1. Chemistry 130 Electrochemistry Dr. John F. C. Turner 409 Buehler Hall
Chemistry 130

2. Redox and electron transfer
A redox reaction is one where electrons are transferred between reactants and products, changing the formal oxidation states of the species involved.
In order to balance a redox reaction, we use half equations to show the change in formal oxidation state for the species involved:
Loss of Electrons is Oxidation – LEO – is a useful mnemonic for remembering the direction of redox reactions.
We term reactions such as a half-reaction or a couple.
Chemistry 130

3. Balancing redox reactions
A stoichiometric redox reaction is composed of two couples – one is an oxidation and one a reduction. Both the oxidation and reduction couple must be present in order to conserve charge.
Balancing a redox reaction is lengthy but straightforward. The key to balancing a reaction is to ensure that the electrons are balanced on both sides, using acid or base to balance the final equation with respect to acid numbers.
Chemistry 130

4. Balancing redox reactions
A stoichiometric redox reaction is composed of two couples – one is an oxidation and one a reduction. Both the oxidation and reduction couple must be present in order to conserve charge.
Balancing a redox reaction is lengthy but straightforward. The key to balancing a reaction is to ensure that the electrons are balanced on both sides, using acid or base to balance the final equation with respect to acid numbers.
Example: Write a balanced equation for the reduction of permanganate ion by thiosulphate in acid solution. The products of the reaction are sulfate ion and
The unbalanced reaction is
Chemistry 130

5. Balancing redox reactions
Example: Write a balanced equation for the reduction of permanganate ion by thiosulphate in acid solution. The products of the reaction are sulfate ion and
Step 1: Assign oxidations states to the species
Step 2: Write down the half-reactions
Chemistry 130

6. Balancing redox reactions
Step 1: Assign oxidations states to the species
Step 2: Write down the half-reactions
Step 3: Use acid and water to balance the number of oxygen atoms present
Chemistry 130

7. Balancing redox reactions
After steps 1, 2 and 3, we have two balanced half-equations:
Step 4: We now multiply them individually to balance the number of electrons:
Chemistry 130

8. Balancing redox reactions
Step 5: We now add the two half- reactions together:
Step 6: Species appear on both sides of the reaction, which we cancel. I particular, 40e- occur on both sides which means that we are correct in terms of the oxidation and reductions. Also, we have
Chemistry 130

9. Balancing redox reactions
Step 6: The final reaction is
Chemistry 130

10. Balancing redox reactions
Example: In basic solution, bromine disproportionates to give bromide ion and bromate ion. Write a balanced equation for this reaction.
The unbalanced reaction is
Step 1: Assign oxidations states to the species
Step 2: Write down the half-reactions:
Chemistry 130

11. Balancing redox reactions
Step 3: Use acid and water to balance the number of oxygen atoms present
We now have two individually balanced half reactions.
Step 4: We now multiply them individually to balance the number of electrons:
Chemistry 130

12. Balancing redox reactions
Step 5: We now add the two half- reactions together:
This is the balanced reaction in acid solution. We need the reaction in basic solution so we add sufficient hydroxide ion to remove the hydronium ion.
Chemistry 130

13. Balancing redox reactions
This is the balanced reaction in acid solution. We need the reaction in basic solution so we add sufficient hydroxide ion to remove the hydronium ion.
Which is the final, balanced equation in basic solution.
Chemistry 130

14. Redox reactions and electrochemical cells
Redox reactions involve the transfer of electrons from one species to another. For the two previous examples in acidic and basic solution, there reactions take place implicitly in the same system.
If we separate the two reaction physically, then no reaction will obviously take place unless there is a way that the electrons can pass between the oxidative and reductive parts of the reaction – the half-reactions.
If we connect the two reactions via a conducting wire, the external electrical contact, then electrons can move through the wire.
This is known as a voltaic cell.
A salt bridge connects the rest of the circuit – the internal electrical contact.
Chemistry 130

