1.11 Electrode Potentials and Cells
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What happens when a rod of metal is dipped into a solution of its own ions ?
An equilibria is set up between solid metal and aqueous metal ions
Write a half equation for Zn (s) to Zn (II)
Zn (s) -> Zn 2+ + 2e-
Write a half equation for Cu (II) to Cu (III)
Cu 2+ -> Cu 3+ + e-
What is the simplest salt bridge made of ?
Filter paper soaked in KNO3
Why are salt bridges necessary ?
• They complete the circuit, but do not react • Ion movement balances the charge.
What symbol is used to represent a salt bridge in standard notation ?
I I
What does I indicate in standard notation ?
A phase boundary ( Solid I Liquid I gas).
How would an aluminium/copper cell be represented in standard notation ?
Al (s) I Al 3+ II Cu 2+ I Cu (s)
What happens at the left hand electrode in a chemical cell (if drawn correctly)
• Left hand electrode is where oxidation occurs • Left hand electrode has the most negative E value.
What happens at the right hand electrode in an electrochemical cell (if drawn correctly)
• Right hand electrode is where reduction occurs • Right hand electrode is the half cell with the more positive E cell value.
What side of the cell has the most negative Eo value ? What occurs there ?
Left hand electrode - oxidation
What are the conditions for the standard hydrogen electrode used ?
• 298K • 100kPa • [H+] = 1dm-3.
What is the standard hydrogen electrode used for ?
• Comparing other cells against Eo • Eo of hydrogen is stated to be 0, so all other values are compared against it.
Why might you use other standard electrodes occasionally?
They would be cheaper and easier to set up, platinum is expensive
If the Eo of a reaction is more negative, what does this mean about the oxidising/reducing power ?
It means it’s a better reducing agent (easier to oxidise)
If the Eo value is more positive, what does this mean about the oxidising/reducing power ?
Better oxidising agent (easier to reduce)
What factors change Eo values
• Temperature • Concentration of ions.
What happens if you reduce the concentration of ions in the left hand side of an electrochemical cell ?
• Equilibrium shifts to oppose the change of removing the ions, this releases more e- • The left Eo becomes more negative, making the EMF more positive.
How do you calculate Ecell from Eo values ?
Ecell = right Eo - left Eo
When would you use a platinum electrode ?
When both the oxidised and reduced versions of a species are in solution
Why is platinum used as an electrode ?
• It’s inert so doesn’t take part in reactions • Good conductor so completes the circuit.
How would you predict if a reaction would occur in chemical cells ?
• Take both half equations • Find the species that is being reduced • Find it’s Ecell value and subtract the oxidation value from it • If 0<, reaction will occur.
What are disposable cells made from ?
Zinc/copper (II)
What are the two reactions that take place in zinc/carbon cells ?
• Zn is oxidised into Zn2+ • NH4+ is reduced to NH3 at carbon electrode