Page 338 - Physical chemistry understanding our chemical world

INTRODUCING HALF-CELLS AND ELECTRODE POTENTIALS 305

SAQ 7.11 Write down an expression for the electrode potential of the
bromine | bromide couple. [Hint: it might help to write the balanced redox
reaction first.]

How do ‘electrochromic’ car mirrors work?

Introducing an orbital approach to dynamic electrochemistry
It’s quite common when driving at night to be dazzled by the
Electrochromic mirrors
lights of the vehicle behind as they reflect from the driver’s new-
are now a common
view or door mirror. We can prevent the dazzle by forming a
feature in expensive
layer of coloured material over the reflecting surface within an
cars.
electrochromic mirror. Such mirrors are sometimes called ‘smart
mirrors’ or electronic ‘anti-dazzle mirrors’.
These mirrors are electrochromic if they contain a substance that changes colour
+
according to its redox state. For example, methylene blue,MB (II), is a chromophore
because it has an intense blue colour. II is a popular choice of electrochromic material
for such mirrors: it is blue when fully oxidized, but it becomes colourless when
reduced according to
0
+
MB −−→ MB + e −
colourless blue (7.24)

N

(H 3 C) 2 N S N(CH 3 ) 2
Cl −
(II)

We can now explain how an electrochromic car mirror operates. The mirror is
constructed with II in its colourless form, so the mirror functions in a normal way.
The driver ‘activates’ the mirror when the ‘anti-dazzle’ state of the mirror is required,
+
and the coloured form of methylene blue (MB ) is generated oxidatively according
to Equation (7.24). Coloured MB blocks out the dazzling reflection at the mirror by
+
absorbing about 70 per cent of the light. After our vehicle has been overtaken and
+
we require the mirror to function normally again, we reduce MB back to colourless
0
MB via the reverse of Equation (7.24), and return the mirror to its
colourless state. These two situations are depicted in Figure 7.6.
An electron is donated
We discuss ‘colour’ in Chapter 9, so we restrict ourselves here to to an orbital during
saying the colour of a substance depends on the way its electrons reduction. The electron
interact with light; crucially, absorption of a photon causes an elec- removed during oxida-
tron to promote between the two frontier orbitals. The separation tion is taken from an
in energy between these two orbitals is E, the magnitude of which orbital.
relates to the wavelength of the light absorbed λ according to the
Planck–Einstein equation, E = hc/λ, where h is the Planck constant and c is the