Types of Chemical Reaction

Types of Chemical Reactions

A chemical reaction is the change of a substance into a new substance that has a different chemical and physical properties. Chemical reaction takes place in different ways.There are different types of chemical reaction. Usually, all the types of chemical reactions are studied under four major topics. They are given below,

• Addition or synthesis or combination reaction
• Decomposition or dissociation or analysis reaction
• Displacement or replacement reaction
• Acid- base or neutralization reaction

Combination Reaction: Combination reaction is a type of chemical reaction in which a single product is formed from the combination of two or more reactants. Combination reaction is also known as synthesis reaction. It can be simply expressed as
Combination reaction→ A + B→AB

Examples:

• Formation of sulphur dioxide when sulphur is burned in air.
  S (s) + O₂(g)→ SO₂ (g)
• Formation of ammonium chloride: When we combine the vapours of ammonia gas with hydrogen chloride gas than ammonium chloride is formed. It is a white coloured solid.
  NH₃(g) + HCl(g)→ NH₄Cl(s)
  
  
• Formation of ferrous sulphide: When we heat the fine pieces of iron with sulphur powder, ferrous sulphide is formed.
  Fe(s) + S(s)→ FeS(s)
• Manufacture of Ammonia:The manufacture of ammonia gas from nitrogen and oxygen is also a combination reaction.
  N₂(g) + 3H₂(g)→ 2NH₃(g)
• Combination of sodium oxide and water;Sodium oxide combines with water to form sodium hydroxide.
  Na₂O(s) + H₂O(l) 2NaOH(s)

Decomposition Reaction:A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more elements or new compounds. This reaction is also called analysis reaction. To carry out decomposition reaction, application of light, heat, electricity, etc are used. Decomposition reaction can be simply expressed as
Decomposition reaction→ AB→A + B

Examples:

• Decomposition of hydrogen peroxide: Hydrogen peroxide decomposes into water and oxygen in the presence of light.
  2H₂O(l) \(\overset{light}{\rightarrow}\) 2H₂O(l) + O₂(g)
• Electrolysis of water: When water is electrolysed, it gives hydrogen and oxygen.
  2H₂O \(\overset{electrolysis}{\rightarrow}\) 2H₂ +O₂
• When lead oxide is heated, it decomposes into lead and oxygen
  2PbO \(\overset{heat}{\rightarrow}\) 2Pb + O₂
• Decomposition of ferric hydroxide: Ferric hydroxide undergoes decomposition in the presence of heat, giving ferric oxide and water molecules.
  2Fe(OH)₃ \(\overset{heat}{\rightarrow}\) Fe₂O₃ + 3H₂O
• Decomposition of calcium carbonate:Calcium carbonate (limestone) decomposes into calcium oxide (quick lime) and carbon dioxide when heated. Quick lime is the major constituent of cement.
  CaCO₃(s)\(\overset{heat}{\rightarrow}\) CaO(s) + CO₂

Displacement Reaction:Displacement reaction is a type of reaction in which an atom or a group of atoms in the molecule is replaced by another atom or a group of atoms. Displacement reaction is also known as substitution or replacement reaction. There are two types of displacement reaction. They are single displacement reaction and double displacement reaction.

Single displacement reaction: A single displacement reaction is also known as single replacement reaction. In single displacement reaction,an element reacts with a compound and takes the place of another element in that compound or an atom in a molecule is replaced by another atom. It can be simply expressed as,
Single displacement reaction→ AB +C→ CB + A
Examples:

• A solution of zinc sulphate and copper is obtained when zinc reacts with copper sulphate solution.
  Zn + CuSO₄→ CuSO₄ + Cu
  Here, Cu⁺⁺ ions are replaced by Zn⁺⁺
• When zinc reacts with hydrochloric acid, hydrogen gas is liberated and zinc chloride is formed. Here, Zn replaces H in HCl.
  Zn + 2HCl→ ZnCl₂ + H₂
  
  

Double displacement reaction: Double displacement reaction is also known as double replacement reaction. In a double displacement reaction, there is a mutual exchange of ions between two compounds that react together to form two other compounds. There are two compounds as reactants and two different compounds as products. It can be simple expressed as,
Double displacement reaction→ AB + CD→ CB + AD
Examples:

• When an aqueous solution of barium chloride is treated with the aqueous solution of copper sulphate, a white precipitate of barium sulphate is formed.
  BaCl₂(aq) + CuSO₄(aq)→ BaSO₄(s) + CuCl₂(aq)
• When we pass hydrogen sulphide gas through copper sulphate solution, a black precipitate of copper sulphide is formed.
  CuSO₄(aq) + H₂S(g)→ CuS(s) + H₂SO₄(aq)

Acid- base Reaction: An acid- base reaction is a type of reaction in which acid reacts with base or acid neutralise with a base to give salt and water. This reaction is also known as neutralization reaction. Here, the characteristics properties of both acid and base are destroyed. Acid- base reaction can be simply expressed as,
Acid- base reaction→ A + B→ S + W
Or,
acid + base→ salt + water
Here, H⁺ion of the acid combines with the OH^- ion of the base to give water.
Examples:

• When hydrogen chloride (hydrochloric acid) reacts with potassium hydroxide, potassium chloride and water is formed.
  HCl + KOH→ H₂O + KCl
• When hydrogen bromide (hydrobromic acid) reacts with potassium hydroxide, potassium bromide and water is formed.
  HBr + KOH→ KBr + H₂O

Endothermic reaction and exothermic reaction

On the basis of heat absorbed and heat released in a reaction, the chemical reactions are divided into two types. They are endothermic reaction and exothermic reaction.

