DR. TANUJA NAUTIYALDEPARTMENT OF

CHEMISTRYSHARDA PUBLIC SCHOOL

ALMORA

Chemical Reactions

Chemical reactionA chemical reaction is the change of a

substance into a new one that has a different chemical identity.

A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change.  Absolute confirmation of a chemical change can only be validated by chemical analysis of the products!

Chemical reaction

The vast number of chemical reactions can be classified in any number of ways.

(1)  combination or synthesis reactions (2)  decomposition reactions (3)  substitution or single replacement reactions (4)  double displacement reactions

Types of  Reactions

  Combination or synthesis               A  +  B ----> AB

Decomposition                                     AB ---->  A  +  B

Substitution or Single Replacement: A  +  BC ---->  B  +  AC

Metathesis or Double Displacement      AB  +  CD ---->  AD  +  CB

Chemical equation

Chem. equation uses symbols to represent a chem. Reaction and shows the relationship between the reactants and products of a reaction.

CH4 + 2O2 → CO2 + 2H2O + ENERGY

CH4 + 2O2 are called reactants, and reactants are always written at left side of equation.

CO2 + 2H2O called products and products are always written at right side of equation.

Chemical equation

An arrow(→) is put between reactants and products to indicate that products are produced or yield from reactants.

The reactants and products contain same type of atoms. Atoms cannot be created or destroyed during chemical reaction.

Chemical equation

A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction. It means relative amounts of reactants and products.

Mole ratio can be converted to masses.

Balanced equation and mole ratio

Law of definite proportions: A compound always contains the same elements in the same proportions regardless of how the compound is made or how much the compound is formed.

:

Chemical reactions are breaking of old bonds from reactant-molecules and formation of new bonds in product-molecules.

Chemical reactions involve changes in energy. Photosynthesis is an endothermic reaction.

Energy and chemical reactions

:

Energy is released (exothermic) during formation of bonds and energy is required for breaking the bonds endothermic)

Energy is conserved in chemical reactions.

The energy that is stored in form of chemical bonds is called chemical energy.

Energy and chemical reactions

Combustion Reaction

A substance such as wood, natural gas, or propane combines with oxygen releasing a large amount of energy and produces carbon dioxide and water.

CH4 + 2O2 ----- CO2 + 2H2O + energy

Methane Oxygen Carbon dioxide

C3H8 + 5O2 ---- 3 CO2 + 4H2O + energy

Propane

Exothermic ReactionIn this type of reaction, generally energy

in form of heat is released.

CH4 + 2O2---- CO2 + 2H2O + energyMethane Oxygen Carbon dioxide For example all combustion reactions are

exothermic.

Exothermic Reaction

Endothermic Reaction

In this type of reaction, generally energy in form of heat is absorbed. • Photosynthesis is an endothermic reaction in which, reaction occur taking CO2 from air, H2O from soil, green pigment chlorophyll present in plants and light energy from sun is absorbed to form sugar (stored in form of product) and oxygen gas that is released into the air.

light energy from sun is absorbed

6CO2 + 6H2O → C6H12O6 + 6O2

Exothermic Reactions

Dissolution Reaction

Dissolution reaction occurs when an ionic compound dissolves in water to make an ionic solution.

NaOH(s)  Na+(aq) + OH-(aq)

MgSO4(s)  Mg2+(aq) + SO42-(aq)

Precipitation reactionA precipitation reaction is a reaction in which

soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate.

AB + CD AD + CB2KOH + CuSO4 K2SO4 + Cu(OH)2

AgNO3(aq) + NaOH (aq)  AgOH(s) + NaNO3(aq)

silver nitrate + sodium hydroxide  silver hydroxide + sodium nitrate

Oxidation-Reduction Reaction

Any chemical change in which one substance is oxidized (loses electrons) and another substance is reduced (gains electrons).

This reaction is also called as Redox reaction Redox equation: 2H2(g) + O2(g) → 2H2O(l)

Oxidation: 2H2(g) + 4OH-(aq) → 4H2O (l) + 4e-

Reduction: O2(g) +2H2O(l) +4e- → 4OH-(aq)

Synthesis reaction

The reaction in which two or more substances combined to form a new compound.

When many Ethylene molecules combine then polyethylene is produced. It is synthesis or addition reaction.

2Na + Cl2 --- 2NaClsodium metal + chlorine gas table salt.

2P + 3Br2 2PBr3

Decomposition reactions

The reaction in which a substance –a bigger molecule is broken apart into smaller substances.

Digestion and cracking oil are decomposition reactions.

2H2O --- H2 + O2

2NaHCO3 ----- Na2CO3 + CO2 + H2O2H2O2 2H2O + O2

Single Displacement reactions

The reaction in which one element or radical takes the place of another element or radical in a compound.

In general, a more reactive element will take place of a less reactive one in single-displacement reaction.

Electroplating of silver or gold is single displacement reaction.

3CuCl2 + 2Al 2AlCl3 + 3 Cu

Copper chloride solution Aluminum Aluminum chloride Copper

Double Displacement reactions

The reaction in which a gas, a solid precipitate, or a molecular compound is formed from the apparent exchange of atoms or ions between two compounds.

Pb(NO3)2 + 2 KI PbI2 + 2K(NO3)

Lead nitrate + Potassium iodide -- Lead iodide + Potassium nitrate

In general ions appear to be exchanged between compounds in double displacement reaction.

DR. TANUJA NAUTIYALDEPARTMENT OF

CHEMISTRYSHARDA PUBLIC SCHOOL

ALMORA

Chemical Reactions