Electrochemical Cells Flashcards
What is standard electrode potential of a half cell
Pd of a half cell compared with a standard hydrogen half cell, measured at 298K, solution concens of 1moldm-3 and gas pressure of 1atm
What measures potential difference
Voltmeter
What is a salt bridge
Filter paper soaked in potassium nitrate stoppered with porous plug (as potassium/nitrate salts soluble so will not interfere with reaction) to allow ions that have built up on either side to move to other side, completes circuit as even with high resistance some electrons do flow, if circuit not complete current would cease
Why use high resistance voltmeter
Stops current flowing from where there are a lot of them to where there are less, means cannot make proper comparison
What happens when metals in contact with their ions in aqueous solution
Once equilibrium established metal atoms lose their electrons leaving them behind on piece of metal equal to rate of zinc ions gaining electrons from the piece of metal and become atoms, here there is a pd between the piece of metal which has negative charge and solution which contains positive ions
What does cell potential indicate
Indicated how it will react either in oxidation or reduction reaction
How are electrodes with very negative standard electrode potential likely to react and where is equilibrium in reduction equation
Indicate strong reducing agents so are oxidised themself, further to the left so to ion and electrons
How is electrode with very positive standard electrode potential likely to react and where does equilibrium lie for reduction equation
Indicates strong oxidising agents, reduced themself, equilibrium further to right than hydrogen so to element
What is standard electrode potential compared to and what is its value
Hydrogen
H+ + e- —-> 1/2 H2
0V
What happens if piece of wire used as salt bridge
It would set up its own equilibria and only allow electrons to pass not ions
What would happen if used silver in salt bridge
Would form precipitate with halogens, interfere with reaction
Where is flow of electrons from
From anode to cathode, more negative standard electrode potential to less
Which blocks undergo oxidation reactions
S/d and some p
Which block undergoes reduction
P
How to represent standard electrode potential
E circle with line through
How to use gas in electrode
Flow of gas directed down and through surface of water where inert platinum electrode used to catalyse the reaction
Do you inverse electrode potential value when going in other direction for reaction
No
What is conventional cell representation
-half cell with most negative potential goes on the left
-the most oxidised species from each half cell goes next to the salt bridge
-salt bridge shown as double line
-state symbols included
How to calculate overall cell emf
Emf (cell) = E circle(right) - E circle (left, more negative)
So minus negative to give positive
How to represent gas/aqueous ions on both sides in cell notation
Comma between the two and line then Pt (s)
What occurs on the right and what electrode
Reduction, cathode
What occurs on the left and what electrode
Oxidation, anode
Where do electrons move in cell
From anode to cathode
Do you put numbers in front of atoms in cell notation
Yes to balance charges on overall redox reaction
What does very positive electrode potential indicate
Better oxidising agent, will oxidise species more negative than themself
What does very negative electrode potential indicate
Better reducing agent, will reduce species less negative than themself
What do single lines show in cell notation
Boundary between phases
When calculating cell potential if there are two moles of a reactant do you multiply electrode potential by two
No it’s stays the same
