What is the origin (e.g. physical, thermodynamic, etc.) of the charge neutrality requirement in solids? Why must solids have charge neutrality in the bulk, yet a surface can be charged? Why can't the bulk have a net charge?© BrainMass Inc. brainmass.com December 24, 2021, 11:10 pm ad1c9bdddf
SOLUTION This solution is FREE courtesy of BrainMass!
Charge Neutrality in Solids
What is the origin (e.g. physical, thermodynamic, etc.) of the charge neutrality requirement in solids? Why must solids have charge neutrality in the bulk, yet a surface can be charged? Why can't the bulk have a net charge?
Good Question! Deals with some of the basic concepts in physical chemistry! We can explain this as follows:
Charge neutrality (e.g. ion-pairing) leads to more ordered state in solid. This means entropy of the system decreases i.e. the entropy change is negative (as entropy is the measure of disorderliness in a system). [Do not confuse this with the entropy of surrounding!]. On the other hand, for a spontaneous reaction or process, free energy change should be negative. Besides, chemical bond formation (can be through ion-pairing) releases energy, so it is an exothermic process, so enthalpy change is negative.
ΔG = ΔH - T ΔS
[G, H and S stands for free energy, enthalpy and entropy respectively, Δ represents change in the thermodynamic parameter]
So from the above mentioned equation, energy cost of forming ion-pair assemblies comes from the entropic contribution that renders the solid charge-neutral (irrespective of signs, the magnitude of ΔH should be higher than the magnitude of TΔS).
The origin of surface charge in solid can be varied. It can either be the unbalanced forces operative on the edges of a solid or due to the 'free electrons' present on surface giving rise to 'work function'.
The bulk of a solid can not have negative charge, because it will give rise to electrostatic repulsion and hence disorderliness in system. So from the above-mentioned equation it will not be an energetically favored process.© BrainMass Inc. brainmass.com December 24, 2021, 11:10 pm ad1c9bdddf>