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Ecell properties

Posted: Fri Feb 03, 2017 4:39 pm
Can somebody please explain the difference between an extensive and intensive property? Also, is Ecell an extensive or intensive one? Why? Thank you :)

Re: Ecell properties

Posted: Fri Feb 03, 2017 5:46 pm
Extensive properties are those that depend on the amount of substance in a system, such as mass, volume, enthalpy, and entropy. (Enthalpy and entropy being extensive is the reason why you need to multiply by the number of moles of each substance when you calculate ΔHrxn or ΔSrxn.)

On the other hand, intensive properties do not depend on the amount of substance, and include properties like temperature, density, and molar mass. E.g. whether you have 1 mole or 1000 moles of water, the molar mass is still ~18 g/mol.

Standard reduction potential, E°, is an intensive property. As for why, I'm not 100% sure but the explanation in the course reader is: "Standard reduction potential (E°) gives voltage difference between two standard electrodes which is always the same."

Re: Ecell properties

Posted: Sat Feb 04, 2017 11:55 am
Standard reduction potential is an intensive property because it does not depend on how many times the reaction occurs. Extensive properties (such as mass, volume, entropy, enthalpy, energy, etc.) will not affect the cell POTENTIAL.

[A common example Dr. Lavelle used in lecture was water density. No matter how much water is present (5.0 mL or 10.0 mL), the water compound has a certain constant density.]

Therefore, changing the stoichiometric coefficients in half reactions does not affect the value of the standard potential.