Quote:
Originally Posted by HUstylez
for electrical interactions in chemical systems, how can you show that the following are equal:
E= (q+ * q-)/(4*pi*episilon*r)
and E = 332 kcal/mol * (q+ * q-)/r
i'm not really sure what to do here
Well its roughly like this;
k*q1*q2/r*N= total potential energy if we have N charge-couples q+q- at distance r each, well kind of, its not perfectly accurate, we neglect neighboring pairs etc they kind of cancel etc
K=9*10^9 J/Cb^2*m in SI units, k the 1/(4pi*e)thing.
So if we were to express this as in units of elementary charge 1.6*10^-19 Cb and say Angstrom distances 10^-10 m we would have total energy per mole (=6.023*10^23 pairs of charges)
per mole total energy =N*k*q1*q2/r=6.023*10^23*9*10^9*(1.6*10^-19)^2/10^-10*(q+q-/r in units of elementary charge^2/Angstrom say) =
1387700 J/mole *(q+q-/r in terms of elementary charge units ^2/(distance in Angstrom units) so if its 2 charges e+ and e- at a distance of 1.5 A its basically 1*1/1.5 that q+q-/r thing etc )
(1 cal = 4.18 J)
Now 1387700 J/mole is also 1387700/(4.18*1000) cal/mole or 332 kcal/mol
as you wanted to have.
So basically all you do now is pick a system (eg NaCl say? ) put how many elementary charges is q+,q- each and what is the distance in A (of the "bond") and you use 332kcal/mol q+q-/r for the energy.
Last edited by masque de Z; 05-22-2013 at 07:28 PM.