Toni L6 Flashcards
(25 cards)
What is the equation for Coulomb’s law and what do each of the letters mean
F= Q1Q2/4piε0εrR2
F - electrostatic interactions
Q1 - charge 1
Q2 - charge 2
R - separated distance between charges
What is meant by double layer
A region existing at the boundary of two phases and assumed to consist of two oppositely charged layers
Define capacitance
Capacitance is the ability of a system to store electrical charge
Outline capacitance
Two conductive plated for area A separated by a small distance d can store charge when a potential is applied between the plates. The charge on one plate will be +Q as a result of electrons flowing round the circuit to make a charge -Q on the other plate
What do each of the symbols mean in the following equation
Q/∆𝜙 = C = A ∊/d
Q - charge
∆𝜙 - electric field
C - capacitance
A - area of electrodes
∊ - permittivity of the medium
d - distance between plates
What are the units of capacitance C
Has SI units coulombs per voltage CV^-1 which is also known as farads F
How can the capacitance per unit electrode area be calculated
∊/d
What does an electrode in solution behave as
An electrode in solution behaves like a capacitor, the charge in the electrode itself is balanced by and accumulate in the solution side of the interface
Describe the two layers of charge
There are two layers of charge: electrons in the metal and ions in the solution. This arrangement is called the double layer
Describe ∆𝜙
The electric field between two plates of charge at a distance d and potential difference is a constant as long as there is no net charge between plates
What does PZC stand for
Potential of zero charge
What is the potential of zero change
Is the potential applied to the electrode of interest (working electrode) with respect to a reference electrode at which there is no charge stored in the double layer of the working electrode
What does it mean if we have positive ∆𝜙
For positive ∆𝜙 the metal surface is positive (deficit of electrons) and an equal amount of anions must be present on the solution side of the interface
Outline the Helmholtz model
If we have a positively charges surface the negative ions in solution will be say as close as possible to the positive surface. The closest they can get to that surface will be its ionic radius this is also known as the outer Helmholtz plane. The solution is assumed to be neutral outside the first layer of counterions
How do we determine the Helmholtz capacitance
CH = ∊/d
Is capacitance potential independent
Capacitance is potential independent under some conditions (high ionic strength or large potentials)
Describe water molecules at the outer Helmholtz plane
Water molecules screen the electric field between capacitors. Water molecules are the OHP do not have the same freedom of movement as in bulk water therefore r for water at the interface might be expected to be lower than in the bulk
What is the problem for the Helmholtz model
The capacitance of electrode varies with the ionic strength of the solution at low ionic strength
Describe the relationship between the surface and the outer Helmholtz plane
There is a linear relationship between surface and outer Helmholtz layer
What is the gouy-chapman theory
If you add more thermal energy you are more likely for ions to diffuse away from the outer Helmholtz plane, this starts to create a slight difference in the relationship of the surface as you move into bulk behaviour. The relationship is no longer linear and is an exponential decay
What is the stern model
The stern model combines the diffuse layer with the distance of closest approach from the Helmholtz model to obtain a reasonable picture of the doublemlayer
What happens as you move experimentally away from the OHP
As you move away from the OHP you enter a diffuse layer and then once you move further away you go to a neutral bulk solution
What are the following letters mean in the Debye length equation
rDL, ∊, R,T, F, Ic
rDL - Debye length, thickness of double layer
∊ - permittibity of the medium
R - gas constant
T - temperature
IC - ionic concentration
What is rDL measured in
Meters
