Cells and Electrode potential

Question 1

What is electron transfer in solution?

Answer 1

When electron(s) from one molecule is transferred to another molecule resulting in the oxidation of one molecule and the reduction of the other molecule.

Question 2

What is happening here? and what is the overall reaction?
A —-> A+ + e-
B + e– —-> B-

Answer 2

A is being oxidised because it is losing one electron and B is being reduced as it gains one electron.
The overall reaction is
A + B —-> A+ + B-

Question 3

The rate of electron transfer are affected by multiple factors such as what?

Answer 3

Identities of A and B, driving forces (eg temperature and pressure), solvent and solvation (other ions present, charge), Distance between A and B, mass transport.

Question 4

Electron transfer can also take place at an electrode. Write out the half equation of molecule A being oxidised on the electrode

Answer 4

A —-> A+ + e-

Question 5

How is electron transfer in electrodes heterogenous?

Answer 5

It turns species in solution to a solid on the the electrode.

Question 6

What factors affect the rate of electron transfer in solution?

Answer 6

Identity of A, identity of electrode, driving force (electrode potential), mass transport, distance between A and the electrode, solvent and solvation (other ions present, charge especially at electrode interface)

Question 7

Write the half equations of the hydrogen fuel cell and hence the full equation.

Answer 7

H2 —-> 2H+ + 2e-
O2 + 4H+ + 4e- —-> 2H2O

2H2 + O2 —-> 2H2O

Question 8

What are the two most important variables in electrochemistry?

Answer 8

Current and potential

Question 9

Current flows due to the…

Answer 9

Electrons in the wire and ions in solution with a salt bridge between the two electrodes.

Question 10

ΔE is the potential difference of electrodes. What is the formula?

Answer 10

ΔE = ΔE(Final) - ΔE(Initial)

Question 11

The Electric potential of Zn 2+ is -0.763 and the electrode potential of Cu 2+ is 0.34. What is the value of ΔE when the electron flow is from Cu to Zn?
And what is Δ when the electron flow goes the other way?

Answer 11

ΔE = -0.763 - 0.34 = -1.1 eV
ΔE = 0.34 - (-0.763) = +1.1 eV

Question 12

What is the formula of GIbb’s energy of reaction?

Answer 12

ΔGr = -nFΔE 
n = stochiometric number of electrons 
F = Faraday's constant (96485 C mol-1)

Question 13

An overall cell reaction between A and B is:
A + B+ —-> A+ + B
ΔE = +2.1 V
Calcualate ΔGr and is the reaction spontaneous?

Answer 13

ΔGr = -1 * 96485 * 2.1 = -202.6 kj mol

ΔGr is negative, therefore it is spontaneous

Question 14

Electrons move from high energies to low energies. In terms of Electric potental what does this mean?

Answer 14

Electrons move from low potentials to higher potentials.

Question 15

In a cell with Ag and Ag+ on one side and Cu and Cu2+ on the other side,
Ag+ + e- —-> Ag E = +0.8 V
Cu2+ + 2e- —-> Cu E = +0.34 V

Which direction do electrons flow?
What is the value of ΔE of this cell?
What is the overall cell reaction?

Answer 15

Electrons flow from Cu to Ag as Cu has the lower energy potential

ΔE = 0.8 - 0.34 = +0.46 V

Cu + 2Ag+ —-> 2Ag + Cu2+

Question 16

How to measure a standard potential?

Answer 16

Put a metal in contact with 1M of its salt in solution

Question 17

What does the standard potential value mean?

Answer 17

It is when a metal in contact with 1M its salt, equilibrium will be established.

Question 18

What happens during equilibrium of a standard solution?

Answer 18

the rate of which the metal dissolves and ionises is equal to the rate of which the metal cation combines with an electron to form a metal ion and the electrons liberated during the reaction remain on the electrode.
Which creates an extra negative charge around the metal, creating a potential difference.

Question 19

What is the purpose of the SHE? (Standard Hydrogen Electrode)

Answer 19

Used as a reference with respect to other electrodes. when electrons liberated from the hydrogen accumulate on the electrode where the potential is assigned as 0.00 V. The potential difference shows whether the other metal is more or less positively charged compared to the hydrogen electrode.

Question 20

Zinc (Zn) has a -0.763 V electrode potential in equlibrium with the hydrogen electrode. What does this mean?

Answer 20

Zinc electrode is more negatively charged compared to the hydrogen electrode.