Lithium/Water Reaction 1

[DonyWane]

I have found several references to the rate of reactions for metal/water reactions, but they have all been qualitative. Does anyone know what the rate of reaction for a lithium/water reaction is (quantitatively)? I know that it is slower than sodium and much slower than cesium, but that is about all I know.

Thanks!

 

[bchoate]

bchoate
I cannot give you quantitative data for the series I elements (lithium, sodium, potassium, rubidium, and cesium).
I can tell you the activation energy for Series I elements decreases with molecular weight as does the 1st ionization energy. Both trends indicate an increase in reactivity as
Li < Na < K < Rb < Cs. Lithium in water releases hydrogen smoothly and sometimes gets hot enough to ignite the hydrogen. Sodium is much more reactive and skips about the water surface and often ignites the hydrogen. While lithium's melting point is 181 C, potassium melts at 63 C.
Potassium dropped into water melts with the heat. The reaction is violent with ignition and detonation. If one wanted to roughly approximate a comparison between lithium and potassium, pieces (weight such that equal number of moles are present) can be dropped in water and timed for complete disapperance.

 

[25362]

From the NIST Chemical Kinetics Database:

For the reaction: H2O + Li -> H + LiOH
Reaction order:2
Temperature range: 200-1000K
Pressure: 0.07- 0.13 Bar

Rate: 1.91x10^-10(cm³/molecule s)(T/298K)^0.50e^{-62442(+/-4373J/mole)/RT}

Is this the information you wanted ?

 

[DonyWane]

Thanks! This seems very close to what I need. The next step is interpreting the rate equation! I notice that the reaction mechanism is not balanced for H2 - instead it is balanced for H. I don't know what impact that has on calculating H2 generation. Do you read the units (cm^3/molecule-s) as saying that for every Li molecule, x cm^3/sec of H2 gas will be generated? Or is it 1/2x cm^3/sec of H2 gas because of the balancing of the equation. I may not be interpreting the units (cm^3/molecule-sec) correctly anyway.

Following the logic of x and not 1/2x, I get 803.75 cm^3/sec of H2 gas for 25 g of Li at 25°C. (25g-->3.6 mole -->2.17E24 molecules.)

Seem right to you?

 

[DonyWane]

In reference to my previous post - I believe that I have figured it out. The rate equation given is the temperature dependent form of the rate constant. From this and the the fact that it is second order, the calculations should be able to be carried out. Unfortunately, the equation appears to be for a gas phase reaction, which means that I really can't use it here. Thanks for you help - really appreciate your pointing out the website. I had never seen it before - good stuff!

 

[25362]

When considering ambient temperature levels, say, 300 K, the lowest pressure 0.07 bar corresponds to a liquid (water) phase reaction, true ?

 

[bchoate]

bchoate
The equation given you from Nist is as you say for the gas phase. The kinetic library for Nist does not contain liquid phase data for the alkali metals and water. The rate given you is in the form of the Arrhenius equation.

rate = A * exp (-EA/RT) where A is a collision frequency parameter and EA is the activation energy. The EA energy maybe the same for both liquid and gas reactions but the collision integral will not be. I have looked extensively for a liquid phase rate constant without success.

In the liquid phase there are two options:
A. small amount of lithium is dropped into a large amount of water - pseudo first order in water
B. water dropped onto large amount of lithium - pseudo first order in lithium.
There is the trivial, near stochiometric case in which the temperature will rise more.

2Li + 2H2O ------> 2LiOH + H2

This is a redox reaction in which Li(0) is oxidized to Li(+) and H(-) is reduced to H(0). The reaction rates are related to the first ionization energies; the relative ease of removing an outer shell electron. These are listed below:
Li 520 KJ/mol
K 418.6 KJ/mol
Rb 402.9 KJ/mol
Cs 375.6 KJ/mol
The lower the ioniziation potential the more likely or faster the reaction. Alkali metals have very low IP's.

I am certain that rate constants have been measured for these reactions but I have not found them todate.

 

[DonyWane]

I tend to agree with your logic about pseudo-first-order. In my case, I am placing a small amount of Li in a large amount of water. I anticipated that I could approximate the rate as first order with respect to Li.

I would guess that the rate equation would be based on something like the shrinking core model. It wouldn't be extremely easy to find because of its dependence on particle geometry. I have found an interesting paper on the reaction of lithium hydride with water - it follows this approach somewhat. I think that LiH + H20 is a faster reaction than Li + H20, but I don't have any data to back that up.