Ask A Scientist© Chemistry Archive

name         Christine K.
status       student
age          30s

Question -   I'm doing a kinetics problem that deals with Activation
Energy.  I have a table of temperatures with corresponding viscosities of
pure water.  What I'm trying to find is the Activation energy of water
given a certain temperature and viscosity:  298 Kelvin.  I've tried
plotting the inverse temperatures and ln of the inverse of viscosities
and finding the y intercept from the slope of the line and from there
plugging in the y intercept equation into the equation:  1/(viscosity) =
Ae^-Ea/RT.  Doing this as well as taking two corresponding viscosities
and temperatures and getting the ratio to find another Ea thus finding
the A still doesn't work.  HELP!!!!
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The simplified hydrodynamic model for the viscosity of liquids predicts
that the temperature dependence of viscosity goes like:

                             viscosity = A * exp(-Ea / RT)

So you should plot log (viscosity) vs. 1/T in kelvin not 1/ (viscosity). In
most cases a good linear relationship is found. However, water is anomalous
because the hydrogen bonded structure changes with temperature. Empirically
this means that the pre-exponential factor,
A, is also temperature dependent. For "well behaved" liquids, Ea ~ 1/3 to
1/4 the heat of vaporization.

Vince Calder
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