15. Redox reactions and electrochemical cells
We can harness the current that moves through the cell to do work and when we do so, we are converting chemical energy into work of some description – a light bulb or a motor are two obvious applications.
In a simple cell based on copper and silver, the half reactions are:
The reaction between copper metal and silver ion occurs because the electrons on copper are less tightly bound – copper is more electropositive than silver.
Spontaneously, the reaction will proceed at the Cu surface to form silver metal and copper ion.
Chemistry 130

16. Electrochemical cells
An electrochemical cell is system that contains an oxidation couple and a reduction couple, together with two electrodes that connect the two reactions and allow electrons to pass and a salt bridge that allows the necessary charge neutrality of the system to be maintained.
The two couples are termed half cells and in general, an oxidative and a reductive cell can be assembled to form the full electrochemical cell.
There is a special nomenclature that is used to describe electrochemical cells.
Oxidation takes place at the anode
Reduction takes place at the cathode
Chemistry 130

17. Cell diagrams Chemistry 130
Cell diagrams are used to represent the cell and is a method that represents the full cell reaction. We denote phase boundaries in the cell with a vertical line: |
and the boundary between the two cells, which contains the external electrical connection (the wire) and the internal electrical connection (the salt bridge) by a double line:
For the silver and copper half-reactions, we write the cell as
Chemistry 130

18. Cell diagrams Chemistry 130
A standard cell is written with the anode on the left and the cathode on the right.
The standard cell potential is then written as
Chemistry 130

19. Potentials and the Gibbs function
For each half reaction, we can write a half-cell potential, such that we can calculate the potential of the full cell by adding these half-cell potentials together.
The half-cell potential is related to the standard Gibbs function for the half reaction via
F is Faraday's constant and which is the charge on 1 mole of electrons. n is the number of electrons passed in the half-cell reaction.
Manipulating half-cell or full cell potentials is equivalent to manipulating the standard Gibbs function for the half-cell or full cell reaction.
Chemistry 130

20. Standard potentials Chemistry 130
The zero point for electrode potentials is the Standard Hydrogen Electrode (SHE)
Platinum acts as a catalyst for the hydrogen-hydronium oxidation, which is why there are two phase boundaries in the cell.
All other electrode potentials are measured against this reaction, with the pressure of hydrogen being 1 atm, all concentrations 1 M and the temperature being 298 K.
Standard half-cell reactions are written as reductions by convention.
Chemistry 130

21. Standard potentials Chemistry 130
Example: Calculate the cell potential for the zinc-copper cell, given the following cell potentials and construct a full cell diagram for the spontaneous reaction.
Step 1: Calculate the change in standard Gibbs function for each reaction
We do not need to include the value of F as we will convert back at the end of the calculation.
Chemistry 130

22. Standard potentials Chemistry 130
Example: Calculate the cell potential for the zinc-copper cell, given the following cell potentials and construct a full cell diagram for the spontaneous reaction.
Step 2: Arrange the equations to give a balanced stoichiometric reaction, changing as necessary
is positive, so as written, the reaction is non-spontaneous, so we reverse the reaction.
Chemistry 130

23. Standard potentials Chemistry 130
Example: Calculate the cell potential for the zinc-copper cell, given the following cell potentials and construct a full cell diagram for the spontaneous reaction.
We now have a spontaneous reaction as is negative.
Step 3: Convert the calculated back to the electrode potential:
Chemistry 130

24. Standard potentials Chemistry 130
Example: Calculate the cell potential for the zinc-copper cell, given the following cell potentials and construct a full cell diagram for the spontaneous reaction.
Step 4: Construct the cell diagram from the balanced, spontaneous reaction:
Chemistry 130

25. Standard potentials Chemistry 130
This method always gives the correct reaction and the correct direction for the spontaneous cell reaction, even when the cell reaction is not obvious. It is equivalent to the Right – Left rule but is simpler as you don't need to identify the cathodic and anodic reaction initially.
Chemistry 130

26. Thermodynamics and electrochemistry
The relationships between the equilibrium constant, the cell potential and the change in the standard Gibbs functions allows us to write a relationship between the cell potential and the equilibrium constant
Chemistry 130

