11/8/2021 0 Comments Convert Entropy Units
This page considers various entropy changes: of the system, of the surroundings, and the total change. The entropy of a substance is usually given as an intensive property either entropy per unit mass (SI. Boltzmanns constant, and therefore entropy, have dimensions of energy divided by temperature, which has a unit of joules per kelvin (JK 1) in the International System of Units (or kgm 2 s 2 K 1 in terms of base units).A heat reservoir (Figure 5.3) is a constant temperature heat source or sink.Because the temperature is uniform, there is no heat transfer across a finite temperature difference and the heat exchange is reversible.This is going to be quite a long page. Heat transfer from, or to, a heat reservoir. 5 Calculation of Entropy Change in Some Basic Processes. Energy with the lowest entropy, you posses the most useful form of energy for conversion to work.5.
Although energetically it might be feasible, it may have a large activation energy barrier that will slow it down, or even prevent it from happening altogether at a particular temperature.If the total entropy change is negative (if entropy decreases) then the reaction isn't feasible.The word spontaneous is often used in place of feasible.In everyday life, something is spontaneous if it happens of its own accord, without any input from outside. An exothermic change heats the surroundings, and increases the entropy of the surroundings.If, for example, the entropy change of the reaction (the system) was +112 J K -1 mol -1, then the total entropy change would beFor a reaction to be feasible, the total entropy has to increase - in other words the sign of the total entropy change must be positive.So what does "feasible" mean in reaction terms?A feasible reaction is one that is possible in terms of energy, but it doesn't mean that it will necessarily happen. But entropy change is quoted in energy units of J.That means that if you are calculating entropy change, you must multiply the enthalpy change value by 1000.So if, say, you have an enthalpy change of -92.2 kJ mol -1, the value you must put into the equation is -92200 J mol -1.Notice that the negative sign in the equation converts the negative exothermic enthalpy change into a positive entropy change. When you quote figures for enthalpy change they will have energy units of kJ. Tecmo bowl free onlineThe total entropy increases and so the reaction is feasible (and again literally spontaneous).Example 3: The reaction between magnesium ribbon and oxygenThis is simple to do as a calculation. So the total entropy change is positive, and the change is feasible - actually also literally spontaneous in this instance.Example 2: The reaction between concentrated ethanoic acid and solid ammonium carbonateThis is another endothermic change which becomes feasible because the increase in entropy due to the gaseous carbon dioxide formed outweighs the fall in entropy of the surroundings. However, If your examiners use the word spontaneous, then you will be expected to as well.Example 1: Dissolving ammonium nitrate in waterThis is a simple example of an endothermic change which nevertheless happens because there is a large increase in disorder when the crystal breaks up into its separate ions and mixes with the water.The entropy change to the surroundings will be negative because of the cooling caused by the ammonium nitrate dissolving, but this is more than made up for by the large increase in the entropy of the system. The energetics are right for a reaction to happen, but there is a huge activation energy.Chemistry counts the reaction between carbon and oxygen as spontaneous! Personally, I think that is daft, and I prefer the word "feasible", which is often used in this topic. It is a spontaneous change.But in chemistry, a spontaneous change doesn't have to be rapid in fact, it can be very, very, very slow indeed - even infinitely slow!For example, carbon burns in oxygen to make carbon dioxide, but a piece of carbon will stay totally unchanged however long you keep it unless you first heat it. You don't need to do anything else - the reaction happens entirely of its own accord. That page also contains all the raw data for the calculations involved. It is positive, and the reaction is feasible.Note: You will find this reaction described and explained in more detail in this page from the RSC's Practical Chemistry project. That is a high value because of the gas and liquid molecules being formed from two more ordered solids.If you use these figures to calculate the total entropy change, and assuming a temperature of 293 K, you should find that it comes to +31.3 J K -1mol -1. Look back at the equation further up the page if you aren't sure.Since the entropy change of the system is also negative, the total entropy change is bound to be negative whatever the temperature you choose. It is quicker just to give it a bit of thought!The entropy change of the surroundings is going to be negative because of the minus sign in the equation. We don't need to do a calculation with this. Convert Entropy Units Free EnergyTo TheThat means that you can write the fairly simple looking equation:The ΔS in this version is always just the entropy change of the system.You will see how this equation is used in the next page of this sequence.Because this is all covered in detail in my calculations book I shan't be setting any questions throughout this section on entropy and free energyTo the entropy and free energy menu. All I want to do for now is to see how this new term comes from what we have already discussed on this page.You also know how the entropy change of the surroundings is related to the enthalpy change of the reaction:Putting these together, and rearranging slightly by multiplying everything by T to get rid of the fraction gives:The term on the left-hand side is known as the Gibbs free energy, and is given the symbol ΔG.
0 Comments
Leave a Reply. |
AuthorMichelle ArchivesCategories |