The enthalpy changes for exothermic and endothermic reactions can be represented by energy level diagrams as shown in Fig. 16.4(a) and (b) respectively. In these diagrams, the energy levels of reactants and products are drawn on a vertical scale according to which a substance with higher heat content is represented higher up in the scale
thermochemical equations the fractional coefficients are also commonly used contrary to our usual practice for balancing the chemical equations. A thermochemical equation can be written in two ways:
(a) Heat effect can be written as one of the term along with the products.
2SO2 (g) + O2 (g) à 2SO3(g) + 694.6 KJ
(b) Heat evolved or absorbed can be expressed in terms of Δ.H.
2SO2(g) + O2(g) à 2SO3(g) ; = – 694 .6 KJ.
It is very important to mention the physical states of various reactants and products while writing thermochemical equations because change of physical state is also accompanied by the enthalpy changes. For example:
When 1 mol of hydrogen gas reacts with f mol of oxygen gas to produce 1 mol of liquid water, 286 kJ of heat is produced.
H2 (g) + 1/2 O2(g) à H2O (l) ; ΔH = – 286 KJ
On the other hand, if 1 mol of water vapours is produced instead of 1 mol of liquid water, the value of ΔH will be different.
H2 (g) + 1/2 O2(g) à H2O(g) ; ΔH = – 249 KJ
Some Important Conventions about Thermochemical Equations
1. The coefficients of various substances of chemical equation represent the number of their respective moles. In thermodynamic interpretation of an equation, we never interpret the coefficients as number of molecules. Hence, it is acceptable to write coefficients in fractions wherever necessary.
2. The value of ΔH in a thermochemical equation corresponds to the enthalpy change taking place when a specified number of moles of various reactants and products (as indicated by the coefficients of the various substances in the chemical equation), are involved in the reaction.
3. In case of the coefficients in the chemical equation are multiplied or divided by some integer, the ΔH value must also be multiplied or divided by the same integer. For example:
H2 (g) + 1/2 O2(g) à H2O(g) ; ΔH = – 286 KJ
If the whole equation is multiplied by 2, the MI for the new thermochemical equation is given as:
2H2 (g) + O2 (g) à 2H2 O (l)
ΔH = 2 x ( – 286) KJ = – 572 KJ
4. When a chemical equation is reversed, the magnitude of the Mf remains same, however, its sign is reversed. For example, if ΔH is +ve for the forward reaction, it would be negative for the reverse reaction.
H2(g) + 1/2 O2(g) à H2O(l) ; ΔH = – 286 KJ
H2O (l) à H2(g) + 1/2 O2 (g) ; ΔH = – 286 KJ
The thermochemical equations for some exothermic and endothermic reactions are given as follows:
Exothermic reactions(ΔH = -ve):
H2(g) + 1/2 O2(g) à H2O(l) ; ΔH = – 286.0 KJ
CH4(g) + 2O2 (g) à CO2(g) + 2H2O(l) ;
ΔH = – 890.4 KJ
N2 (g) + 3H2(g) à 2NH3(g) ; ΔH = – 92.3 KJ
ENTHALPY CHANGES IN SOME PHYSICAL AND CHEMICAL PROCESSES
Whenever a substance undergoes any physical or chemical transformations, energy changes also accompany them. In this section we shall” study the enthalpy changes involved in some physical processes like evaporation, fusion, dissolution, etc. We shall also become familiar with the energy changes involved in some chemical processes, like combustion, hydration, etc.