Final answer:
During exothermic reactions, some of the released heat increases the system's entropy, making it unavailable for work. This is observed when ice melts into water, where the energy input breaks molecular bonds without changing temperature, thereby increasing disorder.
Step-by-step explanation:
For many exothermic reactions, some of the heat released as a result of the enthalpy change goes into increasing the entropy, so it is not available to do work. When ice melts into water, there is an increase in entropy because the highly ordered solid structure converts into a disorderly liquid, which is a change that embodies a higher degree of randomness and disorder. This increase in entropy is a result of the system transitioning from a more ordered state to a more disordered state, wherein the energy becomes more spread out and less available to do work.
It is important to note that although energy is being added to the system, the temperature does not change during the phase change from ice to water. This is because the energy is being used to break the bonds between the molecules of ice, transitioning it into a liquid state, rather than increasing the kinetic energy of the molecules which would raise the temperature.