Enthalpy of Combustion: Energy changes when 1mole of substance is completely burnt in presence of oxygen to give CO2 & H2O.
C + O2 à CO2 ∆Hc=-393.5 KJmol-1
CH4 + O2 à CO2 + H2O ∆Hc=-74.8 KJmol-1
Efficiency of fuel: is measured in terms of its calorific value.
Calorific value: is the amount of heat released when 1gram of fuel is completely burnt.
More in calorific value, more in the efficiency
Example: For wood = 17kJ mol-1
Charcoal = 33kJ mol-1 (better and efficient because of high calorific value)
Hess’s law of heat summation
According to this “total energy change for a reaction is same, whether reaction takes place in one step or in many steps.
Application of Hess law
We can calculate ∆H formation.
Other type of enthalpies
Enthalpy of solution: Energy changes when 1mole of solute get dissolved in specific amount of solvent at constant temperature and pressure.
Example: If we need to dissolve NaCl in H2O.
NaCl(s) –> Na(g)+ + Cl(g)–
we need lattice enthalpy
Amount of energy required to break 1 mole if crystal lattice into constituents.
Na+(g) + H2O -> Na(aq)
Hydration energy is released to dissolve it
∆Hsol= ∆Hlattice + ∆HHydration
lattice enthalpy can be used in calculate ∆hr also.
Born- Haber cycle
It is defined as “enthalpy change diagram “
It consists of various steps like:
Step 1: Dissociation of ionic solid into free ions and the energy required is called lattice enthalpy
Step 2: Hydration of the ions and for this step energy is released called hydration enthalpy.
These steps are shown below in diagrammatic form:
Enthalpies of phase transition:
- Enthalpy of fusion: It is amount of heat changes that occur when 1 mole of solid gets converted into liquid state by heating.
- Enthalpy of vaporisation: It is amount of heat changes that occur when 1 mole of liquid gets converted into gaseous state by heating.
- Enthalpy of sublimation: It is amount of heat changes that occur when 1 mole of solid state gets converted into gaseous state by heating.
It is the enthalpy change in breaking one mole of bonds.
CH4 –> C + 4H ∆H = -1665 KJ mol-2
- For diatomic it is equal to enthalpies of atomisation.
- For polyatomic ions: Bond energy of particular bond is not the same when present in different compounds in them therefore, average is taken.
Average Bond dissociation energy: To break different kind of bonds.
H2O à H + OH 497.8KJmol-1
OH à O + H 428.5 KJ mol-1
= (497.8 + 428.5)
=463.15 KJ mol-1
Bond dissociation energy: Amount of energy required to break one mole of bonds of particular type behind atoms in gaseous state.
H2 –> 2H ∆H = 435.0 KJ mol-1
Application to calculate enthalpies of reaction.
∆Hr =Bond energy of reactants + Bond energy of products
Limitation of 1st law of thermodynamics:
- It tells us about energy exchanges between system & surroundings but it does not tell us about the direction of reaction.
- There are certain reactions that proceed on its own in given direction.
Like: Cup of hot tea cools down itself but never gets warm itself.
So, why these reactions do not proceed in its opposite manner by themselves?
- It proves that certain reaction is spontaneous and certain are non- spontaneous.
- Non- spontaneous means that it will not take place until we do some kind of work on it to make it proceed.
Now question arises that what factor or driving force behind spontaneous processes:
It was suggested that the energy factor in the driving force i.e. any process that leads to decrease in energy, leads to stability & it makes the reaction spontaneous.
That means all exothermic reactions are spontaneous because for them change in enthalpy is negative.
Some reactions are spontaneous but they need initiation.
Like, we to burn wood we ignite it
The afterwards reaction occur on its own
Electric spark passed though H2O –> H2 + O2
But it has been observed that some endothermic reactions are also spontaneous.
Ice –> liquid ∆H =+ve
liquid –> vapour ∆H =-ve
So, it was studied that though these reactions are endothermic but still are spontaneous.
It was seen that these reactions when occur they lead to increase in other factor i.e. Randomness or disorder.
So, it was concluded that any reaction that leads to increase in randomness is called spontaneous.
For reaction to be spontaneous:
- Change in enthalpy is -ve
- Randomness increases
Then another term was introduced that is entropy