Shipwrecks: water undergoing reduction question (1 Viewer)

enigma_1

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From the data sheet, when a question asks you to show what reactions are happening and water is there, how do you know whether to use the equation with reduction potential of E=0.40V with Oxygen in it with water and oxygen undergoing reduction or the E=-0.83V with just water undergoing reduction?

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HeroicPandas

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H2O + e- <-> 1/2 H2 + OH- -0.83V
Use this when water is getting reduced and nothing else

1/2 O2 + H2O + 2e- <-> 2OH- 0.40V
Use this when the environment of the reaction contains oxygen

eg. A wet iron plate left on the beach. Since oxygen exists in the atmosphere, then we use equation 2 (RUST)

However, you may ask, "If water is oxygenated, then why don't we also use the first one?"... I would say that the second equation is more likely to occur compared to the first (0.4V vs -0.83V). The first equation requires -0.83V to occur, whereas, the second one produces 0.40V to occur

Please take this idea and evaluate it's validity before committing it to memory
 
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HeroicPandas

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