CHEMICAL ENERGETICS

You will learn in this chapter about:

*        Exothermic and Endothermic reaction.
*        Heat of Reactions.
*        Measurement of heat of reactions.
*        Heat of Neutralization.

INTRODUCTION

The chemical reactions during which material changes are accompanied with change in heat energy are called thermo chemical reactions. The branch of chemistry which deals with the study of heat changes in chemical reactions is called thermo chemistry. There are two types of thermo chemical reactions i.e. exothermic and endothermic chemical reactions.

10.1 EXOTHERMIC AND ENDOTHERMIC REACTIONS

10.1.1 Exothermic Reaction:

It is Greek word and composed of EXO means out of or to evolve + THERME means heat. An exothermic reaction is the chemical change during which heat is given out or released. The change of heat is represented by AH and it is shown by negative, sign.

Examples:

      i.        The combustion of coal in air is the example of exothermic reaction. 393.7 Kilo joules of heat energy is released when 1 mole of coal is burnt in 1 mole of O2 to produce 1 mole of CO2.

                      C(s) + O2 (g)
------à CO2 (g)     ΔH = - 393.7 K.J/mol

     ii.        Burning of methane in presence of oxygen is another example of exothermic reaction. When 1 mole of methane is burnt in 2 moles of O2 then 1 mole of CO2 and 2 moles of water are formed. During these reaction 890 Kilo joules per mole of heat energy is released.

          CH4 (g) + 2O2 (g)
------à CO2 (g) + 2H2O (1) ΔH = -890 K.J/mol.

    iii.        The formation of water from hydrogen and oxygen is also example of exothermic reaction 286 Kilo joules per mole of heat energy is released, when 1 mole of H2 reacts with ½ moles of O2 to form 1 mole of H2O.

                    H2 (g) + O2 (g)
------à  H2O (1)        ΔH = -286 K.J/mol.

Generally in exothermic reactions, heat flows from the system to surroundings and container becomes hot, this is only possible, when total energy of the reactants is greater than total energy of products. The difference in the energies is the heat supplied by the system to surroundings.

Using Exothermic Reactions to Warm Food:

In modern army, food rations can be warmed without benefit of stove or campfire. The pouch that contains the food is attached to flameless radiation heater. The heater contains chemicals that react with water to produce heat. When the pouch is placed in a bag and water added, temperature of the food reaches to 60C in about 15 minutes.

Mg(s) + 2H2O(1) ------à  Mg (OH) 2(s) + H2 (g) ΔH= -3.53KJ/mol

The reaction of (Mg) with water is slow, because of the formation of film oxide (MgO). The reaction of Mg with water is highly accelerated in the presence of iron (Fe) and ordinary salt (NaCl). Thus, the flameless radiation heater contains a mixture of Mg, Fe and NaCl.

10.1.2 Endothermic Reactions:

It is Greek word and composed of ENDO mean into or to absorb and THERME means heat. Endothermic reaction is the chemical change during which heat is absorbed or taken in. The change of heat energy is represented by Δ H and sign of A H is positive (+Δ H= Absorbs heat). During the endothermic reaction heat is absorbed from the surroundings, it means heat flows from surroundings to the system and container becomes cold. This is because the total energy of the products is greater than the total energy of reactants. Here, the difference in the energies is the heat supplied to the system by the surroundings.

Examples:   

      i.        The decomposition of water into hydrogen and oxygen is example of endothermic reaction. During decomposition of 1 mole of water to 1 mole of hydrogen and half mole of oxygen 286 Kilo joules per mole of heat energy is absorbed.

               H2O (l)
------à  H2 (g) +1 O2 (g) ΔH = +286 KJ/mol

     ii.        1 mole of nitric oxide (NO) is formed by combination of ½   mole of N2 and ½      mole of O2 .This is the example of endothermic reaction and heat absorbed is about 90.25 Kilo joules per mole.

           ½ N2 (g) +½ O2 (g)
------à         NO (g)   ΔH = + 90.25 KJ/mol

10.2 HEAT CONTENTS OF REACTION

Every substance possesses a characteristic internal energy and internal energy depends upon the structure and physical state of that substance. The energy possessed by a substance is called heat contents of that substance. During a chemical reaction, the reactants are converted into products and heat energy is either absorbed or evolved. This is because the heat contents of these respective substances are different. The heat evolved or absorbed at constant pressure is called as enthalpy of the reaction.

The heat content (enthalpy) of a substance is represented by "H" and the change in heat content during a chemical reaction is then represented by ΔH. Here Greek letter Δ (delta) signifying the change in the property. It means the change in heat content during chemical reaction is the difference between the heat content of products and reactants of that reaction.

It is difficult to measure the enthalpy of a reaction, but we can measure the change in enthalpy which is denoted by (ΔH). It is obtained by subtracting the enthalpy of reactant (HJ from enthalpy of products (H2).

             ΔH                 =          (H2                               H1)

Change of Enthalpy       Heat content of product   Heat content of reactions

If enthalpy of products is greater than the enthalpy of reactants, then the sign of A H will be positive and over all reaction is endothermic and heat is absorbed.

½ N2 (g) +½ O2 (g) ------à NO (g)         ΔH = + 90.25 KJ/mol

C(s) + H2O (1) ------à CO (g) + H2 (g)     ΔH = + 118 KJ/mol.

H2O (1) ------à H2 (g) + ½ O2 (g)            ΔH = + 286 KJ/mol.

If enthalpy of product is smaller than the reactants, then the sign of ΔH will be negative and overall reaction is exothermic and heat is evolved. The examples of exothermic reactions are following:

H2O (1) +½ O2 (g) ------à H2O (l)   ΔH = - 286 KJ/mol.

S (g) + O2 (g)          ------à SO2 (g)       ΔH = -296.8 KJ/mol.

CH4 (g) +O2 (g) ------à CO2 (g) +2H2O (1)     ΔH = -890.4 KJ/mol.

N2 (g) + 3H2 (g) ------à 2NH3 (g)    ΔH = -92.0 KJ/mol.

 
    back to previous page  
     
 
 
Home | About Us | Notes | Animation | Career Guidance | Tuition | Wisdom | Contact Us
© All Copyright Reserved to www.mynoteslibrary.com