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Unit 5- Module 2

Unit 5- Module 2 - Module 2 Unit 5 Comparing Free Energies...

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Module 2- Unit 5 Comparing Free Energies John Pollard University of Arizona So far we have established thinking that allows us to make qualitative predictions about the favorability of chemical processes. Now imagine that you are interested in quantitatively determining which of a collection of chemical processes could have occurred on primitive earth. How could you calculate the extent and directionality of a chemical reaction and use it to make predictions about competitive processes? Let’s start b y looking at a few reactions where a qualitative prediction of the extent and directionality is clear. In the combustion of methane CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) one would predict the reaction to be energetically favored (because upon forming products, A-A bonds are transformed into A-B bonds) and entropically favored (because the product mixture of gases is more complex by eliminating the diatomic O 2 ). Therefore, the reaction overall is most certainly favored. Now let’s consider the decomposition of carbon dioxide CO 2 (g) C(s) + O 2 (g) One would predict this reaction to be energetically unfavored (because of the formation of A-A bonds at the expense of A-B bonds) and entropically unfavored (because a solid is produced without an increase in the moles of gas, and the gas produced (O 2 ) is less massive and less complex than the reactant gas ,carbon dioxide). Therefore, the reaction overall is most certainly unfavored. In both of these situations, a qualitative approach works well for predicting the overall favorability of the process. We can still quantify how favorable or unfavorable these processes are, but their respective favorabilities will not change. How about this reaction? N 2 (g) + 3H 2 (g) 2NH 3 (g) Qualitatively, one would predict this process to be energetically favored (A-A to A-B bonds) but entropically unfavored (more moles of gas to less moles of gas). So is the overall reaction favored? What we find is that in processes where energetic and entropic factors work against each other, which occurs when either a reaction has ΔH= – and ΔS= – or ΔH= + and ΔS= +, then the extent and directionality are temperature dependent. In other words, the process might become more or less favored as temperature changes, and this depends on the signs of ΔH and ΔS .
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In general, entropic effects dominate at higher temperatures as compared to energetic effects. In other words, the ΔS contribution becomes more substantial as temperature increases. To more specifically quantify this effect, we rely on a well established equation that relates the enthalpy, entropy and temperature of any process and establishes a value called the Gibbs free energy (ΔG°). ΔG rxn = ΔH rxn TΔS rxn (where T is temperature in K) From a very practical perspective, the sign on ΔG rxn indicates if a reaction is favored or not. If ΔG rxn is negative (at constant T and pressure), then the process is favored (*your text refers to favored processes as being spontaneous) . The larger the value is (more negative) the greater the extent is of the reaction.
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