3.1.7.2 Gibbs Free energy and the concept of entropy OBJECTIVES: Learn the concept of entropy for use in explain transition metal chemistry Know the equation for Gibbs free energy. Starter: https://www.youtube.com/watch?v=gOWt_ Hq3yrE KEY WORDS: Entropy Enthalpy Gibbs Free energy Isotropic 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Enthalpy is not sufficient to explain why some reactions happen spontaneously and some do not. Why is CuSO4.5H2O soluble even though dissolving it is an endothermic reaction? If chemistry is all about rearranging systems to their lowest energy – most stable – point, then why do endothermic reactions happen at all?? In order to explain these processes we must introduce a concept I’ve mentioned before… entropy! 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Entropy is a physical properties of systems – in simplest terms it measures the disorder of a system. Mathematically – the more possible ways of arranging a system, the higher entropy. A disordered system has more possible ways of arranging itself. For example, a brick wall will have say 4 ways of arranging the bricks such that it is still a wall… A pile of bricks can have thousand of possible arrangements and still be a pile… So every time you throw bricks in the air there is a much higher chance they will fall in a disordered pile 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy This can explain simple processes with no enthalpy change associated. For example diffusion… or ligand exchange reactions 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy A higher temperature system will have more entropy – Why? 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Temperature is a measure of the average kinetic energy of the particles in a system, faster moving particles collide more and each collision changes the energy and direction of the particles involved – introducing more randomness. This will become important when we look at Gibbs free energy 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Entropy can explain why heat is transferred from hot to cold objects… 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy And why changing pressure can be used to shift the equilibrium of a reaction 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy 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 entropy because they are relatively ordered systems. Notice that the simpler the system, the lower the entropy (Diamond!!) Changing state will increase entropy – why? The increase from ice to liquid water is lower than expected- why? For each of these reaction predict weather entropy increases and decreases and explain your reasoning 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Review: Entropy is the measure of disorder of a system. Although a system can decrease in entropy, this must be accompanied by an increase the entropy of it’s surrounding. This is why processes that reverse the flow entropy are often exothermic – their heat given off increases the entropy of their surroundings Put mathematically: Ssystem + Ssurroundings > 0 3.1.7.2 Gibbs Free energy and the concept of entropy Entropy Calculating entropy from data values… After all these mind bending concepts the maths we are expected to do around around entropy is simple.. None of that S=Kb ln Ω nastiness… - look up this equation in your own time if your interested in taking this deeper. Change in entropy = entropy of products – entropy of reactants Or ΔS° = ΣS°(products) - ΣS°(reactants) 3.1.7.2 Gibbs Free energy and the concept of entropy Gibbs Free energy So how does entropy solve our original problem?? Weather or not a reaction will happen turns out to be a balancing act of enthalpy and entropy (which is dependant on temperature). This leads us to a new property we can calculate – the Gibbs free energy. Basically Gibbs free energy tells us the feasibility of a reaction occurring at a given temperature If it is negative then the reaction will happen spontaneously, if is positive then it cannot happen at that temperature. NOTE: Just because G is negative doesn’t mean the reaction has to happen, there are other factors that may prevent this e.g. v.high activation energy. 3.1.7.2 Gibbs Free energy and the concept of entropy Gibbs Free energy ΔG° = ΔH° - TΔS° units : Kjmol-1 From this equation you can see that, when change in entropy is very high Gibbs free energy is likely to be negative We can also see that the contribution from entropy is greater at higher temperatures. So if entropy increases – higher temperatures will favour the reaction, but if entropy decreases high temperatures will make the reaction less favourable. This is a more accurate explanation for le chataliers observations on the behaviour of equilibriums. See if you can think of a scenario where the reaction cannot happen at any temperature… What about a reaction that will be spontaneous at any temperature? 3.1.7.2 Gibbs Free energy and the concept of entropy Gibbs Free energy – ΔG° = ΔH° - TΔS° first steps in to really understanding chemistry units : Kjmol-1 This explains a huge amount of chemistry, from why changes of states occur at specific temperatures (the point at which entropic contribution out weighs enthalpic) To why some salts are soluble and some are not. For example – freezing. Freezing is actually a slightly exothermic (negative H), but the entropy change is negative, as we’ve gone from less ordered liquid, to a very structured solid. (less possible rearrangements). This means that at high temperatures the entropic contribution is very positive and outweighs the enthalpy leading to a positive G value. As we lower temperature, the entropic contribution decreases until we hit a point that it’s lower than the enthalpy change – this is the freezing point! For water this is higher than one might expect, this is because the hydrogen bonds formed have little energy (low H), and the difference in entropy between ice and water is surprisingly small due to the temporary structures formed in water. (thanks to those H bonds again)
