   2H_2(g)  + O_2(g) 

Characteristic of Endothermic Reaction:

• In an endothermic reaction, heat is absorbed.
• For an endothermic reaction, the change in enthalpy is positive.
• In an endothermic reaction, the enthalpy of products is more compared to that of reactants.
• Reactants are more stable than the products.

Types of Chemical Reactions:

There are vast numbers of chemical reactions. The study of these reactions becomes easy and convenient by classifying them into various types. Different types of chemical reactions are as follows:

Combination Reactions:

In this reaction, two or more elements or compounds combine to form a single compound.

If the compound is formed by the combination of its constituent elements, then the combination reaction is called synthesis.

N_2(g)  +  3 H_2(g)   →    NH_3(g)    (synthesis)

_PbS(s)  +  2O_2(g)   →    PbSO_4(s)    (combination)

_2Mg(s)  + O_2(g)    →   2MgO_(s)  (synthesis)

2CO_(g)  +  2O_2(g)   →    2CO_2(g)    (combination)

Note: In synthesis reactant side involves elements only.

Decomposition Reactions:

In this reaction, a compound breaks into either elements or simpler compounds. In decomposition, there is no reverse reaction.

CaCO_3(s)     CaO_(s)  + CO_2(g)

KNO_3(s)   KNO_2(s)  + O_2(g) 

Depending the source of energy used to carry out the decomposition, decomposition reactions are further classified as

Thermal Decomposition:

A decomposition reaction that is brought about by heat energy is called thermal decomposition.

CaCO_3(s)      CaO_(s)  + CO_2(g)

KNO_3(s)       KNO_2(s)  + O_2(g) 

ZnCO_3(s)      ZnO_(s)  + CO_2(g)

2 HgO_(s)   2 Hg_(l)  + O_2(g)

2 Pb(NO₃)_2(s)    2 PbO_(s)  + 4 NO_2(g)  + O_2(g)

Photolytic Dissociation: 

A decomposition reaction that is brought about by light energy is called photolytic decomposition.

2 H₂O_2(i)   2H₂O_(l) + O_2(g) 

Electrolytic Decomposition: 

A decomposition reaction that is brought about by electrical energy is called electrolytic decomposition. e.g. electrolysis of water

2H₂O_(l)  2H_2(g)  + O_2(g)

Double Decomposition:

In this reaction, two compounds in solution react to form two other compounds by the mutual exchange of radicals.

AgNO_3(aq) + HCl_(aq) →  AgCl_(aq) + HNO_3(aq)

NaNO_3(aq) + HCl_(aq) →  AgCl_(aq) + NaNO_3(aq)

CaBr_2(aq) + K₂SO_4(aq) →  CaSO_4(aq) + 2KBr_(aq)

NaOH_(aq) + HCl_(aq) →  NaCl_(aq) + H₂O_(l)

Displacement Reactions:

In this reaction the more reactive element displaces the less reactive element from its compound.

The reactivity can be obtained from electrochemical series. For ready reference the order of reactivity decreases in the order  K (most reactive) > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Au > Pt (Least reactive).

Fe_(s)   + CuSO_4(aq)  →  FeSO_4(aq) +   Cu_(s)

Zn_(s)   + CuSO_4(aq)  →  ZnSO_4(aq) +   Cu_(s)

Mg_(s)   + H₂SO_4(aq)  →  MgSO_4(aq) +   Cu_(s)

Revesible Reactions:

A chemical reaction, in which the products formed react with each other to give back the original reactants, is called a reversible reaction. conventionally the reaction proceeding from left to right is called as a forward reaction while that proceeding from right to left is called as a forward reaction.

PCI_5(g)    ⇌    PCl_3(g)   +     Cl_2(g)

N_2(g) +  3 H_2(g)   ⇌    2NH_3(g)

H_2(g) + Cl_2(g)  ⇌  2HCl_(g)

Note: Reversible reactions are discussed in details in topic chemical equilibrium

Neutralization Reactions:

The reaction between acid and base to form salt and water is called neutralization.

NaOH_(aq) + HCl_(aq) →  NaCl_(aq) + H₂O_(l)

2NaOH_(aq)  + H₂SO_4(aq)  → Na₂SO_4(aq) +  2H₂O_(l)

Note: Neutralization reactions are discussed in details in topic Arhrnius acid base theory.

Hydrolysis:

In this reaction salt and water react together to form acidic or a basic solution.

Na₂CO_3(aq) +  2H₂O_(l)    → 2NaOH_(aq)  + H₂CO_3(aq)

NaOH is the strong base and H₂CO₃ is the weak acid. Hence resulting solution is basic in nature

NH₄NO_3(aq) +  2H₂O_(l)    → NH₄OH_(aq)  + HNO_3(aq)

NH₄OH is the weak base and HNO₃ is the strong acid. Hence resulting solution is acidic in nature

Note: Hydrolysis is discussed in details in topic Ionic Equilibrium

Redox Reactions:

According to the electronic concept, a reaction in which gain of electrons by an atom or an ion takes place is called reduction and a reaction in which loss of electrons from an atom or an ion takes place is called oxidation.

Consider a reaction

2HgCl₂ +  SnCl₂ → Hg₂Cl₂ + SnCl₄

HgCl₂ is reduced to Hg₂Cl₂ whereas SnCl₂ is oxidised to SnCl₄.

Thus oxidation and reduction take place simultaneously.  Therefore, all such reactions are called as reduction-oxidation reactions or redox reactions.  In all such reactions, one of the reactants loses the electrons (oxidized) while other gains those electrons (reduced)

Note: Redox reactions are discussed in details in topic oxidation-reduction reactions