Chemical reactions are either accompanied by the evolution or absorption of heat.Energy changes occur in chemical reactions because the reactants and products of any chemical reaction possess different amount of energy.This is called heat content of the reaction.
Note:The amount of chemical energy in a compound or substance cannot be measured rather the energy changes taking place during a chemical reaction can be observed either in the form of heat,light or sound.The most convenient observation of the measure is by heat.
Chemical energy can be released in form of heat called enthalpy which is denoted by letter,H.
The rise or fall in the temperature of the chemical reaction is determined by the heat change,∆H, pronounced as Delta"H",where ∆ stands for change.
Mathematically, enthalpy change,∆H is the difference between the heat content of products,£H(product) and the heat content of reactants,£H(reactants
Types of physical and chemical processes in Heat change
In terms of heat change,∆H,the two types of physio-chemical processes are: Exothermic and Endothermic reaction.
These are changes or reactions in which heat is released to the surrounding of the reactants, leading to a rise in the temperature of the system.This occurs when the enthalpy of the products is less than the reactants.That is,Hp. NaCL(aq) + H2O
B) Dissolution of NaOH or KOH pellets in water.
C) Reaction between calcium oxide and water
CaO(a). +. H2O(l). ----------->. Ca(OH)2(aq)
D)Addition of concentrated acid to water.
H2SO4 (aq). + H2O. ----------->. H2SO4 (aq) + H2O
E) Displacement reaction