Now, standard mobile entropy represented by S0 is the entropy possessed by 1 mole of a substance at standard conditions. And remember, standard conditions means we're at 25°C and one atmosphere. Now here's something important to note. Different phases of a substance can exist simultaneously at standard conditions. For example, water. We know that water exists in its liquid phase predominantly between 0°C and 100°C. Here we're at standard conditions of temperature, which is 25°C. Even at that temperature, if you were looking at some liquid water, there would be some water vapor involved. Even in the room that you're standing in, there's water vapor all around us.
We may not see it, but it exists, and because of that we can find standard molar entropies of liquid water and gaseous water within our books. Now, solid water, though, the temperature is too high for it to exist under these standard conditions, so you won't find a standard molar entropy for solid water. Now here, if we're comparing standard mode entropies, we first have to look at the states of matter that are involved. If we're going from solid to liquid to gas, we see that the molecules are more and more spread out. As we transition from solid, liquid to gas, then becoming more spread out means they're more random in their orientation and arrangements, and therefore they're more chaotic. This would mean that we're increasing our standard molar entropies here.
If they're in the same phase, like you're comparing 2 solids, or comparing 2 liquids, or comparing 2 gases, then we look at what happens when the states of matter are the same. The first thing we do is we look at complexity. We're going to say the greater the complexity, the greater the standard molar entropy. For example, let's say we're looking at oxygen, the gaseous phase, and sulfur in the gaseous phase. Since they're both in the same phase, we then look at their complexity. Here O2 is made-up of two atoms of oxygen. An S8 is made-up of eight atoms of sulfur. S8 has more atoms involved, therefore it's more chaotic and therefore it's standard molar entropy would be higher.
But let's say that the two substances that I'm comparing are in the same phase. They have the same complexity. The last thing we look at is Massachusetts. The greater the mass, the greater the standard molar entropy. So for example, let's say we're looking at Br2 and iTunes, and let's say they both were in the same phase. They're both in the same phase. So next we look at it is our complexity. They both are composed of two atoms. Br2 is composed of two atoms of bromine. I2 is composed of two atoms of iodine, so the tiebreaker would look at their mass. If you look at their molar masses, you would see that I2 weighs more than Br2, and because of that I2 would be more chaotic and therefore a higher standard molar entropy.
All right, so just remember we can compare the standard mode entropies of different substances by first looking at the phases that they're in. Gases have the most entropy, followed by liquids, then solids. If they're tied, then look at the complexities, so the number of atoms composed within that substance. If they're still tied, use the mass as the final tiebreaker.