1.11 Electrode potentials
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How do electrochemical cells work?
• Use redox reaction between metals and the flow of electrons. • Transfer of electrons produces a potential difference, which is measured.
What is a half cell?
• A rod of metal dipped into a solution of its own ions.
How do you make an electrochemical cell?
Two half cells are connected together by a salt bridge.
What does the salt bridge do?
Transfers free ions to opposite electrodes (for red/ox)
What are the conditions of a salt bridge?
Needs to be made of a salt that doesn’t react with any ions in the half cells.
Which electrode is positive?
Cathode
Which electrode is negative?
Anode
At which electrode does reduction occur?
Cathode
At which electrode does oxidation occur?
Anode
What is the standard hydrogen electrode (SHE) ?
A half cell that is used to compare other metals, to see their tendency to release electrons.
How does a standard hydrogen electrode work?
• H₂ is bubbled through concentrated HCl. • H₂ is unreactive, so has no conduction, so a platinum wire is used as the electrode. • This half cell is connected to another metal half cell with a salt bridge.
What are the conditions of the SHE?
• 298K • 1 atm • [H⁺] in HCl = 1 moldm¯³
What is the potential of the SHE?
0
What is the EMF?
The potential difference between the SHE and a metal.
What is the EMF of a better reducing agent than the other half cell?
It has a more negative EMF.
What are the rules to follow when representing half cells?
SOLID LINE: a phase boundary, between different states. DOUBLE LINE: represents the salt bridge. The highest oxidation state goes next to the salt bridge. If there is no phase change, use a comma. SHE is always on the left of the salt bridge. (the most -ve EMF is on the left of the salt bridge)
How do you find the E-cell of an electrochemical cell?
Most positive EMF - Most negative EMF (E,red - E,ox)
How do you predict the redox reaction equation?
• Write the most -ve reduction equation on top. • Write the less -ve reduction equation on top. • The top equation goes from RHS - LHS. • The bottom equation goes from LHS - RHS. • DO this to see which way the equation is, then recombine and balance.
What must the E,cell be for the reaction to be feasible?
Positive
When does the E,cell increase?
When the forward reaction is favoured.