kinetics vs thermo

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Deena Doan 2F
Posts: 100
Joined: Sat Aug 17, 2019 12:17 am

kinetics vs thermo

Postby Deena Doan 2F » Sun Mar 08, 2020 11:04 pm

How do you determine whether it's kinetics or thermodynamics in regard to controlling a reaction? Is there a "hack" or a quick way to determine?

Alice Chang 2H
Posts: 101
Joined: Fri Aug 30, 2019 12:18 am

Re: kinetics vs thermo

Postby Alice Chang 2H » Sun Mar 08, 2020 11:20 pm

I'm not sure if these sites help, but here:

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Chemical_Equilibria/Principles_of_Chemical_Equilibria/Kinetically_vs_Thermodynamically_Stable
Summary:
- kinetics describes the rate of a reaction and how fast it reaches equilibrium, but no info after equilibrium
- thermodynamics gives info about the reaction after equilibrium/after the reaction takes place, but not during the reaction

https://en.wikipedia.org/wiki/Thermodynamic_versus_kinetic_reaction_control
Summary:
- short reactions = kinetics; long reactions = thermodynamics
- temperature change causes change in product ratio with 2nd equation with deltaG remaining fairly constant = kinetics
- temperature change causes change in product ratio with 1st equation with deltaE remaining fairly constant = thermodynamics

lilymayek_1E
Posts: 107
Joined: Sat Aug 17, 2019 12:16 am

Re: kinetics vs thermo

Postby lilymayek_1E » Sun Mar 08, 2020 11:30 pm

So in cases where kinetics dictate reaction rates, it is typically when there's a large energy barrier between the reactants and products & the reaction wouldn't happen in regular conditions & is typically irreversible. the example Lavelle gave was the reaction from Carbon (diamond) to Carbon (graphite). the gibbs free energy for the reaction is -3 kJ, very large & something that won't occur under standard conditions. Thermodynamically, this is unstable while in kinetics it's stable.
Additionally, under thermo conditions, the product will be more stable (lower energy level from reactants). Kinetics, the product will form faster.
I'm not sure if we'll have to differentiate between whether a reaction is kinetics or thermodynamic, but that general idea behind the products is useful. I found this differentiation from this link! hhttps://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Chemical_Equilibria/Principles_of_Chemical_Equilibria/Kinetically_vs_Thermodynamically_Stable

Frank He 4G
Posts: 50
Joined: Tue Nov 12, 2019 12:19 am

Re: kinetics vs thermo

Postby Frank He 4G » Sun Mar 08, 2020 11:35 pm

Since kinetics is about the rate of the reaction and thermodynamics tells you about how favorable it is, I would say that if a reaction is determined as favorable (spontaneous), then what's left is determining how fast it happens. Since a reaction like diamond turning into graphite can be favorable and yet not happen much in nature because the activation energy is so high (which means the rate is slow), then we could say it's controlled by kinetics. What's limiting the reaction from proceeding is the rate, or kinetics. Otherwise, it'd be a fast reaction and we might say it's controlled by thermodynamics.


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