![]() ![]() Naturally, ordered, entropically lower systems have fewer possible microstates than disordered, entropically higher systems. Chemists define a microstate as a specific arrangement of matter and energy. The main way to quantify the orderliness of matter and energy involves summing the microstates a given system can have. Mathematical Representations of Entropy Statistical Definition The relatively low attraction between gas particles allows each molecule to move freely, resulting in a random dispersion. In chemistry, a gas provides another good example of an entropically-high system. Any motion and heat, similarly, are dispersed, resulting in a relatively consistent temperature and unpredictable movements of trees and animals. The mass of the trees, plants, rocks, and animals remains random and widely dispersed. The lattice energy of the crystal limits the motion of its particles, resulting in a perfectly geometric shape.Ī system high in entropy, by contrast, involves widely dispersed mass and energy. In chemistry, a solid mass of crystal provides another good example of a entropically-low system. Any heat energy remains controlled as well, with certain cold pockets, like the refrigerator, and hot pockets, like the oven, with different temperatures that don’t spread to the rest of the house. Any mechanical energy of motion, such as water and gas moving through pipes, remains tightly controlled and directed. The mass that makes up the house is ordered and exact to form the walls and furniture. Low EntropyĪ system low in entropy involves ordered particles and directed motion. The letter “ ” serves as the symbol for entropy.Īs we’ll find out in a later section, entropy has a lot of use for chemists and physicists in determining the spontaneity of a process. This means that as a system changes in entropy, the change only depends on the entropies of the initial and final states, rather than the sequence (“path”) taken between the states. Importantly, entropy is a state function, like temperature or pressure, as opposed to a path function, like heat or work. Topics Covered in Other ArticlesĮntropy is a measure of how dispersed and random the energy and mass of a system are distributed. In this article, we discover the meaning of entropy and its importance in thermodynamics, in both the universe and within a system. For the purposes of this page, you can ignore any reference to the word "system".īecause this is all covered in detail in my calculations book I shan't be setting any questions throughout this section on entropy and free energy You will find examples on pages 260 to the top of page 262, and in problems 15 and 16 in the end-of-chapter questions. There are lots in my calculations book if you have a copy. That's because there is a decrease in the total number of gas molecules present.Īnd that is all there is to it! You will, of course, need to practise doing this until you are completely confident, but you will need to find your own examples. The entropy has decreased - as we predicted it would in the earlier page. Total entropy at the end = 214 + 2(69.9) = 353.8 J K -1mol -1Įntropy change = what you end up with - what you started with.Įntropy change = 353.8 - 596 = -242.2 J K -1mol -1 You ended up with 1 mole of carbon dioxide and two moles of liquid water. You started with 1 mole of methane and 2 moles of oxygen. In the introductory page we looked at the following reaction and worked out that there would be a decrease in entropy. Where Σ (sigma) simply means "the sum of". Change in entropy = what you end up with - what you started with You add up the entropies for everything you end up with, and take away the entropies of everything you started with. Working out entropy changes for a reaction is very easy. ![]() In an exam, you will be given values for all the standard entropies you need. The thing you must be most careful about is the fact that entropy is measured in joules, not kilojoules, unlike most of the other energy terms you will have come across. 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 less recent sources. If your syllabus doesn't mention all these different sorts, just ignore this comment.Įntropy is given the symbol S, and standard entropy (measured at 298 K and a pressure of 1 bar) is given the symbol S°. Entropy change to the surroundings and the total entropy change are dealt with on another page. This page deals only with entropy changes to the system. Note: If you haven't already read the page about introducing entropy, you should do so before you go on. This page looks at how you can calculate entropy changes during reactions from given values of entropy for each of the substances taking part. ![]()
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