Page 313 - Physical chemistry understanding our chemical world

280 ELECTROCHEMISTRY

While it feels as though all the mouth fills with this pain, in fact
The word ‘amalgam’
probably comes from the pain only manifests itself through those teeth filled with metal,
the Greek malagma the metal being silver dissolved in mercury to form a solid – we
meaning ‘to make callita silver amalgam. Corrosion of the filling’s surface causes it
soft’, because a metal to bear a layer of oxidized silver, so the tooth filling also represents
becomes pliable when a redox couple, with silver and silver oxide coexisting.
dissolved in mercury. An electrochemical cell is defined as ‘two or more half-cells in
Another English word contact with a common electrolyte’. We see from this definition
from the same root is how a cell forms within the mouth, with aluminium as the more
‘malleable’. positive pole (the anode) and the fillings acting as the more negative
pole (the cathode). Saliva completes this cell as an electrolyte.All
A cell comprises two the electrochemical processes occurring are contained within the
or more half-cells in boundaries of the cell.
contact with a common Oxidation proceeds at the anode of the cell according to
electrolyte. The cell is
the cause of the pain.
Al (s) −−→ Al 3+ (aq) + 3e − (7.1)

and occurs concurrently with a reduction reaction at the cathode:
Oxidation reactions
occur at the anode.
0
−
Ag O (s) + 2e −−→ 2Ag (s) + O 2− (7.2)
2
Reduction reactions
occur at the cathode. The origins of the words ‘anode’ and ‘cathode’ tell us much.
‘Anode’ comes from the Greek words ana, meaning ‘up’, and
hodos means ‘way’ or ‘route’, so the anode is the electrode to which electrons travel
from oxidation, travelling to higher energies (i.e. energetically ‘uphill’). The word
‘cathode’ comes from the Greek hodos (as above), and cat meaning ‘descent’. The
English word ‘cascade’ comes from this same source, so a cathode is the electrode
to which the electrons travel (energetically downhill) during reduction.
The oxidation and reduction reactions must occur concurrently because the electrons
released by the dissolution of the aluminium are required for the reduction of the
silver oxide layer on the surface of the filling. For this reason, we need to balance
the two electrode reactions in Equations (7.1) and (7.2) to ensure the same number of
electrons appear in each. The pain felt at the tooth’s nerve is a response to this flow
of electrons. The paths of electron flow are depicted schematically in Figure 7.1.
Each electron has a ‘charge’ Q. When we quantify the number of electrons produced
or consumed, we measure the overall charge flowing. Alternatively, we might measure
the rate at which the electrons flow (how many flow per unit time, t): this rate is
termed the current I. Equation (7.3) shows the relationship between current I and
charge Q:
dQ
I = (7.3)
dt

So ultimately the pain we feel in our teeth comes from a flow of current.