Gibbs free energy change ΔG and entropy change ΔS

• Entropy = The amount of disorder or randomness in a system.
• The higher the absolute energy value, the higher the degree of disorder of a substance.
• Gases are more disorders than liquids and solutions which are more disordered than solids. Therefore, melting or evaporation increases the entropy value. 
• The higher the Mr the higher the entropy value.
• ΔSΘ = Sum of entropy values of products - Sum of entropy values of reactants   
• Feasible reaction = A reaction that can occur at a particular temperature.
• For a reaction to be feasible, ΔG must be equal to or less than 0.
• ΔGΘ = Enthalpy change - (Temperature x entropy change)
• There is a difference between a reaction being feasible and spontaneous. A feasible reaction may not occur at a particular temperature as the activation energy is too high, a spontaneous reaction is one that is feasible and occurs under standard conditions. For a reaction to be spontaneous, ΔG must be less than or equal to 0 and the activation energy must be low enough.