When a compound is broken down into smaller compounds then, the entropy of system increases. Other example include the dissolution process in which the "solute particles" gets separated from each-other when a solution is formed. The entropy of a system will never decrease but one hypothetical situation will be "Heat transfer occuring from cold reservoir to hot reservoir".In this case the total entropy of system decreases. The four processes occuring in carnot-cycle are "Isothermal expansion, adiabatic expansion, isothermal compression and isothermal expansion". The efficiency of a carnot cycle = 1 - (Tc / Th) If Tc = Th Then efficiency will be 100%