How is it that a strongly non-spontaneous reaction (AG is large and positive) can still occur? Reactions such as photosy

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How Is It That A Strongly Non Spontaneous Reaction Ag Is Large And Positive Can Still Occur Reactions Such As Photosy 1 (111.66 KiB) Viewed 31 times

How is it that a strongly non-spontaneous reaction (AG is large and positive) can still occur? Reactions such as photosynthesis, biological processes and others fall into this category. If a non-spontaneous reaction is coupled with a spontaneous one, the combination can lead to an overall spontaneous change. We have previously seen this sort of combination with other state functions such as AH (Hess's Law of Heat Summation). Consider the following inorganic reaction: MnO₂ (s) → O₂ (g) + Mn (s) The reactant here is a transition metal oxide, a form of rust. Thus, this equation describes manganese rust (MnO₂) spontaneously going back to form pure Mn. This is not the direction that common experience would predict. Most metals tend to spontaneously rust in air. Element or Compound Fe₂O₂ (s) Fe (s) 0₂ (9) Al(s) Al (5) Al₂O3 (5) p Mn (s) Mey MnO₂ (s) Bez Cr (s) Cr₂O3 (s) Cu (s) CuO (s) AH, (in kJ/mol) - 824.2 0.0 0.0 0.0 -1675.7 0.0 0:0 -520.0 0.0 - 1139.7 0.0 - 157.3 sº (in J/molk) 87.4 27.3 205.2 28.32 Banan 50.9 K 32.0 Baca 53.1 23.8 81.2 33.2 42.6 a) Prove that the reaction for MnO₂ converting back into Mn and oxygen is non-spontaneous by calculating the AG at 41°C for this reaction. k] b) If this reaction was coupled to another more favorable reaction (one whose AG is negative enough to overcome the positive AG® of the first), the overall change could be made spontaneous. Consider the oxidation of aluminum metal to form aluminum oxide. 2 Al (s) + 3/2 0O₂ (g) → Al₂O3 (s) What is the AG for this reaction at 41°C? kJ c) If these two reactions are combined (make sure that the O₂ molecules cancel), you get the overall reaction for MnO₂ (s) reacting with Al (s) to yield Mn (s) and Al₂O3 (s). This reaction is known as a thermite reaction. By coupling the two reactions together, the conversion of MnO₂ back into pure manganese has been made spontaneous. It is important to realize that in order to couple two reactions, the reactants must actually directly interact with each other; the free energy can not just fly through space from one to the other. What is the overall AG for this thermite reaction at at 41°C? KJ