What is the reaction called in which an element is displaced by another element?

Reactions in which two substances combine and form one substance are termed as combination reactions. Whereas the reactions in which one substance forms two products are termed as decomposition reactions. 

Explanation: 

Displacement Reaction:  

When one element of a reactant gets displaced by another element it is said to be a displacement reaction.  

A  +  BC    →    AC    +    B 

In such a reaction, the element and compound combine. An element separates from the compound and combine with the element to give a new compound as a product. The element is replaced in the compound. For that reason it is also called the replacement reaction. Since, single element is replaced or displaced we call this type of reaction a single displacement reaction.  

Examples: 

1. Used in welding. 

1. In the process of extraction of metals. 

1. Used in acid indigestion. 

Activity: 

• When zinc metal pieces are added to a small amount of HCl, we immediately see fumes coming out. 
• When a burning match stick is brought near to it, it puts off with a pop sound.  
• This confirms the formation of hydrogen gas.      

Conditions for displacement:  

• Displacement occurs when an element with higher reactivity is treated with an element with lower reactivity. 
• Zn has higher reactivity than H so Zinc can displace Hydrogen from HCl. 
• For example: Cu + ZnSO4  →  CuSO4 + Zn  
• This reaction is not possible because reactivity order is Cu < Zn 

Reactivity of metals: 

According to reactivity of elements, metals are categorised to highly, moderately and least reactive metals. 

Reactivity Series: 

The arrangement of elements or metals either in the increasing order or decreasing order of their creativity is called reactivity series. 

Importance: 

1. Reactivity series helps in knowing the reactivity of metal and relative order of reactivity. 

1. It helps in knowing the feasibility of a reaction. 

Double Displacement Reactions: 

A chemical reaction in which exchange of ions or elements takes place to form two new compounds is known as a double displacement reaction. 

Or 

If in a reaction two elements are replaced from the reactants to form two new products we call it the double displacement reactions. It is also called double replacement reactions. 

The general equation for the double displacement reaction is:  

AB    +    CD    →    AD    +    BC 

Types of Double Displacement: 

Double displacement reactions can be of two types: 

• Neutralisation Reactions 
• Precipitation Reactions 

Neutralisation Reactions: 

The reaction between an acid and a base is neutralisation.  

Or 

The neutralisation of an acid with base gives salt and water. 

Examples: 

HCl  +  NaOH    →    NaCl    +    H2O 

Acid        Base             Salt             Water 

HBr    +    KOH    →    KBr    +    H2O 

Acid           Base            Salt           Water 

Neutralisation reactions in our body: 

• Acids produced in the body, like HCl, get neutralised by the alkaline food that we intake as a part of our diet. 
• Excess acid causes acidity pain which can be relieved by an antacid which is a base. 

Importance of acid-base reactions: 

• Acid- base reactions are useful in forming salts. 
• Acid-base reactions help in maintaining the balance in pH. 
• Acid-base reactions promote metabolic activities. 

Precipitation reaction: 

When two solutions containing soluble salts are mixed and form an insoluble substance in the solution, such reaction is a precipitation reaction.  

CuSO4     +    2NaOH →    Na2SO4    +     Cu(OH)2  ↓ 

Na2SO4   +    SrCl2        →    2NaCl      +    SrSO4  ↓ 

AgNO3    +    NaCl         →    NaNO3     +    AgCl ↓ 

CdSO4     +     K2S          →    K2SO4      +    CdS  ↓ 

Differences: 

Summary

• A chemical reaction in which one element displaces the other is called single displacement treaction.
• Chemical reaction in which both elements exchange other is called double displacement reaction.
• When acid and base react to form salt and water, it is known as neutralization reaction.
• If one of the products in double displacement is an insoluble precipitate, then it’s a precipitation reaction.

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