electrode potentials and cells Flashcards
describe what happens in a zinc/copper electrochemical cell
-zinc loses electrons more easily than copper so in the half cell on the left, zinc is oxidised to form Zn2+ ions. This releases electrons into the external circuit (the wire)
-in the other half cell, the same amount of electrons are taken in from the wire, reducing the Cu2+ to copper atoms
what is a salt bridge
filter paper dipped in KNO3 -allows ions to flow through and balance out the charges
when is platinum electrode used and why
-used in the SHE or a solution with 2 aqueous ions e.g. Fe2+/Fe3+
-platinum is an inert metal so won’t react with the ions
which half equation for electrochemical cells goes which way
-the reduction reaction goes in the forwards direction (the more positive electrode)
what does the electrode potential of a cell say about its ability to be oxidised/reduced
a metal that is easy to oxidise has a very negative electrode potential
it is a good reducing agent
what is the standard electrode potential of a half cell
-the voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
-if standard conditions are maintained, the reading on the voltmeter when a half-cell is connected to the SHE is its standard electrode potential
what are standard conditions
any solutions must have a concentration of 1 moldm-3
the temp must be 298K
the pressure must be 100kPa
how to work out Ecell or EMF when 2 half cells are connected
electrode potential of the electrode being reduced - electrode potential of the electrode being oxidised
what is the conventional way to draw electrochemical cells
reduced form|oxidised form||oxidised form|reduced form
e.g. Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)
more positive electrode on right except when there is a SHE which is always on the left
when is a reaction feasible
Ecell must be positive
what factors affect the redox of transition metals
pH:
-metals in high oxidation states tend to be reduced in acidic solution
-metals in low oxidation states tend to be oxidised in alkaline conditions
the ligand surrounding the metal ion
what happens when the conc of ions at one electrode changes
equilibrium shifts to oppose change, changing the emf
give the half equations of each half cell in a rechargeable lithium cell
negative electrode:
Li –> Li+ + e-
positive electrode:
Li+ + CoO2 + e- –> Li+[CoO2]-
advantages and disadvantages of hydrogen fuel cells
-only waste product is water (non toxic and non polluting)
-efficient
-doesn’t need to be recharged- will work as long as hydrogen and oxygen are present
-hydrogen is flammable and explosive- issues with storing
-need a constant supply of hydrogen and oxygen
-expensive to manufacture
benefits and risks of cells
-portable source of energy
-waste issues
