Chemistry Spontaneity and Enthalpy

Topics Covered :

● Spontaneity
● Enthalpy as Criterion for Spontaneity

Spontaneity :

`=>` The first law of thermodynamics tells us about the relationship between the heat absorbed and the work performed on or by a system.

● It puts no restrictions on the direction of heat flow. However, the flow of heat is unidirectional from higher temperature to lower temperature.

● In fact, all naturally occurring processes whether chemical or physical will tend to proceed spontaneously in one direction only.

`color{red}("Example ")` A gas expanding to fill the available volume, burning carbon in dioxygen giving carbon dioxide.

● But heat will not flow from colder body to warmer body on its own, the gas in a container will not spontaneously contract into one corner or carbon dioxide will not form carbon and dioxygen spontaneously. These and many other spontaneously occurring changes show unidirectional change.

● We may ask ‘what is the driving force of spontaneously occurring changes?

● What determines the direction of a spontaneous change?

`=>` In this section, we shall establish some criterion for these processes whether these will take place or not.

`=>` Let us first understand what do we mean by spontaneous reaction or change?

● You may think by your common observation that spontaneous reaction is one which occurs immediately when contact is made between the reactants.

● Take the case of combination of hydrogen and oxygen. These gases may be mixed at room temperature and left for many years without observing any perceptible change. Although the reaction is taking place between them, it is at an extremely slow rate. It is still called spontaneous reaction.

● So spontaneity means ‘having the potential to proceed without the assistance of external agency’. However, it does not tell about the rate of the reaction or process. Another aspect of spontaneous reaction or process, as we see is that these cannot reverse their direction on their own. We may summarise it as follows :

`color{green)("A spontaneous process is an irreversible process and may only be reversed by some external agency")`.

Is decrease in enthalpy a criterion for spontaneity?

`=>` If we examine the phenomenon like flow of water down hill or fall of a stone on to the ground, we find that there is a net decrease in potential energy in the direction of change.

`=>` By analogy, we may be tempted to state that a chemical reaction is spontaneous in a given direction, because decrease in energy has taken place, as in the case of exothermic reactions.

● For example :

`color{purple}(1/2 N_2 (g) +3/2 H_2 (g) = NH_3 (g) ; Delta_r H^(⊖) = - 46.1 kJ mol^(-1))`

`color{purple}(1/2 H_2 (g) +1/2 Cl_2 (g) = HCl (g) ; Delta_r H^(⊖) = -92.32 kJ mol^(-1))`

`color{purple}(H_2 (g) +1/2 O_2 (g) → H_2O (l) ; Delta_r H^(⊖) = -285.8 kJ mol^(-1))`

`=>` The decrease in enthalpy in passing from reactants to products may be shown for any exothermic reaction on an enthalpy diagram as shown in Fig. 6.10(a).

●Thus, the postulate that driving force for a chemical reaction may be due to decrease in energy sounds ‘reasonable’ as the basis of evidence so far.

`=>` Now let us examine the following reactions :

`color{purple}(1/2 N_2 (g) +O_2 (g) → NO_2 (g) ; Delta_r H^(⊖) = +32.2 kJ mol^(-1))`

`color{purple}(C_text{(graphite, s)} + 2 S(l) → CS_2(l); Delta_r H^(⊖) = +128.5 kJ mol^(-1))`

● These reactions though endothermic, are spontaneous.

● The increase in enthalpy may be represented on an enthalpy diagram as shown in Fig. 6.10(b).

`=>` Therefore, it becomes obvious that while decrease in enthalpy may be a contributory factor for spontaneity, but it is not true for all cases.