Wednesday 9 December 2015

Lecture 2 – Endothermic Reactions

ACKNOWLEDGEMENT:

Figure
Source of Figure
Figure 2
http://chemistry.tutorvista.com

Endothermic Reactions:

 Endothermic reactions are defined as those reactions in which energy/heat is provided as a reactant of the reaction to form products.

 Figure 2:
Figure 2 shows the energy diagram of an endothermic reaction. It can be seen that:
  1. the internal energy of the reactants is less than the internal energy of the products.
  2. Therefore, the enthalpy or ∆H of such reactions is always positive.
  3. Products formed by such reactions are less stable as compared to their parent reactants.
  4. The bonds formed are weak and can easily be broken by the application of heat.
  5. This makes the products fairly unstable and, 
  6. therefore, in terms of bonds; Bond breaking is Endothermic.

Examples of Endothermic Reactions:




Table 1: Summary and comparison of Exo and Endothermic Reactions.
Exothermic
Endothermic
Heat exchange
Heat is released to the environment
Heat is absorbed from the
environment
Internal Energy of Reactants
Higher than that of Product(s)
Lower than that of Product(s)
Internal Energy of Products
Lower than that of Reactant(s)
Higher than that of Reactant(s)
Stability
Product(s) are more stable than Reactant(s)
Reactant(s) are more stable than 
Product(s)
Enthalpy Change – Using respective examples

&

In term of Bond making and breaking
(the unit of energy if kJ)


Bond energy of C – H bond = 413 kJ
Total C – H bonds = 4
Total energy absorbed in breaking CH4 = 4 x 413kJ = 1652kJ
Bond energy of  O = O bond = 498kJ
Total O = O bonds = 2
Total energy absorbed in breaking O2 = 2 x 498kJ = 996kJ
Total energy absorbed in bond breaking = 1652 kJ + 996 kJ = 2618 kJ

Bond energy of C = O bond = 799 kJ
Total C = O bonds = 2
Total energy evolved in making CO2 = 2 x 799 kJ = 1598 kJ
Bond energy of H – O bond = 467 kJ
Total H – O bonds = 4
Total energy evolved in making H2O = 4 x 467 kJ = 1864 kJ
Total energy evolved in bond making = 1598 kJ + 1864 kJ = 3462 kJ

Enthalpy Change = ∆H 
= Energy of Reactants – Energy of Products
= 2618 kJ – 3462 kJ
= – 844kJ

  1. Enthalpy change is negative
  2. Since, the energy evolved in Bond Making is Greater than energy absorbed in Bond breaking,
  3. Therefore – Bond Making/Formation is Exothermic 


Bond energy of C = O bond = 799 kJ
Total C = O bonds = 6 x 2 = 12
Total energy absorbed in breaking CO2 = 12 x 799 = 9588 kJ
Bond energy of H – O bond = 467 kJ
Total H – O bonds = 6 x 2 = 12
Total energy absorbed in breaking H2O = 12 x 467 kJ = 5604 kJ
Total energy absorbed in bond breaking = 9588 kJ + 5604 kJ = 15192

Bond energy of C6H12O6 = 9467 kJ
Bond energy of O = O bond = 498 kJ
Total O = O bonds = 6
Total energy absorbed in breaking O2 = 6 x 498 = 2988 kJ
Total energy evolved in bond making = 9467 kJ + 2988 kJ = 12455 kJ


Enthalpy Change = ∆H 
= Energy of Reactants – Energy of Products
= 15192 kJ – 12455 kJ
= 2737 kJ
  1. Enthalpy change is positive
  2. Since, the energy evolved in Bond Making is Lesser than energy absorbed in Bond breaking, 
  3. Therefore – Bond Breaking is Endothermic