what entropy changes occur whin a substance melts?
A process that absorbs energy will be spontaneous only if an increase in entropy of the system also occurs that is large enough to compensate for the increase in energy. (e.g. Ice spontaneously melts at 20°C.)
When a substance melts, there are two main entropy changes that occur:
1. Entropy change of the substance: When a solid transitions to a liquid phase, the particles gain more freedom of movement and become more disordered. This increase in molecular randomness leads to an increase in entropy. Therefore, the entropy of the substance generally increases when it melts.
2. Entropy change of the surrounding environment: When a substance melts, it typically absorbs heat from its surroundings. This heat transfer increases the entropy of the surroundings, as the energy disperses into a larger number of particles. This entropy change is often referred to as the "transfer of heat entropy" or the "entropy change of fusion."
To calculate the total entropy change during melting, you can sum up the individual entropy changes of the substance and the surroundings:
ΔS_total = ΔS_substance + ΔS_surroundings
The value of ΔS_substance can be determined by considering the molecular arrangement and characteristics of the substance. For example, substances with more complex molecular structures usually have higher values of ΔS_substance, as they exhibit greater disorder upon melting.
The value of ΔS_surroundings can be calculated using the equation:
ΔS_surroundings = q/T
where q represents the heat absorbed during the melting process, and T is the temperature at which the melting occurs. Note that the heat absorbed (q) is typically given in units of joules (J) or calories (cal), and the temperature (T) is measured in Kelvin (K).
By evaluating the entropy changes of the substance and surroundings, you can determine the overall entropy change that occurs when a substance melts.