electrochemical cells are based on the fact that if you place two different metals in a salt solution and connect them together, a current flows as electrons move from the more reactive metal to the less reactive metal, creating a potential difference
electric potential of one electrode cannot be measured by itself, instead connect two electrodes together and measure the difference in potential between them
by measuring the potential difference between two electrodes, you can measure how good of a reducing agent the metals are, as reducing agents release electrons
the circuit in an electrochemical cell is made up of two electrodes of different metals, in salt solutions of their own ions, joined together by a salt bridge between solutions
a salt bridge is a tube of unreactive ions that can move between the solutions to carry the flow of charge, but will not affect the reaction
each one of the solutions is a half cell, which has a cell potential, this can be used to work out if it will be oxidised or reduced when connected to another half cell
conventional cell representation rules:
half cell with most negative potential goes on the left
most oxidised species from each half cell goes next to the salt bridge
salt bridge is shown with a double line
include state symbols
the standard hydrogen electrode is used as a measuring standard for half cell potentials
the standard hydrogen electrode has a potential of 0.00V measured under standard conditions
standard conditions for the SHE are:
pressure of 100kPa
temperature of 298K
concentration of 1.00 moldm^-3
the standard hydrogen electrode is made up of hydrochloric acid, hydrogen gas, and platinum electrodes, which are inert and conduct electricity so do not interfere with the reaction
if you measure the potential of a half cell under standard conditions, you can compare this value with the SHE, to get a numerical value for the potential of the half cell
negative potentials = more easily oxidised then the SHE, will lose electrons
positive potentials = more easily reduced than the SHE, will gain electrons
standard half cell potentials can be used to calculate the overall cell EMF
EMF = E(right) - E(left)
EMF = most positive E - most negative E
if the overall EMF is positive, the reaction is spontaneous and favourable
the more positive the EMF, the more favourable the reaction
half-equations can be combined to give the overall cell reaction, always flip the half-equation with the more negative potential, electrons should be balanced, then combine
positive potentials are better oxidising agents
negative potentials are better reducing agents
electrochemical cells can be used commercially as a source of energy
the types of commercial electrochemical cell are:
non-rechargeable, rechargeable, fuel
rechargeable cells are based on a reversible reaction which allows the reactants to reform from the products, the reforming of reactants is the recharging
an example of a rechargeable cell is the lithium ion cell, commonly used in phones, laptops and cars, they are made up of a lithium cobalt oxide anode, and a carbon cathode, with lithium salt in organic solvent
at the anode of a lithium ion cell, the equation is:
Li(+) + CoO2 +e- -> Li(+)[CoO2](-)
at the cathode of a lithium ion cell, the equation is:
Li -> Li(+) + e-
in a lithium ion cell, the reverse of the reactions happen when the cell is recharging, this is caused by applying an external voltagegreater than the EMF to drive the electrons in the opposite direction
non-rechargeable cells are based on reactions which are not reversible, therefore they cannot be recharged by applying an external voltage
fuel cells are used to generate a current without needing to be recharged
an example of a fuel cell is the hydrogen fuel cell, which uses a supply of hydrogen and oxygen from the air, and the only waste product it produces is water
at the anode of a hydrogen fuel cell, the equation is:
O2 + 2H2O + 4e- -> 4OH-
at the cathode of a hydrogen fuel cell, the equation is:
2H2 + 4OH- -> 4H2O + 4e-
the overall reaction in a hydrogen fuel cell is:
H2 + 1/2(O2) -> H2O
hydrogen fuel cells have disadvantages, hydrogen is highly flammable, and they are expensive to produce
a fuel cell does not need to be electrically recharged because the reactants are supplied continuously
an advantage of using a methanol fuel cell over a hydrogen fuel cell is that methanol is a liquid whereas hydrogen is a gas, so methanol is easier to transport
when writing cell representations, if it involves the SHE that always goes on the left, and the species with the highest charge on each side goes next to the salt bridge, so the SHE is always Pt|H2|H+||
the advantage of using hydrogen in a fuel cell over an ICE is that a higher proportion of energy from combustion is converted into kinetic energy