• Endothermic reactions: Endothermic reaction is a type of reaction in which heat is absorbed from the surrounding during the reaction. Here, the compounds formed by absorption of heat are called endothermic compounds. Some of the common examples of endothermic reaction are given below,
  N₂ + O₂ + heat→ 2NO
  2KClO₃ + heat→ 2KCl + 3O₂
  CaCO₃ + heat→ Cao + CO₂↑
• Exothermic reactions: Exothermic reaction is a type of reaction in which heat energy is released to the surrounding during the reaction. Here, the compounds formed by exothermic reactions are called exothermic compounds.Some of the common examples of exothermic reaction are given below,
  CaO(s) + H₂O(l)→ Ca(OH)₂(aq) slaked lime + heat
  Zn(s) + 2HCl→ ZnCl₂(aq) + H₂(g) + heat
  C(s) + O₂(g)→ CO₂(g) + heat

Information conveyed by balanced chemical equation

• It tells about the type of chemical reaction.
• It provides information about the ratio of the masses of the reactants and products.
• It gives symbols, name and molecular formulae of the chemical substance involved as reactants and formed as products.
• It gives the relative number of atoms and molecules of the products formed in the reaction.
• It provides information about the relative number of atoms and molecules of the reactants that take part in the reaction.

The information that we obtain by a chemical equation can be easily understood by the following example:

Ammonium chloride + Calcium hydroxide→ Calcium chloride + Ammonia + Water
2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O
[2(14 + 1 \(\times\) + 35)] + [40 + (16 +1) \(\times\) 2] → [40 + (35 \(\times\) 2)] + [2(14 +1 \(\times\) 3)] + [2(1 \(\times\) 2 + 16)]
106 amu + 74 amu→ 110amu + 34 amu + 36 amu

From the given equation, we can obtain the following information,

• Two molecules of ammonium chloride react with one molecule of calcium hydroxide to give one molecule of calcium chloride, two molecules of ammonia and two molecule of water.
• Here, 106 amu or 106 gram of ammonium chloride reacts with 74 amu or 74 gram of calcium hydroxide to give 110 amu or 110 gram of calcium chloride, 34 amu or 34 gram of ammonia and 36 amu or 36 gram of water.
  From this, wecan calculate the amount of ammonium chloride required to give certain amount of calcium chloride.

Limitation of Chemical Equations

A chemical equation does not give all the information. It fails to give us the information on certain conditions:

• The physical states of the reactants and products.
• The concentration of the reactants and products.
• The reaction is reversible or not.
• The chemical equation does not tell about nature of reactions. Some reaction may be explosive.
• The conditions such as temperature, pressure or catalyst etc which affect the reaction.
• The physical state of reactants and the products.
• It does not tell about the speed in which the reaction takes place.

Modification of Chemical Reaction

• The necessary conditions like temperature, pressure, catalyst, light etc required for reactions are written above or below the arrow.
• A single way arrow (→) pointing towards the product for the reactant, is used for irreversible reaction and two-way half arrows(\(\rightleftharpoons\)) between reactants and products is used for the reversible reaction.
• A positive sign (+Δ) is used for endothermic reaction and negative sign (-Δ) is used for the exothermic reaction.
• The upright arrow(↑) after the product indicates that the gaseous product escapes out while the downward arrow(↓) indicates the setting down or precipitation of the products.
• The concentration of reactants is denoted by dil. for dilute and conc. for the concentrated solution of the reactants.
• In a chemical reaction, the physical state of reactants and products of the chemical reaction are expressed by 's' for solid, 'l' for liquid and 'g' for gas and 'aq'. for aqueous solution.

Catalyst

A chemical reaction occurs in different speed. Some reaction occurs in high speed while some reaction occurs in slow speed. By using some chemical substances, we can increase the speed of chemical reaction that occurs slowly and decrease the speed of chemical reaction that occurs at high speed as per our needs. These chemical substances are known as a catalyst. A catalyst changes the rate of a reaction without changing their chemical properties. They are obtained with same (original) chemical properties after the completion of the reaction. Therefore, a catalyst can be defined as a chemical substances that changes the rate of chemical reaction without itself being consumed or without changing its chemical properties during the chemical reaction. Catalysis is the change in the rate of chemical reaction by the use of chemical substances called catalysts. There are two types of catalyst. They are positive catalyst and negative catalyst. The positive catalyst increases the rate of chemical reaction and the negative catalyst decreases the rate of chemical reaction. Some of the examples of positive catalyst and negative catalyst are given below,

Examples:
Positive catalyst: During the preparation of ammonia from N₂ and H₂, iron is used as a positive catalyst and molybdenum as a promoter. Here, iron increases the rate of chemical reaction.
N₂(g) + 3H₂(g)\(\overset{Fe(s), MO}{\rightarrow}\) 2NH₃(g)

Negative catalyst: During the decomposition of hydrogen peroxide,glycerin is used as a catalyst. It decreases the rate of chemical reaction.
2H₂O₂(l)\(\overset{Glycerin (l)}{\rightarrow}\) 2H₂O(l) + O₂(g)