ENERGY CHANGE IN A REACTION

A spontaneous reaction is the one which has the potential to occur on its own without the assistance of any external agent. An example is the reaction of glucose with oxygen in the living tissues. By itself, a spontaneous reaction is irreversible except work is done on the system. The two factors which can affect the spontaneity of a chemical reaction are entropy and the free energy of the system undergoing the change. 

It is the measure of the degree of disorderliness or randomness of a system. The standard entropy change is a state function i.e.
ΔS_ѳ = S_ѳproduct – S_ѳreactant.
The S.I. unit of entropy is JK-1mol^-1
The entropy increases from solid to gaseous state and vice versa. When entropy increases, ΔS_ѳ tends to be positive but when it decreases, ΔS_ѳ tends to be negative. Generally, an increase in temperature of a system increases the entropy of the system. A perfectly ordered system at 0K has an entropy of zero.

ENTROPY CHANGE IN REVERSIBLE PROCESSES:

  the second law of thermodynamics states that a spontaneous process occurs only if there is an increase in entropy of the system and its surrounding. At constant temperature and pressure, the change in entropy of a reversible reaction is given as
ΔS = ^ΔH⁄_T;
Where;
ΔH = enthalpy change
T = absolute temperature
ΔS = entropy change
 

GIBB’S FREE ENERGY:

The free energy of a chemical system is the energy which is available for doing work. It is the driving force that brings about a chemical change. The standard free energy change is a state function and is given as;
ΔG = G_ѳ product – G_ѳ reactant.
Free energy (G), entropy (S) and enthalpy (H) are related by the following equation;
G = H – TS
Hence, ΔG_ѳ = ΔH_ѳ – TΔS_ѳ
ΔGѳ helps us to predict whether a chemical reaction will occur or not provided that the entropy and enthalpy change involved in the reaction are known.

• If ΔG_ѳ is negative, the reaction will occur (spontaneous)
• If ΔG_ѳ is positive, the reaction will not occur (non-spontaneous)
• If ΔG_ѳ is zero, the reaction is at equilibrium.

  Note: An increase in the temperature of a system leads to an increase in ΔSѳ and a decrease in ΔG_ѳ. For a reaction to be thermodynamically feasible (spontaneous), ΔSѳ must be positive and ΔG_ѳ must be negative.