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Entropy and the Second Law of Thermodynamics

Spontaneous Process

So far we have learnt from Chapter 2 of the Physical Chemistry book that the First Law of Thermodynamics is essentially the law of conservation of energy, i.e. energy can neither be created nor destroyed but it may change from one form to another.

Entropy

By considering various spontaneous (or natural) processes, we can see that there is a tendency for materials to become more physically disorganized. This thermodynamic function of state is called entropy, S – and it is referred to the level of randomness, disorder or chaos of a system.

The entropy of system and surroundings together, increases during all natural or irreversible processes. For a reversible process, the total entropy is unchanged.

A small change of entropy ΔS can be measured in terms of the thermal energy qrev added to a system during a reversible process, and the constant temperature T, at which it is performed:

ΔS = Δqrev/T

It is said that the higher the randomness of a system, the higher is its entropy. And in a system that is more organized, its entropy is lower. This can be explained by the example below.

When a solid system is heated, the molecules or ions will start moving from each other as a result of electrostatic attraction or repulsion. Now in the liquid state, the molecules or ions display less order than in the solid state.

The entropy in the liquid state is said to be higher than that in the solid state. A gas will have an even higher disorder than a liquid hence the gaseous state has higher disorder and entropy.

The entropy of a substance increases as the temperature is increased because the addition of heat results in an increase in molecular randomness. Likewise, cooling a substance makes it more ordered, hence a decrease in entropy.

The Second Law of Thermodynamics

The Second Law of Thermodynamics is detailed in Chapter 3 of the Physical Chemistry book. It states that when energy is transferred there will be less energy available at the end of the transfer process than at the beginning.

For example, when you burn coal to make steam to drive a generator or to make electricity or to heat a home – you will end up with less heat than you originally started from the coal.

The Second Law is also said to be directional because it points the direction of change, i.e. the natural flow of heat or energy is always from high temperature or from being concentrated in one place to becoming diffused and spread out, if it is not hindered from doing so.

Second Law of Thermodynamics in Everyday Processes

Our daily lives consist of a multitude of more or less familiar occurrences, most of which are so common that we do not stop to consider the direction of these changes.

A drop of dye when placed in a beaker of water will eventually result in an evenly coloured solution, even without stirring. The dye molecules diffuse and spread itself as evenly as possible throughout the volume of water.

Making snowballs will always make one’s hands colder, because thermal energy always flows in a ‘hot-to-cold’ direction. No material can spontaneously gain thermal energy from colder surroundings. Why does the opposite process not occur?

The answer lies here. It would require the energy to become spontaneously more concentrated in order for the opposite process to occur. That is, this would involve a spontaneous increase in organization of molecules or energy which would mean the opposite of energy spreading out.

Written by: chemmum

Shaliza Dewa has authored 11 more articles.

I hold a Ph.D. in Chemistry and a B.Sc. in Biochemistry from the University of Sussex, England. I am a professional stay-at-home mom to three kids. I love the wonderful world of Chemistry and its practicality. Prior experiences in research and industry will be the stimulus for helping others in their understanding of Chemistry.

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