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Topic: Heating vs Stirring - reaction rate  (Read 969 times)

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Offline pcm81

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Heating vs Stirring - reaction rate
« on: June 24, 2018, 11:58:03 AM »
I remember reading some place that reaction rate between solid and liquid doubles with increase in temperature of like every 4 degrees. This makes sense for, diffusion, molecular dynamics etc. in a static liquid. Does temperature play equally important role vs reaction rate in case of stirred liquid? I realize some reactions can only happen above certain temperature or need energy input to drive the reaction, hence heat plays an additional role there. This question is not about those endothermic cases, but is about simple stuff like acid metal or acid metal-oxide reactions and reaction rate effect of heating vs stirring.

Thanks

mod edit - fixed typo in title
« Last Edit: June 24, 2018, 03:51:07 PM by billnotgatez »

Offline Corribus

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Re: Heating vs Stirring - reaction rate
« Reply #1 on: June 24, 2018, 12:28:20 PM »
I remember reading some place that reaction rate between solid and liquid doubles with increase in temperature of like every 4 degrees.
These are really just rules of thumb, and poor ones at that. The influence of T on reaction rate is complex and depends highly on the activation energy for the reaction. Obviously heat conduction/transfer in the medium will also play a role, but for reasonably small scale reactions it's not really the limiting factor.



mod edit - fixed typo in title
« Last Edit: June 24, 2018, 03:51:46 PM by billnotgatez »
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Offline pcm81

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Re: Heating vs Stirring - reaction rate
« Reply #2 on: June 24, 2018, 03:58:08 PM »
Long story short i am dissolving some metals and metal oxides in acid. I am hesitant to leave my hot plate on without watching it; but i do leave stirrer on. Just trying to decide if its worth while to monitor and stir at higher temperature, vs just letting it stir overnight...

Offline Enthalpy

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Re: Heating vs Stirring - reaction rate
« Reply #3 on: June 25, 2018, 06:37:31 AM »
The essential role of heat is not to help mixing the liquids. Heat provides energy to the molecules so the reaction proceeds.

When you see a law like exponential, or Arrhenius... you can be nearly certain it's a matter of activation energy. Diffusion would only accelerate as the square root of the temperature.

Typical reactions take place at a temperature where the mean energy is several times lower than the main energy barrier in the reaction. This is why a reaction takes seconds or days to proceed while molecules collide in a liquid at a picosecond timescale. That's also why reaction speeds are so sensitive to the temperature.

So: stirring can't replace heat.

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