Electrochemistry - Digression On Water Flashcards
H2O is a molecule which forms what?
It forms a dipole.
What properties in the context of FCs does the dipole nature of H2O influence?
This polar structure makes water:
- absorb easily on charged surfaces like charged metals, ionic crystals
- mix with charged molecules like short chained alcohols
- Penetrate charged pores like those in ionomers - polymers with charged side chains - used as electrolytes for fuel cells
What does the polar structure of water lead to in general?
- The polar structure leads to hydrogen bonds (dt. WasserstoffbrΓΌckenbindung), a loose binding between dipoles of water and other polar fluids such as alcohols.
- These hydrogen bonds give rise to the high enthalpy of evaporation of water and the comparatively low vapor pressure of this small molecule, since the hydrogen bonding energy has to be overcome when evaporating.
- Hydrogen bonds arise through the attractive forces from an H-atom attached to an electronegative atom (like in water) and an electronegative atom like N, O or F from another molecule. (-N-H groups play a role in a fuel cell electrolyte).
- Hydrogen bonds are in the range of some kJ/mol
How does water start having ionic conductivity?
Ions dissolve in water giving rise to ionic conductivity. In an electric or chemical field they migrate.
Formula for electrostatic force that an ion is subject to.
πΉ_E = π§β πβ πΈ
π§ is ionic charge, π is elementary charge, πΈ is electric field in V/m
Formula for frictional force from the induced movement of ions.
πΉ_f = 6πππv
π: viscosity; r: radius of the species hydration shell considered; v: speed of migration
Formula for ion mobility.
U = (ze)/(6πππ)
UE gives the migration speed v
Why does the conductivity of ions decrease with decreasing size?
The relevant size for transportation though increases: the size of the ion including its water shell. Hence, whereas the ionic radii decrease from K over Na to Li,the increasing hydration shell size overcompensates this by far, hence the conductivity decreases.
What is water drag?
Hydronium ions also form hydration shells. Hydronium ions contribute to a minor degree to electric conductance by migrating in a field. Viscous forces limit the speed of this process down. When they move along a chemical gradient like in fuel cells or along an electrical gradient like in an electrolysis cells, this leads to undesired water transport from one elecrode to the other, called water drag.
What is the grotthus mechanism or hopping mechanism?
In a similar way the hydronium ion was formed by hopping of a proton from a water molecule, the proton can move away from a hydronium ion to another water molecule creating a new hydronium ion. It leaves the former hydronium ion as a water molecule. This transfer is very fast. The speed is believed to be limited by the rotation of the proton receiving water molecule and free of the limitation from viscous forces. It always happens randomly. Once a chemical potential gradient (e.g. fuel cell) or an electric field (e.g. electrolyzer) is applied the movement gets directed giving rise to ionic conductivity. This transport mechanism is called Grotthus Mechanism or Hopping Mechanism. Hence, it represents the dominant transportation mechanism of acidic electrochemical devices.
Formula for pH.
pH = βlog[H+]
For example, for 10^-1 mol/Liter H+, the pH is 1.
Formula for dissociation constant K.
K = [H+]*[OH-]
It is in mol/l
Formula for conductivity of non-ideal system.
π_i = π_i^id - k_i*sqrt(c_i)
K_i is empirical factor, c_i is concentration of component in each case
Conductivity of all ions sum up.
Formula for resistivity.
π =π
β
(A/l)
In ohm*m
Formula for conductivity.
ΞΊ = 1/π
In S/m
