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Introduction
Transition metal complexes are typically very colorful because the differences between
the d-orbital energy levels match the energy of visible radiation. Cobalt(II) is a d7 transition
metal that changes color when involved in the following reaction
Co(H2O)62+(aq) + 4 Cl-(aq)  CoCl42-(aq) + 6 H2O(l)
(1)
where the octahedral aqua complex appears pink and the tetrahedral chloride complex appears
blue. Crystal Field Theory explains the color change as a result of two effects that change the
orbital splitting energy. The largest effect is the conversion from the octahedral to the tetrahedral
complex, attenuating the splitting energy to 4/9 of its original value. A secondary consideration
is the displacement of a relatively weak-field ligand (H2O) with a very weak-field ligand (Cl-)
(1). The color observed is the complementary color of the absorbed color, so if [Co(H2O)6]2+ has
a larger splitting energy, its absorbed wavelength will be bluer and its observed wavelength will
be redder compared [CoCl4]2-.
color absorbed
2.5
temperature dependent.
Absorbance
The reaction under investigation is also
Adding heat shifts the
reaction to the blue complex and removing heat
to
measure
some
1.5
1
0.5
shifts it to the pink complex (Figure 1), making it
possible
color observed
2
0
400
thermodynamic
500
600
700
800
Wavelength / nm
T=68.4 ºC
T=65.3 ºC
T=59.8 ºC
T=46.1 ºC
T=35.3 ºC
T=30.1 ºC
T=24.8 ºC
T=20.2 ºC
T=17.1 ºC
T=8.4 ºC
parameters of this reaction, namely entropy and Figure 1. Spectral scans of the hexaaqua (520 nm)
and tetrachloro (600-700 nm) complexes of cobalt.
enthalpy. The equilibrium constant is related to the
Gibb’s Free Energy of the reaction via
-Grxn = RT ln Keq
(2)
Since G is very temperature dependent, it can be replaced by enthalpy and entropy
G = H - TS
(3)
Substituting equations 2 into 1 and dividing both sides by RT yields a linear equation.
ln K eq 
y 
 ΔH 1 ΔS
 
R T
R
m  x b
(4)
By measuring the Keq as a function of temperature, equation 4 yields the H and S from the
slope and y-intercept.
Others who have studied this reaction have only focused on it as an example of
Le Chatelier’s Principle in a purely qualitative manner (2) (3) (4). These studies have students
setup a series of test tubes, to which they add various reagents that directly affect the
concentration of the reaction species and observe which way the reaction shifts. From these
qualitative observations students can draw make conclusions about whether the reaction is
exothermic, antropic, etc. While these experiments are interesting, they do not involve the use of
complicate data collection equipment or require much data processing – both of which are highly
valued skills that our laboratory program seeks to develop. The purpose of this experiment is to
quantitatively determine the entropy and enthalpy of the cobalt reaction by studying the
equilibrium as a function of temperature.
References
1. Miessler, G.; Tarr, D. Inorganic Chemistry, 3rd ed.; Prentice Hall: Upper Saddle River, 2004;
pp 360, 368.
2. Grant, A. J. Chem. Educ. 1984, 61, 446.
3. Orphardt, C. J. Chem. Educ. 1980, 57, 453.
4. Barrera, N.; McCarthy, J.; Dragojlovic, V. Chem. Educator 2002, 7, 142-145.