AN INTRODUCTION TO ENTROPY This page provides a simple, non-mathematical introduction to entropy suitable for students meeting the topic for the first time. At this level, in the past, we have usually just described entropy as a measure of the amount of disorder in a system. A very regular, highly ordered system (diamond, for example) will have a very low entropy. A very disordered system (a mixture of gases at a high temperature, for example) will have a high entropy. We now expand on this a bit, but luckily not too much! Let's look at this with a couple of thought experiments . . . Suppose you held a stack of ten coins between your finger and thumb. That's a fairly ordered state for them to be in. And then you dropped them on the floor. Every time you did this, you would get a different random pattern of coins on the floor - arranged just by chance. That is now a disordered system. 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. Technically, entropy applies to disorder in energy terms - not just to disordered arrangements in space. But we often just quickly look at how disordered a system is in space in order to make a judgement about its entropy. A system which is more disordered in space will tend to have more disorder in the way the energy is arranged as well. 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. And then you let them go and do what molecules do - move around, and bump into each other and the walls of the container. Each collision between two molecules will cause them to change direction, and it will probably speed up one of them, and slow down the other. After a very short time, their arrangement in space will be chaotic, and so will the way energy is shared between them. The faster moving particles have more energy; the slower ones less. The entropy has increased in terms of the more random distribution of the energy. In essence . . . "a system becomes more stable when its energy is spread out in a more disordered state". That is really all you need to know. 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. Entropy changes during physical changes Changes of state This includes solid to liquid, liquid to gas and solid to aqueous solution. Entropy is given the symbol S, and standard entropy (measured at 298 K and a pressure of 1 bar) is given the symbol S°. You might find the pressure quoted as 1 atmosphere rather than 1 bar in older sources. Don't worry about it - they are nearly the same. 1 bar is 100 kPa; 1 atmosphere is 101.325 kPa. Use whatever units the examiners give you. Here are some standard entropies for a few solids, all with the units J K-1mol-1: These all have low entropies because they are highly ordered solids, but notice that the entropy usually increases as the solid gets more complicated. What happens during change of state? The following figures are for the standard entropy of water in different states. The entropy increases as the molecules become more disordered as you go from solid to liquid to gas. Notice that there isn't very big jump in entropy when ice turns to water. That's because the hydrogen bonding between the liquid molecules imposes a fair amount of order on them even in the liquid. . . . and for benzene: Notice that the benzene values are bigger than those of water-steam. This is because benzene is a more complicated molecule. There are more ways of arranging the energy of the molecule in a disordered way over bigger molecules than smaller ones. What happens when an ionic solid dissolves in water? The ionic solid is highly ordered, and so has a relatively low entropy. Pure liquid water also has a certain amount of order as explained above. 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. Entropy increases.  September 6, 2000 -Christian Right Lobbies To Overturn Second Law Of ThermodynamicsPast Regents Questions on Entropy Jan 2010 44 Given the balanced equation representing a phase change: (4) It is exothermic, and entropy increases. Aug 2009- 45 The entropy of a sample of H2O increases as the sample changes from a(1) gas to a liquid (3) liquid to a gas (2) gas to a solid (4) liquid to a solid June 2008-45 Which 1-mole sample has the least entropy? June 2008- 17 A thermometer is in a beaker of water. Which statement best explains why the thermometer reading initially increases when LiBr(s) is dissolved in the water?(1) The entropy of the LiBr(aq) is greater than the entropy of the water.(2) The entropy of the LiBr(aq) is less than the entropy of the water.(3) The dissolving of the LiBr(s) in water is an endothermic process. (4) The dissolving of the LiBr(s) in water is an exothermic process. Jan 2007-27 In terms of energy and entropy, systems in nature tend to undergo changes toward(1) higher energy and higher entropy(2) higher energy and lower entropy(3) lower energy and higher entropy (4) lower energy and lower entropy Aug 2006- 49 Which list of the phases of H2O is arranged in order of increasing entropy?(1) ice, steam, and liquid water(2) ice, liquid water, and steam(3) steam, liquid water, and ice (4) steam, ice, and liquid water Jan 2006- 8 Given the balanced equation: (4) decreases because the particles are more randomly arranged Aug 2005-46 At STP, a sample of which element has the highest entropy? June 2005- 20 Systems in nature tend to undergo changes toward(1) lower energy and lower entropy(2) lower energy and higher entropy(3) higher energy and lower entropy (4) higher energy and higher entropy Jan 2005- 43 Which of these changes produces the greatestincrease in entropy? (1) CaCO3(s) --> CaO(s) + CO2(g) (2) 2 Mg(s) + O2(g) --> 2 MgO(s) (3) H2O(g) --> H2O(l) (4) CO2(g) --> CO2(s)Aug 2004-21 Even though the process is endothermic, snow can sublime. Which tendency in nature accounts for this phase change? (1) a tendency toward greater entropy (2) a tendency toward greater energy(3) a tendency toward less entropy(4) a tendency toward less energyJan 2004- June 2005- June 2008- 22 Systems in nature tend to undergo changes toward(1) lower energy and less disorder(2) lower energy and more disorder(3) higher energy and less disorder (4) higher energy and more disorder June 2004- June 2003-50 As carbon dioxide sublimes, its entropy 41 Which phase change represents a decrease in entropy?(1) solid to liquid (3) liquid to gas Aug 2002-39 Which sample has the lowest entropy? June 2002-44 Which process is accompanied by a decrease in entropy? (1) boiling of water(2) condensing of water vapor(3) subliming of iodine (4) melting of ice |
Which of the following statements correctly explain the increase in entropy that occurs when substance changes from solid to liquid or liquid to gas?
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