Bond Energy
or Bond Enthalpies
When a bond is formed
between atoms, energy is released. Obviously same amount of energy will be
required to break the bond. The energy required to break the bond is termed bond
dissociation energy. The more precise definition is,
?The amount
of energy required to break one mole of bond of a particular type between the
atoms in the gaseous state, i.e., to separate the atoms in the gaseous state
under 1 atmospheric pressure and the specified temperature is called bond
dissociation energy.?
For example, \[H-H(g)\to
2H(g);\] \[\Delta H=+\,433\,kJ\,mo{{l}^{-1}}\]
\[Cl-Cl(g)\to
2Cl\,(g);\] \[\Delta H=+\,242.5\,kJ\,mo{{l}^{-1}}\]
\[H-Cl(g)\,\to
H(g)+Cl(g);\]\[\Delta H=+\,431\,kJ\,mo{{l}^{-1}}\]
\[I-I(g)\to
2I(g);\] \[\Delta H=+\,15.1\,kJ\,mo{{l}^{-1}}\]
\[H-I(g)\to
H(g)+I(g);\] \[\Delta H=+\,299\,kJ\,mo{{l}^{-1}}\]
The bond dissociation
energy of a diatomic molecule is also called bond energy.
However, the bond dissociation energy depends upon the nature of bond and also
the molecule in which the bond is present. When a molecule of a compound
contains more than one bond of the same kind, the average value of the
dissociation energies of a given bond is taken. This average bond
dissociation energy required to break each bond in a compound is called bond
energy.
Bond energy is also
called, the heat of formation of the bond from gaseous atoms constituting the
bond with reverse sign.
\[H(g)+Cl(g)\to
H-Cl\,(g);\]\[\Delta H=-\,431\,kJ\,mo{{l}^{-1}}\]
Bond energy of \[H-Cl=-\]
(enthalpy of formation) \[=-(-431)=+\,431\,kJ\,mo{{l}^{-1}}\]
Consider the
dissociation of water molecule which consists of two \[O-H\] bonds. The
dissociation occurs in two stages.
\[{{H}_{2}}O(g)\to
H(g)+OH(g);\,\]\[\Delta H=497.89\,kJ\,mo{{l}^{-1}}\]
\[OH(g)\to
H(g)+O(g);\] \[\Delta H=428.5\,kJ\,mo{{l}^{-1}}\]
The average of
these two bond dissociation energies gives the value of bond energy of \[O-H.\]
Bond energy of \[O-H\]
bond \[=\frac{497.8+428.5}{2}=463.15\,kJ\,mo{{l}^{-1}}\]
Similarly, the bond
energy of \[N-H\] bond in \[N{{H}_{3}}\] is equal to one ? third of the energy
of dissociation of \[N{{H}_{3}}\] and those of \[C-H\] bond in \[C{{H}_{4}}\]
is equal to one ? fourth of the energy of dissociation of \[C{{H}_{4}}.\]
Bond energy of \[C-H=\frac{1664}{4}=416\,kJ\,mo{{l}^{-1}}\]
\[[C{{H}_{4}}(g)\to
C(g)\,+4H(g);\]\[\Delta H=1664\,kJ\,mo{{l}^{-1}}]\]
Applications
of bond energy
(1) Heat of a
reaction \[=\Sigma \]Bond energy of reactants ? \[\Sigma \] Bond energy of
products.
Note :
In case of atomic species, bond energy is replaced by heat of atomization.
Order
of bond energy in halogen \[Cl>Br>{{F}_{2}}>{{I}_{2}}\]
(2) Determination of
resonance energy : When a compound shows resonance, there is considerable
difference between the heat of formation as calculated from bond energies and
that determined experimentally.
Resonance energy
= Experimental or actual heat of formation ~
Calculated heat of formation.
The system and the surroundings are separated by real or imaginary boundaries. The boundary also defines the limits of the system. The system and the surroundings can interact across the boundary.
(2) Types of systems
(i) Isolated system : This type of system has no interaction with its surroundings. The boundary is sealed and insulated. Neither matter nor energy can be exchanged with surrounding. A substance contained in an ideal thermos flask is an example of an isolated system.
(ii) Closed system : This type of system can exchange energy in the form of heat, work or radiations but not matter with its surroundings. The boundary between system and surroundings is sealed but not insulated. For example, liquid in contact with vapour in a sealed tube forms a closed system. Another example of closed system is pressure cooker.
(iii) Open system : This type of system can exchange matter as well as energy with its surroundings. The boundary is not sealed and not insulated. Sodium reacting with water in an open beaker is an example of open system.
(iv) Homogeneous system: A system is said to be homogeneous when it is completely uniform throughout. A homogeneous system is made of one phase only. Examples: a pure single solid, liquid or gas, mixture of gases and a true solution.
(v) Heterogeneous system: A system is said to be heterogeneous when it is not uniform throughout, i.e., it consist of two or more phases. Examples : ice in contact with water, two or more immiscible liquids, insoluble solid in contact with a liquid, a liquid in contact with vapour, etc.
(vi) Macroscopic system : A macroscopic system is one in which there are a large number of particles (may be molecules, atoms, ions etc. )
Note : All physical and chemical processes taking place in open in our daily life are open systems because these are continuously exchanging matter and energy with the surroundings.
(3) Macroscopic Properties of the System : Thermodynamics deals with matter in terms of bulk (large number of chemical species) behaviour. The properties of the system which arise
