Introduction:

The term spontaneous signifies voluntary or occurring devoid of external assistance or influence. A spontaneous reaction can thus be stated as a reaction which takes place devoid of external help or incitement. For illustration sodium metal reacts explosively via water. This is why sodium metal is kept in oil to avoid contact with water. Calcium carbide generally termed as carbide by welders is kept away from water. This is as well to stop the spontaneous reaction of carbide via water.

There various forms of energy and the energy changes in physical and chemical methods. The concept of heat content (that is, enthalpy) was introduced and chemical changes were categorized as exothermic or endothermic methods based on whether heat absorption or evolution accompanied the reactions. A few illustrations of methods are illustrated below.

H₂ (g) + 1/2 O₂ (g) → H₂O (l) ΔH = - 285.6 KJmol^-1

                                               ΔH = - 285.6 KJmol^-1

NH₄NO₃ (s) + H₂O (l) → NH₄⁺ (aq) + NO₃^- (aq)

                                              ΔH = 26.5 KJmol^-1

The first method, the reaction of hydrogen with oxygen is an exothermic reaction however yet the reaction of hydrogen with oxygen is not a spontaneous reaction. Remember that for a reaction to take place in a jar of hydrogen gas a spark should be introduced that is, reaction is aided to take place. The other illustration is the reaction of nitrogen and oxygen that is as well an exothermic reaction. Such two gases N₂ and O₂ are present in the air and no reaction takes place. The reaction of nitrogen and oxygen will not take place at ordinary temperature.

The second method, the dissolution of ammonium trioxonitrate (v) is the endothermic reaction. Whenever the solid dissolves, the beaker cools down appreciably. This method, via endothermic takes place spontaneously.

Ice melts whenever removed from the freezer compartment and placed outside spontaneously however it is an endothermic method. The above illustrations and numerous others recommend which enthalpy data alone are not adequate to predict whether a change will be spontaneous or not.    

Entropy and Entropy Change:

Entropy is the measure of the disorder of a system.  The greater the disorder the greater the entropy. The entropy change is the measure of the change in disorder which accompanies a chemical of physical procedure. The entropy and entropy change are represented by the symbols S and ΔS correspondingly, that is similar to enthalpy.

ΔS_reaction = S_P - S_R and for numerous reactants and products

ΔS_reaction = ΣS_P - ΣS_R

For a method that is accompanied by greater disorder the entropy increases and ΔS is positive. If a procedure is accompanied by less disorder (more order) the entropy decreases and ΔS are negative. The change in entropy is a driving force behind the reactions and methods. This is a measure of the capacity for a spontaneous change. Entropy is measured in units of energy. The SI unit is the joules.

Entropy is affected by numerous factors:

1) The physical state: The solid state by its ordered structure consists of the least entropy whereas the gas state consists of the highest. The liquid is intermediate.

Ice → Water   ΔS +ve

Water → Steam ΔS +ve

Solid-liquid, liquid-gas and solid-vapor transitions are accompanied via increased entropy.

2) Temperature: The higher the temperature, the higher the average kinetic energy of the particles, the more the arbitrary motion and collisions leading to greater disorder. Entropy increases with increasing temperature. For a gas sample whenever there is a change of temperature and pressure the entropy change based on the volume change. Entropy rises with increasing volume of a gas sample.

3) Change in the number of gas molecules: There is more arbitrary motion in a gas than in a liquid or solid. The more the number of gas molecules resultant from a reaction the greater the entropy of such a reaction.

Illustration:

N₂O₄ (g) → 2NO₂ (g) ΔS +ve

CaCO₃ (s) → CaO + CO₂ (g) ΔS +ve

H₂ + 1/2 O₂ (g) → H₂O (l) ΔS -ve

Na (s) + H₂O (l) → NaOH (aq) + H₂ (g) ΔS +ve

4) Mixing: There is more disorder in the mixture of samples than in each of the pure samples. Whenever a solid dissolves in liquid entropy increases (ΔS + ve). If two gases mix, there is as well raise in entropy.

