Electrochemistry Flashcards
What are the symbols for electrode potential? How can potential difference of a half cell be calculated?
ΦMetal - electrical potential of an electrode or metal wire
ΦSolution - the electrical potential of the solution
Potential difference = ΦMetal – ΦSolution
What is electrical potential? And current?
The work required (joules) to bring up a charge of one coulomb from an infinite distance to the object
Current: the flow of charge induced by an electrical potential
How is a gas phase thermodynamic equilibrium established?
How is a liquid/solution phase thermodynamic equilibrium established?
How is an electrochemical thermodynamic equilibrium established?
Same µ as chemical potential, so later, as with the solution equilibrium, expressions can be derived
How can the electrochemical equilibrium for Fe2+/Fe3+ be derived from thermodynamics?
What happens to solids in a Nernst equation? And gases?
They are not included in the equilibrium equation, concentrations effectively constant
Gases use partial pressure
What are the two types of experiments associated with an electrode potential?
Either an electrode is placed in solution and a potential is established to comply with predictions from the Nernstian equation
Or a potential is applied to a system, and concentrations realign to match the Nernstian predictions
What measurement is used in Nernstian equations for non-ideal solutions?
Activity, rather than concentrations
Gases remains with partial pressures
Why do we use a standard hydrogen electrode rather than measure the potential of one system?
It is impossible to measure the drop in potential for any one electrode solution system, and instead only differences between two systems can be measured
What conditions are used for the standard hydrogen electrode? Why?
Activity H+ = 1, [HCl]=1.18M
No current passed through
Black Pt to catalyse H2/H+ equilibrium via providing adsorption sites
H2 gas pumped in, 1atm
What equations are used alongside the SEH?
How does cell notation work?
Electrode potential of right relative to left (right - left)
|| = salt bridge
e.g Cu(s) | Cu2+(aq) (a=1) || Zn2+ (aq) (a=1) | Zn(s)
Follows what happens, i.e oxidation left reduction right
= boundary surface
How via calculation has a standard electrode potential been calculated for a system relative to hydrogen?
How can you write the Nernst equation for any chemical system?
1) Write out the reductions for both sides, with only 1 electrons
2) Of these equations, right - left
3) Nernst equation:
E cell = E cell standard - RT/F ln (Kc)
with activities/partial pressures, as dictated by the new equation formed
But E cell from the original cell, so right - left
What is the Nernstian equation for this reaction ? Cd(s) |Cd2+ (aq) || Pb2+
(aq) | (Pb(s) |
Remember, E cell is for reduction potentials so don’t flip any signs
What does it mean if a reaction occurs when a cell is short circuited?
The reaction is thermodynamically feasible
Delta ΔG<0
From ΔG = −FE
And so the cell potential must be positive
How can the dG<0 for a feasible reaction be proved from the second law of thermodynamics?
How can dG=dw additional be proved?
How can the formula relating ΔG and E be derived from dG=dw additional?
How can the relationship between G, E, H, and ΔS be derived?
How has the equation linking the equilibrium constant and E been derived?
How is the chemical potential of a non-ideal solution measured? How is activity calculated?
µ A = µ A ° + RT lnaA
Using activity
a= γ [A]
where γ is the activity coefficient
Or instead of [A] using molarity of A but aqueous effectively the same as water is 1L/Kg roughly
Why does non ideality arise in solutions and what evidence is there?
When there are grossly dissimilar intermolecular forces e.g interaction between Na+ - Cl-, Cl- Cl-, water and the ions, all differ in strength
Shown by the enthalpy of mixing expected to be 0, but not for non-ideal
What is the equation for ΔG with non ideal solutions?
ΔG = −RT lnγ
γ<1
Charge stabilises a solution
What is the idea of the Debye-Huckel theory?
The motion of ions in solution is not entirely random
There is a diffuse cloud of opposite charge surrounding an ion, arising from this attraction of opposite ions
An ionic atmosphere, arising from stabilisation of + — -
What is Ohm’s law?
K= 1 / p
Conductivity = 1/Resistivity
= l / R x A
Where l=length, R= Resistance, A=Area
What is Fick’s 1st Law?
Flux = Diffusion Coefficient x conc grad
What are the steps for finding an overall cell equation with one electrode?
As usual, right - left
Usually will simplify
If an acid e.g HCl, keep as it is, then simplify to H+ and Cl -
What are the general steps for finding the solubility product with electrode potentials?
Determine the equation of the cells given
Generally when you subtract them, they will give an equation for the solubility product
Then calculate electrode potentials for these cells under standard conditions (if not given) using their Nernst equation
Calculate standard change in gibbs energy use -FE standard, with the difference in the two electrode potentials converted to G
Then use G standard -= -RTlnK
