4 Entropy

Free-energy change (G) and entropy change (S) A problem with ∆H A reaction that is exothermic will result in products that are more thermodynamically stable than the reactants. This is a driving force behind many reactions and causes them to be spontaneous (occur without any external influence). Some spontaneous reactions, however, are endothermic. How can this be explained? We need to consider something called entropy A SPONTANEOUS PROCESS (e.g. diffusion) will proceed on its own without any external influence. Entropy, S˚ Entropy is a description of the number of ways atoms can share quanta of energy. If number of ways of arranging the energy (W) is high, then system is disordered and entropy (S) is high. Substances with more ways of arranging their atoms and energy (more disordered) have a higher entropy. Elements …tend to have lower entropies than… Compounds Simpler compounds Complex compounds Pure substances Mixtures solid Liquid gas Temperature Entropy Solids have lower entropies than liquids which are lower than gases. When a solid increases in Temperature its entropy increases as the particles vibrate more. There is a bigger jump in entropy with boiling than that with melting. Gases have large entropies as they are much more disordered Predicting Change in entropy ‘∆S’ Qualitatively Balanced chemical equations can often be used to predict if ∆S˚ is positive or negative. In general, a significant increase in the entropy will occur if: -there is a change of state from solid or liquid to gas – there is a significant increase in number of molecules between products and reactants. NH4Cl (s)  HCl (g) + NH3 (g) ∆S˚ = +ve •change from solid reactant to gaseous products •increase in number of molecules both will increase disorder Na s + ½ Cl2 g  NaCl s ∆S˚ = -ve •change from gaseous and solid reactant to solid •decrease in number of molecules both will decrease disorder An increase in disorder and entropy will lead to a positive entropy change ∆S˚ = +ve Calculating ∆S˚ quantitatively Data books lists standard entropies (S˚) per mole for a variety of substances. It is not possible for a substance to have a standard entropy of less than zero. Elements in their standard states do not have zero entropy. Only perfect crystals at absolute zero (T = 0 K) will have zero ∆S entropy. At 0K substances have zero entropy. There is no disorder as particles are stationary. Note: the entropy change is very positive as a large amount of gas is being created increasing disorder