A process which under some conditions may take place by itself or by initiation independent of the rate is called spontaneous process.
A process which can take place by itself or has an urge or tendency to take place is called spontaneous process.
A spontaneous process is simply a process which is feasible.
The rate of the process may vary from extremely slow to extremely fast.
Examples of processes which take place by themselves :
1) Dissolution of common salt in water
2) Evaporation of water in an open vessel
3) Flow of heat from hot end to cold end or from a hot body to a cold body
4) Flow of water down a hill
5) Combination of nitric oxide and oxygen to form Nitrogen dioxide
2NO ( g ) + O2 ( g ) ———–> 2NO2 ( g )
Examples of processes which take place on initiation
1) Lightning of candle involving burning of wax
2) Heating of calcium carbonates to give calcium oxide and carbon dioxide
CaCO3 ( s ) ————–> CaO ( s ) + CO2 ( g )
3) Combination of hydrogen and oxygen to form water when initiated by passing an electric spark
H2 ( g ) + ½ O2 ( g ) ———> H2O ( g )
A process which can neither take place by itself nor by initiation is called a non spontaneous process.
1) Flow of water up a hill
2) Flow of heat from cold body to a hot body
3) Diffusion of gas from low pressure to a high pressure
4) Dissolution of sand in water
The force which is responsible for the spontaneity of a process is called a driving force
Nature of driving force
In order to acquire maximum stability, every system tends to have minimum energy.
Tendency for minimum energy
1) A stone lying at height has a tendency to fall down so as to have minimum potential energy
2) Water flows down a hill to have minimum energy
3) A wound watch spring has tendency to unbind itself to decrease its energy to minimum
4) Heat flows from hot body to cold body so that heat content of the hot body becomes minimum
All the processes are spontaneous because they have tendency to acquire minimum energy.
Consider the following exothermic reaction, all of which are spontaneous :
1) H2 ( g ) + ½ O2 ( g ) ———–> H2O ( l ) ΔrH° = -285.8 KJ mol-1
2) N2 ( g ) + 3H2 ( g ) ———–> 2NH3 (g) ΔrH° = -92.2 KJ mol-1
3)C ( s ) + O2 ( g ) ————> CO2 ( g ) ΔrH° = -285.8 KJ mol-1
All these reactions are accompanied by evolution of heat. The heat content of the products is less than those of the reactants. These reactions are spontaneous because they are accompanied by decrease of energy.
A tendency to attain minimum energy i.e. a negative value of enthalpy change, might be responsible for a process or a reaction to be spontaneous or feasible.
limitation of the criteria for minimum energy
1) A number of reactions are known which are endothermic i.e. for which ΔH is positive but still they are spontaneous.
1) Evaporation of water or melting of ice
It takes place by absorption of heat from the surrounding.
H2O ( l ) ———-> H2O ( g ) Δvap H° = 40.8 KJ mol-1
H2O ( s ) ———-> H2O ( l ) Δfus H° = 6 KJ mol-1
2) Dissolution of salts like NH4Cl , KCl
NH4Cl (s) + aq ——–> NH4 +(aq) + Cl– (aq ) ΔrH° = 15.1 KJ mol-1
3) Decomposition of calcium carbonate on heating
CaCO3 ( s ) ————> CaO (s ) + CO2 ( g ) ΔrH° = 177.8 KJ mol-1
4) Decomposition of N2O5 at room temperature
2N2O5 ( g ) ———–> 4NO2 ( g ) + O2 ( g ) ΔrH° = 219 KJ mol-1
5) Decomposition of mercuric oxide on heating
2HgO ( s ) ———–> 2Hg ( l ) + O2 ( g ) ΔrH° = 90.8 KJ mol-1
2) A number of reactions are known for which ΔH is zero but still they are spontaneous
Reaction between acetic acid and ethyl alcohol
CH3COOH ( l ) + C2H5OH ( l ) ———> CH3COOC2H5 ( l ) + H2O ( l )
2) Expansion of an ideal gas into vacuum
3) Even those reactions for which ΔH is negative, rarely proceed to completion even though ΔH remains negative throughout.
4) Reversible reactions also occur.
H2 ( g ) + I2 ( g ) ——>2HI (g )
2HI (g ) ———-> H2 ( g ) + I2 ( g )
The energy factor or enthalpy factor cannot be the sole criterion for predicting the spontaneity or the feasibility of a process.
Tendency for maximum randomness
Suppose the two gases are enclosed in bulbs A and B connected to each other by a tube and kept separated by a stopcock. If the stock cock is opened the two gases mix completely. The gases which were confined to bulbs A and B separately are no longer in order. A disorder has come in or the randomness of the system has increased.
A spontaneous process for which ΔH = 0 is the spreading of a drop of ink in a beaker filled with water.
Second factor which is responsible for the spontaneity of a process is the tendency to acquire maximum randomness.
1) Evaporation of water take place because the gaseous water molecules are more random than the liquid water molecules. The process is spontaneous because it is accompanied by increase of randomness. Melting of ice is a spontaneous process because liquid state is more random than the solid state.
2) Dissolution of Ammonium Chloride is spontaneous because in the solid, the ions are fixed but when they go into the aqueous solution, they are free to move about.
3) Decomposition of solid calcium carbonate is spontaneous because CO2 produced is more random than the solid CaCO3.
4) Decomposition of N2O5 is spontaneous because 2 moles of gaseous N2O5 give 5 moles of gaseous product .
5) Decomposition of solid Mercury oxide is spontaneous because the liquid Mercury and gaseous oxygen formed are more random than solid HgO.
The overall tendency for a process to occur depends upon the resultant of the following two tendencies :
1) Tendency for minimum energy
2) Tendency for maximum randomness
The resultant of the above 2 tendencies which gives the overall tendency for a process to occur is called the driving force of the process.
For example :
E represents the tendency for minimum energy
R represents the tendency for maximum randomness.
D represent the overall tendency
1) Evaporation of water
H20 ( l ) ———-> H2O ( g ) ΔH = 44 kJ mol-1
E opposes( process being endothermic), R favours (because gas is more random than liquid).Since the process is known to be spontaneous hence R must be greater than E.
2)Dissolution of NH4Cl in water
NH4Cl ( s ) + aq ————>NH4+ (aq ) + Cl– (aq) ΔH = 15.1 kJ mol-1
In this process E opposes and R favour. Here again , the spontaneity of the process is explained by suggesting that R> E.