![]() Think back to having some gas particles in a container with a high degree of disorder both in their spacial arrangement, and in their energies - they are moving at all sorts of different speeds, changing all the time. Obviously if increasing the temperature involves a change of state in the material (from solid to liquid or liquid to gas), then you have increased the entropy. The increase in disorder in space and movement is associated with an increase in disorder in energy terms, but is much easier to imagine. I have gone straight from an increase in disorder in space and movement to an increase in entropy without specifically thinking about it in energy terms. ![]() Note: Notice that I have taken a short cut here. But when the solid dissolves in water, the whole system becomes highly disordered as the crystal breaks up and the ions find their way between the water molecules. Pure liquid water also has a certain amount of order as explained above. The ionic solid is highly ordered, and so has a relatively low entropy. What happens when an ionic solid dissolves in water? There are more ways of arranging the energy of the molecule in a disordered way over bigger molecules than smaller ones. This is because benzene is a more complicated molecule. Notice that the benzene values are bigger than those of water-steam. That's because the hydrogen bonding between the liquid molecules imposes a fair amount of order on them even in the liquid. Notice that there isn't very big jump in entropy when ice turns to water. The entropy increases as the molecules become more disordered as you go from solid to liquid to gas. What happens during change of state? The following figures are for the standard entropy of water in different states. These all have low entropies because they are highly ordered solids, but notice that the entropy usually increases as the solid gets more complicated. Here are some standard entropies for a few solids, all with the units J K -1mol -1: carbon (as diamond) Use whatever units the examiners give you. 1 bar is 100 kPa 1 atmosphere is 101.325 kPa. Don't worry about it - they are nearly the same. You might find the pressure quoted as 1 atmosphere rather than 1 bar in older sources. This includes solid to liquid, liquid to gas and solid to aqueous solution.Įntropy is given the symbol S, and standard entropy (measured at 298 K and a pressure of 1 bar) is given the symbol S°. If you look in textbooks or on the web, you will find explanations of increasing difficulty - some very scary indeed! Don't waste time on these at this level. "a system becomes more stable when its energy is spread out in a more disordered state". The entropy has increased in terms of the more random distribution of the energy. The faster moving particles have more energy the slower ones less. After a very short time, their arrangement in space will be chaotic, and so will the way energy is shared between them. And then you let them go and do what molecules do - move around, and bump into each other and the walls of the container.Įach collision between two molecules will cause them to change direction, and it will probably speed up one of them, and slow down the other. ![]() Suppose you managed to arrange some gaseous molecules in a container so that they were all exactly evenly spaced and so that they all had exactly the same energy - a fairly ordered state. A system which is more disordered in space will tend to have more disorder in the way the energy is arranged as well. But we often just quickly look at how disordered a system is in space in order to make a judgement about its entropy. Technically, entropy applies to disorder in energy terms - not just to disordered arrangements in space. Now, it is just imaginable that when you dropped them, by chance they would fall into a neat stack of coins like the one you started with, but the probability of that happening, compared to all the other ways that the coins might fall, is so very, very tiny that you would be totally amazed if it happened. Every time you did this, you would get a different random pattern of coins on the floor - arranged just by chance. ![]() That's a fairly ordered state for them to be in. Suppose you held a stack of ten coins between your finger and thumb. We now expand on this a bit, but luckily not too much! Let's look at this with a couple of thought experiments. A very disordered system (a mixture of gases at a high temperature, for example) will have a high entropy. A very regular, highly ordered system (diamond, for example) will have a very low entropy. This page provides a simple, non-mathematical introduction to entropy suitable for students meeting the topic for the first time.Īt this level, in the past, we have usually just described entropy as a measure of the amount of disorder in a system. ![]()
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