Free Energy and the Free Energy Change:

Free energy is stated as the energy that is accessible to do work. Free energy is associated to the enthalpy and entropy.

G = H - TS, here G is the free energy and H, S and T are enthalpy, entropy and the absolute temperature for a change, physical or chemical at a constant temperature.

ΔG = ΔH - TΔS. For the standard state

ΔG^o = ΔH^o - TΔS^o where T = 298 K

The free energy finds out whether a reaction will take place spontaneously or not. For a spontaneous change ΔG is less than zero that is ΔG -ve. If ΔG is greater than zero (ΔG +ve) the procedure is not spontaneous. Such reactions will take place by supplying a driving force example by heating. At equilibrium ΔG = 0

Applications of the Free Energy Equation:

Five cases will be taken.

An exothermic reaction accompanied by an increase in entropy:

For the above kind of reaction:

ΔH is - ve and ΔS is + ve

Example:

2Na (s) + 2H₂O (l) → 2 NaOH (aq) + H₂ (g)

CaC₂ (g) + H₂O → CaO(s) + C₂H₂ (g)

The reactions above are exothermic reactions and are accompanied through greater disorder. The free energy change for such reactions at any temperature is less than zero that is, even at absolute zero temperature ΔG is still less than zero. The reactions are for all time spontaneous. The only way to prevent these reactions would be to keep the reactants apart. This is why carbide and sodium are kept away from the water.

An endothermic reaction accompanied via a decrease in entropy:

Here ΔH is positive and ΔS are negative. It follows that ΔG must be positive. The reaction is thus unlikely to take place. The reverse of the reactions will serve as:

NaOH (aq) + H₂ (g) → Na (s) + 2H₂O (l)

CaO (s) + C₂H₂ (g) → CaC₂ (s) + H₂O (l)

For the reactions above even at absolute zero, ΔG is more than zero. They are not spontaneous.

Exothermic reaction accompanied via a decrease in entropy:

For the group above of reactions:

ΔH is negative and ΔS is negative

Water → Ice ΔH -ve

Steam → Water ΔH -ve

In both the methods, ΔH is negative and ΔS are negative. The disorder reduces as a result of the processes. As ΔS is negative.

- TΔS is positive and ΔH is negative. For such methods there is an equilibrium temperature if ΔG = 0. This is the case, when TΔS = ΔH

T_eq = (ΔH/ΔS) T_eq - equilibrium temperature

Above the T_eq, ΔG is positive and the methods are non spontaneous. Beneath T_eq the ΔG is negative and the methods are spontaneous. The tendency for a spontaneous method reduces by increasing temperature for the procedure. The equilibrium temperatures are the melting and boiling points correspondingly.

Endothermic reaction accompanied via an increase in entropy:

The ΔH is positive and ΔS is positive

NH₄NO₃ (s) + H₂O (l) → NH₄⁺ (aq) + NO₃^- (aq)

Ice → water

As ΔS > 0

- TΔS < 0 however ΔH is positive. There is an equilibrium temperature (T_eq) as in the last illustration.

T_(eq) = ΔH/ΔS

Above the T_eq ΔG is negative and the reactions are spontaneous. Beneath T_(eq) the ΔG is positive and the reactions are non-spontaneous reaction. The tendency for a spontaneous reaction rises with increase in temperature. The solubility of ammonium trioxonitrate (v) increases by increasing temperature. For this reaction to have occurred spontaneous at lab temperature recommends that the equilibrium temperature is beneath the lab temperature. In case of the ice → water reaction the equilibrium temperature is the melting point that is around 0°C. As the laboratory temperature is above 0°C, the melting procedure is spontaneous and takes place as soon as the ice is eliminated from the freezer and put outside the laboratory.

Reaction in which the enthalpy change is zero:

For a few reactions the enthalpy is extremely negligible. The driving force for these reactions is the entropy change alone. Whenever the reaction is accompanied through an increase in entropy, then the reaction is spontaneous.

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