Studied by 1 person
AQA A Level Chemistry
salt bridge
filter paper soaked in/ inert tube filled with an ionic solution (usually KNO3) that has one end in cathode solution and one in anode solution
function of salt bridge
to maintain electrical neutrality within the cell by providing ions to either side which completes the circuit by allowing ions to pass through it
electrode potential
a measure of the ability of a metal to lose electrons
Standard Hydrogen Electrode (SHE)
baseline used to compare electrode potential on half cells to when they are connected to it
negative voltmeter value when connected to SHE
other metal is a better reducing agent (is being oxidised)
positive voltmeter value when connected to SHE
SHE is a better reducing agent
describe SHE
hydrogen gas is passed down to a platinum electrode in a solution of 1 mol dm-3 H+ at 298K and 100kPa
role of voltmeter in a cell
measures the difference in electron build up either side of it
conventional representation of cells
reduced species on the right, vertical line for phase boundary, double line for salt bridge, most oxidised species closest to salt bridge
E⦵ values
show equilibria when a half cell is connected to SHE, if value is positive the ions are being reduced and if negative they’re being oxidised
E⦵ cell
overall electromotive force of a whole cell, found by finding difference between E⦵ of both half cells, if positive the reaction is feasible
E⦵ cell equation
E⦵ cell = E⦵right(reduction) - E⦵left(oxidation)
why platinum is used as an electrode
it is inert and conducts electricity and acts as a source of electrons
more positive E cell value
more reduced species, on right side
more negative E cell value
more oxidised species, left side
lithium cell positive electrode half equation
Li+ + CoO2 + e- → Li+[CoO2]-
lithium cell negative electrode half equation
Li → Li+ + e-
alkaline hydrogen cell positive electrode half equation
O2 + 2H2O + 4e- → 4OH-
alkaline hydrogen cell negative electrode half equation
2H2 + 4OH- → 4H2O + 4e-
hydrogen cell overall equation
O2 + 2H2 → 2H2O
electrochemical series
the list of half cell reactions and their electrode potentials in numerical order
half cell
one half of an electrochemical cell, constructed of a metal dipped in its ions or a platinum electrode with 2 aqueous ions
advantages of hydrogen oxygen fuel cells
more efficient (less energy lost as heat), don’t need to be recharged, only waste product is water, no carbon dioxide produced
disadvantages of hydrogen oxygen fuel cells
hydrogen is highly flammable, expensive to transport and store hydrogen, energy is required to make hydrogen and oxygen
what happens when voltmeter in a cell is replaced by a bulb
EMF decreases to 0V because the concentration of ions in each solution becomes equal
why an EMF value for a commercial cell is different to the theoretical value
non-standard conditions
change made to a cell to allow the cell reaction to go to completion
replace the voltmeter with a lamp/ wire
