Kinetically controlled reactions

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Kinetically controlled reactions

Postby Kyung_Jin_Kim_1H » Sat Mar 17, 2018 10:17 pm

How can we tell from a free energy diagram whether a reaction is kinetically or thermodynamically controlled? My thought process is that if that if the curve is really tall, it's more responsive to temperature, and therefore be thermodynamically controlled, but I saw a worksheet that said otherwise.

Caitlin Mispagel 1D
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Re: Kinetically controlled reactions

Postby Caitlin Mispagel 1D » Sat Mar 17, 2018 10:23 pm

Even if a reaction is exothermic and seems to be thermodynamically favorable, it may happen at a very slow rate in which case it is not kinetically favorable. It also might have a high activation energy which is difficult to achieve which would effect the spontaneity of the reaction.

Nhan Nguyen 2F
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Re: Kinetically controlled reactions

Postby Nhan Nguyen 2F » Sat Mar 17, 2018 10:46 pm

Kinetically controlled rxn has lower Ea than thermodynamically controlled rxn at higher speed and higher collision frequency as seen in pg 644.

Masih Tazhibi 2I
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Re: Kinetically controlled reactions

Postby Masih Tazhibi 2I » Sat Mar 17, 2018 10:54 pm

Suppose you have two possible products, A and B. If A is kinetically favorable, it means that it has a lower activation energy, and a higher rate constant. If B is thermodynamically favorable, it means that as a product, it resides at a lower energy state, and thus is more stable. The problem is, that there are certain cases, like with carbon, where diamond is more thermodynamically stable, and theoretically carbon should be turning into diamond, but instead it stays as graphite. The reason is because graphite is more kinetically stable, that is, you have to input much less energy to overcome the activation energy to form graphite, whereas the activation energy for diamond is so high that it can't be easily overcome. In this case, you would say carbon is in Kinetic control because kinetic factors dominate thermodynamic factors.

Thus, there are two main metrics -- at least for our intents and purposes -- to see whether something is in Kinetic or Thermodynamic control.

The first is temperature. At low temperatures, Kinetic control tends to dominate because there is not enough energy in the form of heat to overcome the large activation energy required to reach the thermodynamically favorable state. At high temperatures, thermodynamic control takes over as there is an abundance of heat energy to overcome the activation energy to produce the more stable product.

We can also manipulate time. Over the long term, all reactions will eventually reach the thermodynamically favorable state. Because the activation energy is high relative to the kinetically favorable state, it will take a longer time, but in our example, the graphite will eventually turn into diamond. The rate constant is much lower, but the reaction still will proceed over the longterm. In a short time period however, expect kinetic control to dominate as the rate is much higher, and not enough time has elapsed for the large activation energy to be overcome for the thermodynamically favorable product.

Hope this helped!

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