27. Non-standard cell potentials
The dependency of the Gibbs function with concentration is
where Q is the reaction quotient. Including this in the equation for the cell potential gives the Nernst equation:
Chemistry 130

28. Non-standard cell potentials
The Nernst equation allows us to relate changes in concentration to the cell potential. For the reaction
is written for concentrations of 1 mol l-1. In general, the potential of the cell will vary with concentration as
Chemistry 130

29. Non-standard cell potentials
Because of the logarithmic relationship between concentration and the cell potential, we can measure very precisely very large or very small quantities.
Values of Keq that span 50 or 100 orders of magnitude result in changes in the cell potential of a few volts, allowing us to measure very subtle changes in concentration very accurately.
In order to use the Nernst equation, we first calculate the standard cell potential for the balanced stoichiometric reaction.
Chemistry 130

30. Non-standard cell potentials
Example: Calculate the cell potential based on the couples when
Step 1: Write down the half reactions for the two couples:
Step 2: From the balanced half cell reactions, calculate the change in the standard Gibbs function in terms of F
Chemistry 130

31. Non-standard cell potentials
Example: Calculate the cell potential based on the couples when
Step 3: Arrange the equations to give a balanced stoichiometric reaction, changing as necessary
Step 4: Ensure that is negative and therefore the reaction is spontaneous
Chemistry 130

32. Non-standard cell potentials
Example: Calculate the cell potential based on the couples when
Step 5: Calculate the standard cell potential from the change in standard Gibbs function.
Step 6: Calculate the value of the reaction quotient for the reaction
Chemistry 130

33. Non-standard cell potentials
Example: Calculate the cell potential based on the couples when
Step 7: Use the Nernst equation to calculate the cell potential
Chemistry 130

34. Chemistry 130 Nuclear Chemistry Dr. John F. C. Turner 409 Buehler Hall
Chemistry 130

35. Radioactivity Chemistry 130
The majority of the chemical elements are stable in that their nuclei do not show any form of decay.
In general, from hydrogen (Z = 1) to bismuth (Z = 83) all elements have at least one stable nucleus with the exceptions of technetium (Z = 43) and promethium (Z = 61).
After Bi, all the elements do not have a stable nucleus.
Even for the elements that have at least one stable isotope, many radioactive isotopes are also known and are produced artificially.
Examples include
Chemistry 130

36. The nucleus
The nucleus is a very complicated object. Part of the difficulty of describing the nucleus is the strength of the forces involved and the level at which they couple to each other.
The two forces in the nucleus that are important in terms of stability are the strong nuclear force and the electromagnetic force.
Electrons are bound in the atom by the electromagnetic force and the equilibrium size of the atom represents the magnitude of the force involved – atoms have a radius of ~ 1-2 Å or ~ m.
The nuclear radius is ~ 1-2 fm or ~ m or 5 orders of magnitude smaller than the atom.
Chemistry 130

37. The nucleus
The constituents of the nucleus are the neutron and the proton.
The neutron has no electric charge, a spin of ½ and a mass very similar to that of the proton.
The proton has a single positive charge and a spin of ½ as well. Both the neutron and the proton are therefore fermions and obey the Pauli principle in a manner analogous to electrons in the atom:
The wavefunction of a fermion must change sign on interchange of particles or
No more than two fermions can occupy the same state
Chemistry 130

38. The nucleus
Though a description of the nucleus is extremely hard, several features are immediately obvious.
There is a force that is attractive and extremely powerful – the strong nuclear force.
The strong force is powerful enough to overcome the electrostatic repulsion of the protons in the nucleus and so the nucleons in the nucleus are bound – elements after hydrogen are usually stable up to Z = 83.
The size of the nucleus is dictated by the balance between these two forces.
The definition of the nuclear surface and the nuclear radius and size is ambiguous and depends on the particle.
Chemistry 130

39. This week
Revise kinetics and acids, in addition to the electrochemistry
Chemistry 130