4/15/2024 0 Comments Absolute entropy pictureWe know that if we leave an ice cube out on the table at room temperature, it will spontaneously melt, and that the liquid water that results is more disordered than the crystalline solid. There's more chaos in water, and that's higher entropy. While the water molecules in liquid water are spaced just about as far apart as in ice (they're actually a little closer), they are relatively free to rotate, and those bonds are transient – breaking and reforming frequently. When water ice melts, a very well-ordered tetrahedral lattice of water molecules, each sharing four hydrogen bonds, is disrupted. Phase changes are another instance where entropy changes are usually obvious. It will be the combination of $\Delta H$ and $\Delta S$ that will be our ultimate predictor of spontaneity. The entropy of this system has clearly increased, so $\Delta S = S_f - S_i \gt 0.$ Such decomposition reactions can also be endothermic or exothermic. The decomposition, NH 4NO 3 → N 2O + 2H 2OĬonverts one mole of a substance into three moles of two different compounds. so there is something about negative $\Delta S$ and negative $\Delta H$ that might predict that this reaction is spontaneous. This reaction is actually known to be spontaneous and quite exothermic (It's the reaction that lifts some rockets into space). In running this reaction, we've imposed more order on these molecules: The products are more ordered than the reactants, therefore their entropy is lower. In this reaction, three moles of "particles" (H 2O and O 2) are converted into two moles of product (H 2O). First consider a synthesis reaction, 2 H 2 + O 2 → 2 H 2O Here are a couple of examples of entropy changes in chemical reactions. We can, however, work with the change in entropy, $\Delta S.$ Measuring absolute entropy is difficult (how do you put a number on disorder?) and we'll tackle that below.
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