electrode potentials Flashcards
(28 cards)
how can an electrochemical cell be made?
made from 2 different metals dipped in salt solutions of their own ions and connected by a wire
2 types of reaction that take place in a cell
- oxidation
2. reduction
what produces 0v?
when you use the same metal in an electrochemical cell
electrochemical cells involving different metals
metals have different reactivities.
more reactive metal loses electrons, releases electrons into external circuit, less reactive metal accepts electrons
salt bridge
allows the flow of ions to maintain a balance in charge
voltmeter
shows voltage between 2 half cells= cell potential/ EMF
cells involving solutions of the same element
e.g. Fe2+ and Fe3+
the conversion of Fe2+ to Fe3+ or vice versa takes place on a platinum electrode
why are electrodes made out of platinum?
it is an inert metal so wont react with the ions
what type of reaction occurs at each electrode?
reversible reaction
when writing half equations the reduction reaction goes in the forward direction
electrode potentials
measure how easily a metal is oxidised
metals that are easy to oxidise have a negative electrode potential
one that is harder to oxidise has a less negative electrode potential
half equation for metal that is oxidised is reversed when writing overall equation
standard hydrogen electrodes
impossible to measure the potential of a single electrode so you measure the electrode potential of a half cell against standard hydrogen electrode
if standard conditions are maintained the reading on the voltmeter when a half cell is connected to the standard hydrogen electrode will be the standard electrode potential of that half cell
what factors affect cell potential?
- temperature
- pressure
- concentration
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.
what are the standard conditions for electrode potentials?
any solution must have a conc of 1 moldm-3
temp= 298K
pressure= 100 kpa
calculating standard cell potentials
voltage of reduced metal- voltage of oxidised metal
drawing electrochemical cells
reduced form I oxidised form II oxidised form I reduced form
ROOR
oxidised metal followed by reduced metal
for those that use a Pt electrode Pt goes on far left
Positive electrode potential values
the reaction will be spontaneous
batteries
types of electrochemical cell which which provide electricity to power things. e.g. phones
2 types of cell
- rechargeable
2. non rechargeable
lithium cells
-made up of a lithium cobalt oxide electrode LiCoO2
- the other electrode is graphite
-electrolyte is a lithium salt in an organic solvent
Li= Li+ +e-
Li+ + CoO2+ e- = Li+(CoO2)-
how is a lithium cell recharged
a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions.
the reactions that take place in non-rechargeable batteries are difficult/impossible to reverse in this way
what is the difference between a normal cell and a fuel cell?
most cells have the chemicals that are needed to produce electricity present on the electrode/ in the electrolyte
in a fuel cell the chemicals are stored separately outside the cell and fed in when electricity is required.
alkaline hydrogen oxygen fuel cell
- hydrogen and oxygen are fed into 2 separte platinum electrodes
- the electrodes are separated by an anion exchange membrane that allows anions (OH-) and water to pass through it but not hydrogen or oxygen
- the electrolyte is an aqueous alkaline solution e.g. KOH
equation for negative electrode in a fuel cell
2H2 + 4OH- = 4H2O + 4e-
