It's important to emphasize that this negative temperature isn't some state "below" absolute zero. It's only the negative temperature that keeps the cloud of atoms from collapsing in this set of circumstances. This created something that could be viewed as an "anti-pressure," which should cause the collection of atoms to collapse. But the authors were able to switch things so that the atoms had attractive interactions. Under normal circumstances, these atoms repel, and thermodynamics behaves as we've come to expect. To create one of these systems, the authors set up an optical lattice of potassium atoms, chilled to near absolute zero. Because negative temperature systems can absorb entropy while releasing energy, they give rise to several counterintuitive effects, such as Carnot engines with an efficiency greater than unity." "In thermal contact," the authors write, "heat would flow from a negative to a positive temperature system. Strange things would happen if you bring it together with a system that has a normal temperature. If you could maximize the entropy in the system, temperature becomes discontinuous-it jumps from positive to negative infinity. This has some pretty bizarre consequences. In thermodynamic terms, you've reached negative temperatures. As this happens, entropy actually starts to go down, since an increasing fraction of the atoms begin to occupy the identical energy state. As you add more energy, more and more atoms start occupying the maximum energy state. Now imagine a system where there's an upper limit on the energy state an atom can occupy. This in turn increases the entropy of the system, since fewer and fewer atoms are in the same energy state. With more energy, they start spreading out evenly among these states. If you start with a system at absolute zero and add energy, the atoms or molecules it contains start occupying higher energy states. In a normal system, there's a lower limit on energy content-absolute zero-but no upper limit. To understand how temperatures can go negative, you have to think in terms of thermodynamics, which is governed by energy content and entropy. They found that the negative temperature system was stable for hundreds of milliseconds, raising the prospect that we can study a radically different type of material. Yesterday, a team of German researchers reported that they were actually able to produce a system with exactly that. But absolute zero is as low as a temperature can get, and we can't actually reach it, so progress will ultimately be limited.Īs thermodynamics defines temperature, it's theoretically possible to have a negative value. This has allowed them to study unusual states of matter, like Bose-Einstein condensates, which behave quite differently from the materials we're familiar with. If ∆ S total for a reaction is positive, the reaction will be feasible, if negative it will not be feasible.Over the past decades, researchers have made significant progress in cooling objects closer to absolute zero, the temperature at which all molecular motion reaches its minimum. ∆ S total = ∆ S system + ∆ S surroundings So if we want to predict the direction of a chemical reaction we must take account of the total entropy change of the system and the surroundings, and that includes the effect on entropy of any heat change from the system to the surroundings (or in the other direction, heat taken in from surroundings to system). So the total entropy change (of the Universe, ie system + surroundings) brought about by the reaction is +307 J K ‑1 mol -1. ∆ S surroundings = 591 J K -1 mol -1, more than enough to outweigh the value of ∆S system of –284 J K ‑1 mol -1. the same amount of energy dumped into the surroundings will make more difference at lower temperature – this rationalises the ‘division by T’įor the ammonia / hydrogen chloride reaction at 298 K:.the more negative the value of ∆ H, the more positive the entropy increase of the surroundings.the negative sign means that an exothermic reaction (∆ H is negative, heat given out) produces an increase in the entropy of the surroundings.The formal derivation is complex but leads to the expression ∆ S of the surroundings (∆ S surroundings) = –∆ H / T. But how can we evaluate the entropy change caused by ‘dumping’ 176 kJ mol -1 of heat energy into the surroundings? (Notice that this is 176 000 J mol -1.) It must be positive as more quanta of energy lead to more possible arrangements and it must be more than 284 J K -1 mol -1. It is negative as we have calculated (and predicted from the reaction being two gases going to a solid). As we have seen above, the entropy change of the ammonia / hydrogen chloride reaction (‘the system’) is –284 J K -1 mol -1.
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