# Types Of Chemical Reactions-I

Chemical Reaction

The reaction in which a chemical substance transforms into another chemical substance is known as chemical reaction. A chemical change involves the formation of new substances whereas a physical change involves a change in color or state and no new substances are formed. During a chemical reaction, the substances that react are known as reactants whereas the substances that are formed during a chemical reaction are known as products.

Types of Chemical Reactions

Types of chemical reactions

Chemical reaction can be classified as:

1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Double Displacement reaction
5. Precipitation Reaction

Combination reaction

• A reaction in which two or more reactant combine to form a single product is known as combination reaction.
• It takes the form of X + Y → XY
• Combination reaction is also known as synthesis reaction.
• Example of combination reaction: 2Na + Cl2 → 2NaCl

Decomposition reaction

• A reaction in which a single compound breaks into two or more simpler compounds is known as decomposition reaction.
• It takes the form of XY → X + Y
• Decomposition reaction is just the opposite of combination reaction.
• Example of decomposition reaction: CaCO3 → CaO + CO2
• The reaction in which a compound decomposes due to heating is known as thermal decomposition reaction.

Displacement reaction

• A chemical reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution.
• It takes the form X + YZ → XZ + Y
• It is also called as substitution reaction.
• Example of displacement reaction: Zn + CuSO4 → ZnSO4 + Cu

Double Displacement reaction

• A chemical reaction in which ions gets exchanged between two reactants which forms a new compound is called as double displacement reaction.
• It takes the form of XY + ZA → XZ + YA
• It is also called as metathesis reaction.
• Example of double displacement reaction:

$~~~~~~~~~~~$BBaCl2 + Na2SO4 → BaSO4 + 2NaCl

Precipitation Reaction

A chemical reaction that involves the formation of an insoluble product (precipitate; solid) is called Precipitation reaction. The reactants are soluble, but the product formed would be insoluble and separates out as a solid. The chemical equation by which a chemical change is described is adequate for reaction in solutions, but for reactions of ionic compounds in aqueous solution (water), the typical molecular equation has different representations. A molecular equation may indicate formulas of reactants and products that are not present, and eliminate completely the formulas of the ions that are the real reactants and products. If the substance in the molecular equation that are actually present as dissociated ions are written in the form of their ions, the result is an ionic equation.

A precipitation reaction occurs when a solution, originally containing dissolved species, produces a solid, which generally is denser and falls to the bottom of the reaction vessel.

The most common precipitation reactions occurring in aqueous solution involve the formation of an insoluble ionic compound when two solutions containing soluble compounds are mixed. Consider what happens when an aqueous solution of NaCl is added to an aqueous solution of AgNO3. The first solution contains hydrated Na+ and Cl− ions and the second solution, Ag+ and NO3 − ions.

NaCl(s) S Na+(aq) + Cl−(aq)

AgNO3(s) S Ag+(aq) + NO3 −(aq)

When mixed, a double displacement reaction takes place, forming the soluble compound NaNO3 and the insoluble compound AgCl. In the reaction vessel the Ag+ and Cl− ions combine, and a white solid precipitates from the solution. As the solid precipitates, the Na+ and NO3 − ions remain in solution. The overall double displacement reaction is represented by the following balanced equation:

NaCl(aq) + AgNO3(aq) S AgCl(s) + NaNO3(aq)

Example Problem

1. When aqueous solutions of Pb(NO3)2 and KI are mixed, does a precipitate form?
2. Write a balanced equation for the precipitation reaction that occurs when aqueous solutions of copper(II) iodide and potassium hydroxide are combined.

Solution:

You are asked to predict whether a precipitate will form during a chemical reaction and towrite a balanced equation for a precipitation reaction.

You are given the identity of two reactants.

1. Yes, a solid precipitate, PbI2, forms when these solutions are mixed:

Pb(NO3)2(aq) + KI(aq) S PbI2(s) + 2KNO3(aq)

2. The two products of the reaction are insoluble copper (II) hydroxide and soluble potassium iodide.

CuI2(aq) + 2 KOH(aq) S Cu(OH)2(s) + 2 KI(aq)