Rate constant with varying temperature

[HopefulMDclass2020]

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According to EQ, an increase in temperature will always increase the rate constant. However, what if the reaction is exothermic, wouldn't raising the temperature cause the reaction to shift to the left, and lower the rate constant?

Thanks in advance.

 

[mik30102]

Your getting two separate ideas confused. The reaction rate will indeed increase (if we are starting with all reactants) compared to a lower temperature even for a exothermic reaction. The exothermic/endothermic part will just determine where equilibrium happens, but this does not effect the actual reaction rate. Remember even at complete equilibrium reactions are still occurring in both directions.

 

[chemist16]

To make the above post more explicit, you're confusing kinetics with thermodynamics. Whether a reaction is endo- or exo-thermic is a thermodynamic concept. The rate of a reaction is a kinetic construct. Just look at the Arrhenius equation. It takes the form of k = e^-x/T where x is some Ea/R. When you increase T, the exponential term just becomes larger.

 

[HopefulMDclass2020]

Perfect, thank you! One last question, I would assume that the addition of enzymes to an uncatalzyed reaction would increase the rate constant. I seems very intuitive that it would. If so, we can basically say only an increase in temperature or the addition of enzymes (to an otherwise uncatalzyed rxn) will increase the rate constant?

 

[HopefulMDclass2020]

To make the above post more explicit, you're confusing kinetics with thermodynamics. Whether a reaction is endo- or exo-thermic is a thermodynamic concept. The rate of a reaction is a kinetic construct. Just look at the Arrhenius equation. It takes the form of k = e^-x/T where x is some Ea/R. When you increase T, the exponential term just becomes larger.

Perfect, makes complete sense. Thank you 🙂

 

[mik30102]

Perfect, thank you! One last question, I would assume that the addition of enzymes to an uncatalzyed reaction would increase the rate constant. I seems very intuitive that it would. If so, we can basically say only an increase in temperature or the addition of enzymes (to an otherwise uncatalzyed rxn) will increase the rate constant?

Yup.