The ammonia molecule (NH3) has a dipole moment of 5.0×1030Cm. Ammonia molecules

Question

The ammonia molecule (NH3) has a dipole moment of 5.0×1030Cm. Ammonia molecules in the gas phase are placed in a uniform electric field E  with magnitude 1.9×106 N/C .

a.What is the change in electric potential energy when the dipole moment of a molecule changes its orientation with respect to E  from parallel to perpendicular?

Express your answer using two significant figures.

b.

At what absolute temperature T is the average translational kinetic energy 32kT of a molecule equal to the change in potential energy calculated in part (a)? (Note: Above this temperature, thermal agitation prevents the dipoles from aligning with the electric field.)

Express your answer using two significant figures.

Explanation / Answer

A)

V = V2-V1

V1 = -PE*cos 0 = -PE

V2 = -PE*cos 90 = 0

dV = PE = 5*10^-30*1.9*10^6 =9.5*10^-24 J

B) dV = 1.5kT, so T = 9.5*10^-24/(1.5*1.38*10^-23) = 0.458 K

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