Lecture 4

Question 1

What is a pourbaix diagram?

Answer 1

Pourbaix are plots of electrode potential (y axis) versus pH (x axis). They map out the possible stable phases of an aqueous electrochemical system and compare this to the redox reactivity of water

Question 2

If m=n then dE/dpH equals..

Answer 2

-0.059V pH-1

Question 3

Describe the standard electrode potential for proton reduction

Answer 3

At sufficiently negative potential, protons are susceptible to reduction;
2H+ + 2e = H2
m=n=2
1 proton per electron reaction, therefore when move from pH 0 to higher pH the electrode potential for this reaction changes with a gradient of -59mV pH-1

Question 4

Describe the standard electrode potential for water oxidation

Answer 4

• At sufficiently high potentials, water is susceptible to oxidation
• Again, 1 proton per electron reaction, so pH causes electrode potential change of -59mV pH-1
• water stability region sits in the middle of the pourbaix diagram
• top region become O2 in oxidation, bottom region becomes H2 in reduction reaction

Question 5

What do the lines in a Pourbaix diagram mean?

Answer 5

• Vertical solid lines indicate protonation state changes
• Horizontal or diagonal solid lines indicate redox state changes. Top species most oxidised, bottom species most reduced

Question 6

What is an ‘overpotential’?`

Answer 6

In batteries, a kinetic barrier to the reaction manifests itself as an Ecell value that is smaller than the thermodynamic value, this is quantified as ‘overpotential’

Question 7

Give the equation for Ecell taking into account Eoverpotential

Answer 7

Ecell= E cathode - E anode - E overpotential